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An Overview of Blockchain Technology: Architecture, Consensus, and Future Trends

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• Published: 04 July 2019

A systematic review of blockchain

• Min Xu   ORCID: orcid.org/0000-0002-3929-7759 1 ,
• Xingtong Chen 1 &
• Gang Kou 1  

Financial Innovation volume  5 , Article number:  27 ( 2019 ) Cite this article

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Blockchain is considered by many to be a disruptive core technology. Although many researchers have realized the importance of blockchain, the research of blockchain is still in its infancy. Consequently, this study reviews the current academic research on blockchain, especially in the subject area of business and economics. Based on a systematic review of the literature retrieved from the Web of Science service, we explore the top-cited articles, most productive countries, and most common keywords. Additionally, we conduct a clustering analysis and identify the following five research themes: “economic benefit,” “blockchain technology,” “initial coin offerings,” “fintech revolution,” and “sharing economy.” Recommendations on future research directions and practical applications are also provided in this paper.

Introduction

The concepts of bitcoin and blockchain were first proposed in 2008 by someone using the pseudonym Satoshi Nakamoto, who described how cryptology and an open distributed ledger can be combined into a digital currency application (Nakamoto 2008 ). At first, the extremely high volatility of bitcoin and the attitudes of many countries toward its complexity restrained its development somewhat, but the advantages of blockchain—which is bitcoin’s underlying technology—attracted increasing attention. Some of the advantages of blockchain include its distributed ledger, decentralization, information transparency, tamper-proof construction, and openness. The evolution of blockchain has been a progressive process. Blockchain is currently delimited to Blockchain 1.0, 2.0, and 3.0, based on their applications. We provide more details on the three generations of blockchain in the Appendix . The application of blockchain technology has extended from digital currency and into finance, and it has even gradually extended into health care, supply chain management, market monitoring, smart energy, and copyright protection (Engelhardt 2017 ; Hyvarinen et al. 2017 ; Kim and Laskowski 2018 ; O'Dair and Beaven 2017 ; Radanovic and Likic 2018 ; Savelyev 2018 ).

Blockchain technology has been studied by a wide variety of academic disciplines. For example, some researchers have studied the underlying technology of blockchain, such as distributed storage, peer-to-peer networking, cryptography, smart contracts, and consensus algorithms (Christidis and Devetsikiotis 2016 ; Cruz et al. 2018 ; Kraft 2016 ). Meanwhile, legal researchers are interested in the regulations and laws governing blockchain-related technology (Kiviat 2015 ; Paech 2017 ). As the old saying goes: scholars in different disciplines have many different analytical perspectives and “speak many different languages.” This paper focuses on analyzing and combing papers in the field of business and economics. We aim to identify the key nodes (e.g., the most influential articles and journals) in the related research and to find the main research themes of blockchain in our discipline. In addition, we hope to offer some recommendations for future research and provide some suggestions for businesses that wish to apply blockchain in practice.

This study will conduct a systematic and objective review that is based on data statistics and analysis. We first describe the overall number and discipline distribution of blockchain-related papers. A total of 756 journal articles were retrieved. Subsequently, we refined the subject area to business and economics, and were able to add 119 articles to our further analysis. We then explored the influential countries, journals, articles, and most common keywords. On the basis of a scientific literature analysis tool, we were able to identify five research themes on blockchain. We believe that this data-driven literature review will be able to more objectively present the status of this research.

The rest of this paper is organized as follows. In the next section, we provided an overview of the existing articles in all of the disciplines. We holistically describe the number of papers related to blockchain and discipline distribution of the literature. We then conduct a further analysis in the subject field of business and economics, where we analyze the countries, publications, highly cited papers, and so on. We also point out the main research themes of this paper, based on CiteSpace. This is followed by recommendations for promising research directions and practical applications. In the last section, we discuss the conclusions and limitations.

Overview of the current research

In our research, we first conducted a search on Web of Science Core Collection (WOS), including four online databases: Science Citation Index Expanded (SCI-EXPANDED), Social Sciences Citation Index (SSCI), Arts & Humanities Citation Index (A&HCI), and Emerging Sources Citation Index (ESCI). We chose WOS because the papers in these databases can typically reflect scholarly attention towards blockchain. When searching the term “blockchain” as a topic, we found a total of 925 records in these databases. After filtering out the less representative record types, we reduced these papers to 756 articles that were then used for further analysis. We extracted the full bibliographic record of the articles that we identified from WOS, including information on the title, author, keywords, abstract, journal, year, and other publication information. These records were then exported to CiteSpace for subsequent analysis. CiteSpace is a scientific literature analysis tool that enables us to visualize trends and patterns in the scientific literature (Chen 2004 ). In this paper, CiteSpace is used to visually represent complex structures for statistical analysis and to conduct cluster analysis.

Table  1 shows the number of academic papers published per year. We have listed the number of all of the publications in WOS, the number of articles in all of the disciplines, and the number of articles in business and economics subjects. It should be noted that we retrieved the literature on March 25, 2019. Therefore, the number of articles in 2019 is relatively small. The number of papers has continued to grow in recent years, which suggests that there is a growing interest in blockchain. All of the extracted papers in WOS were published after 2015, which is seven years after blockchain and bitcoin was first described by Nakamoto. In these initial seven years, many papers were published online or indexed by other databases. However, we have not discussed these papers here. We only chose WOS, representative high-level literature databases. This is the most common way of doing a literature review (Ipek 2019 ).

In the 756 articles that we managed to retrieve, the three most common keywords besides blockchain are bitcoin, smart contract, and cryptocurrency, with the frequency of 113 times, 72 times, and 61 times, respectively. This shows that the majority of the literature mentions the core technology of blockchain and its most widely known application—bitcoin.

In WOS, each article is assigned to one or more subject categories. Therefore, we use CiteSpace to visualize what research areas are involved in current research on blockchain. Figure  1 shows a network of such subject categories. The most common category is Computer Science, which has the largest circle, followed by Engineering and Telecommunications. Business and Economics is also a common subject area for blockchain. Consequently, in the following session, we will conduct further analysis in this field.

Disciplines in blockchain

Articles in business and economics

Given that the main objective of our research was to understand the research of blockchain in the area of economics and management, we conduct an in-depth analysis on the papers in this field. We refined the research area to Business and Economics, and we finally retrieved 119 articles from WOS. In this session, we analyzed their published journals, research topics, citations, and so on, to depict the research status of blockchain in the field of business and economics more comprehensively.

There are several review papers on blockchain. Each of these paper contains a summary of multiple research topics, instead of a single topic. We do not include these literature reviews in our paper. However, it is undeniable that these articles also play an important role on the study of blockchain. For instance, Wang et al. ( 2019 ) investigate the influence of blockchain on supply chain practices and policies. Zhao et al. ( 2016 ) suggest blockchain will widely adopted in finance and lead to many business innovations and research opportunities.

The United States, the United Kingdom, and Germany are the top three countries by the number of papers published on blockchain; the specific data are shown in Table  2 . The United States released more papers than the other countries and it produced more than one-third of the total articles. As of the time of data collection, China contributed 11 papers, ranking fourth. The 119 papers in total are drawn from 17 countries and regions. In contrast, we searched “big data” and “financial technology” in the same way, and found 286 papers on big data that came from 24 countries, while 779 papers on fintech came from 43 countries. This shows that blockchain is still an emerging research field, and it needs more countries and scholars to join in the research effort.

We counted the journals published in these papers and we found that 44 journals published related papers. Table  3 lists the top 11 journals to have published blockchain research. First is “Strategic Change: Briefings in Entrepreneurial Finance,” followed by “Financial Innovation” and “Asia Pacific Journal of Innovation and Entrepreneurship.” The majority of papers in the journal “Strategic Change” were published in 2017, except for one in 2018 and one in 2019. Papers in the journal “Financial Innovation” were generally published in 2016, with one published in 2017 and one in 2019. All five of the papers in the journal “Asia Pacific Journal of Innovation and Entrepreneurship” were published in 2017.

Cited references

Table  4 presents the top six cited publications, which were cited no less than five times. The list consists of three books and three journal articles. Some of these publications introduce blockchain from a technical perspective and some from an application perspective. Swan’s ( 2015 ) book illustrates the application scenarios of blockchain technology. In this book, the author describes that blockchain is essentially a public ledger with potential as a decentralized digital repository of all assets—not only tangible assets but also intangible assets such as votes, software, health data, and ideas. Tapscott and Tapscott’s ( 2016 ) book explains why blockchain technology will fundamentally change the world. Yermack ( 2017 ) points out that blockchain will have a huge impact and will present many challenges to corporate governance. Böhme et al. ( 2015 ) introduce bitcoin, the first and most famous application of blockchain. Narayanan et al. ( 2016 ) also focus on bitcoin and explain how bitcoin works at a technical level. Lansiti and Lakhani ( 2017 ) argue it will take years to truly transform the blockchain because it is a fundamental rather than destructive technology, which will not drive implementation, and companies will need other incentives to adopt blockchain.

Most influential articles

These 119 papers were cited 314 times in total, and 270 times without self-citations. The number of articles that they cited are 221, of which 197 are non-self-citations. The most influential articles with more than 10 citations are listed in Table  5 . The most popular article in our dataset is Lansiti and Lakhani ( 2017 ), with 49 citations in WOS. This suggests that this article has had a strong influence on the research of blockchain. This paper believes there is still a distance to the real application of the blockchain. The other articles describe how blockchain affects and works in various areas, such as financial services, organizational management, and health care. Since blockchain is an emerging technology, it is particularly necessary to explore how to combine blockchains with various industries and fields.

By comparing the journals in Tables 4 and 5 , we find that some journals appeared in both of the lists, such as Financial Innovation. In other words, papers on blockchain are more welcomed in these journals and the journal’s papers are highly recognized by other scholars. Meanwhile, although journals such as Harvard Business Review have only published a few papers related to blockchain, they are highly cited. Consequently, the journals in both of these lists are of great importance.

Research themes

Addressing research themes is crucial to understanding a research field and exploring future research directions. This paper explored the research topic based on keywords. Keywords are representative and concise descriptions of article content. First, we analyzed the most common keywords used by the papers. We find that the top five most frequently used keywords are “blockchain,” “bitcoin,” “cryptocurrency,” “fintech,” and “smart contract.” Although the potential for blockchain applications goes way beyond digital currencies, bitcoin and other cryptocurrencies—as an important blockchain application scenario in the finance industry—were widely discussed in these articles. Smart contracts allow firms to set up automated transactions in blockchains, thus playing a fundamentally supporting role in blockchain applications. Similar to the literature in all of the subject areas, studies in business and economics also frequently use bitcoin, cryptocurrency, and smart contract as their keywords. The difference is that many researchers have combined blockchain with finance, regarding it as an important financial technology.

After analyzing the frequency of keywords, we conducted a keywords clustering analysis to identify the research themes. As shown in Fig.  2 , five clusters were identified through the log-likelihood ratio (LLR) algorithm in Citespace, they are: cluster #0 “economic benefit,” cluster #1 “blockchain technology,” cluster #2 “initial coin offerings,” cluster #3 “fintech revolution,” and cluster #4 “sharing economy.”

Disciplines and topics

Many researchers have studied the economic benefits of blockchain. They suggest the application of blockchain technology to streamline transactions and settlement processes can effectively reduce the costs associated with manual operations. For instance, in the health care sector, blockchain can play an important role in centralizing research data, avoiding prescription drug fraud, and reducing administrative overheads (Engelhardt 2017 ). In the music industry, blockchain could improve the accuracy and availability of copyright data and significantly improve the transparency of the value chain (O'Dair and Beaven 2017 ). Swan ( 2017 ) expound the economic value of block chain through four typical applications, such as digital asset registries, leapfrog technology, long-tail personalized economic services, and payment channels and peer banking services.

The representative paper for cluster “blockchain technology” was published by Lansiti and Lakhani ( 2017 ), who analyze the inherent features of blockchain and pointed out that we still have a lot to do to apply blockchain extensively. Other researchers have explored the characteristics of blockchain technology from multiple perspectives. For example, Xu ( 2016 ) explores the types of fraud and malicious activities that blockchain technology can prevent and identifies attacks to which blockchain remains vulnerable. Meanwhile, Aune et al. ( 2017 ) propose a cryptographic approach to solve information leakage problems on a blockchain.

Initial coin offering (ICO) is also a research topic of great concern to scholars. Many researchers analyze the determinants of the success of initial coin offerings (Adhami et al. 2018 ; Ante et al. 2018 ). For example, Fisch ( 2019 ) assesses the determinants of the amount raised in ICOs and discusses the role of signaling ventures’ technological capabilities in ICOs. Deng et al. ( 2018 ) argue the outright ban on ICOs might hamper revolutionary technological development and they provided some regulatory reform suggestions on the current ICO ban in China.

Many researchers have explored blockchain’s support for various industries. The fintech revolution brought by the blockchain has received extensive attention (Yang and Li 2018 ). Researchers agree that this nascent technology may transform traditional trading methods and practice in financial industry (Ashta and Biot-Paquerot 2018 ; Chen et al. 2017 ; Kim and Sarin 2018 ). For instance, Gomber et al. ( 2018 ) discuss transformations in four areas of financial services: operations management, payments, lending, and deposit services. Dierksmeier and Seele ( 2018 ) address the impact of blockchain technology on the nature of financial transactions from a business ethics perspective.

Another cluster corresponds to the sharing economy. A handful of researchers have focused on this field and they have discussed the supporting role played by blockchain in the sharing economy. Pazaitis et al. ( 2017 ) describe a conceptual economic model of blockchain-based decentralized cooperation that might better support the dynamics of social sharing. Sun et al. ( 2016 ) discuss the contribution of emerging blockchain technologies to the three major factors of the sharing economy (i.e., human, technology, and organization). They also analyze how blockchain-based sharing services contribute to smart cities.

In this section, we will discuss the following issues: (1) What will be the future research directions for blockchain? (2) How can businesses benefit from blockchain? We hope that our discussions will be able to provide guidance for future academic development and social practice.

What will be the future research directions for blockchain?

In view of the five themes mentioned in this paper, we provide some recommendations for future research in this section.

The economic benefits of blockchain have been extensively studied in previous research. For individual businesses, it is important to understand the effects of blockchain applications on the organizational structure, mode of operation, and management model of the business. For the market as a whole, it is important to determine whether blockchain can resolve the market failures that are brought about by information asymmetry, and whether it can increase market efficiency and social welfare. However, understanding the mechanisms through which blockchain influences corporate and market efficiency will require further academic inquiry.

For researchers of blockchain technology, this paper suggests that we should pay more attention to privacy protection and security issues. Despite the fact that all of the blockchain transactions are anonymous and encrypted, there is still a risk of the data being hacked. In the security sector, there is a view that absolute security can never be guaranteed wherever physical contact exists. Consequently, the question of how to share transaction data while also protecting personal data privacy are particularly vital issues for both academic and social practice.

Initial coin offering and cryptocurrency markets have grown rapidly. They bring many interesting questions, such as how to manage digital currencies. Although the majority of the previous research has focused on the determinants of success of initial coin offerings, we believe that future research will discuss how to regulate cryptocurrency and the ICO market. The success of blockchain technology in digital currency applications prior to 2015 caught the attention of many traditional financial institutions. As blockchain has continued to reinvent itself, in 2019 it is now more than capable of meeting the needs of the finance industry. We believe that blockchain is able to achieve large-scale applications in many areas of finance, such as banking, capital markets, Internet finance, and related fields. The deep integration of blockchain technology and fintech will continue to be a promising research direction.

The sharing economy is often defined as a peer-to-peer based activity of sharing goods and services among individuals. In the future, sharing among enterprises may become an important part of the new sharing economy. Consequently, building the interconnection of blockchains may become a distinct trend. These interconnections will facilitate the linkages between processes of identity authentication, supply chain management, and payments in commercial operations. They will also allow for instantaneous information exchange and data coordination among enterprises and industries.

How can businesses benefit from blockchain?

Businesses can leverage blockchains in a variety of ways to gain an advantage over their competitors. They can streamline their core business, reduce transaction costs, and make intellectual property ownership and payments more transparent and automated (Felin and Lakhani 2018 ). Many researchers have discussed the application of blockchain in business. After analyzing these studies, we believe that enterprises can consider applying blockchain technology in the four aspects that follow.

Accounting settlement and crowdfunding

Bitcoin or another virtual currency supported by blockchain technology can help businesses to solve funding-related problems. For instance, cryptocurrencies support companies who wish to implement non-cash payments and accounting settlement. The automation of electronic transaction management accounting improves the level of control of monetary business execution, both internally and externally (Zadorozhnyi et al. 2018 ). In addition, blockchain technology represents an emerging source of venture capital crowdfunding (O'Dair and Owen 2019 ). Investors or founders of enterprises can obtain alternative entrepreneurial finance through token sales or initial coin offerings. Companies can handle financial-related issues more flexibly by holding, transferring, and issuing digital currencies that are based on blockchain technology.

Data storage and sharing

As the most valuable resource, data plays a vital role in every enterprise. Blockchain provide a reliable storage and efficient use of data (Novikov et al. 2018 ). As a decentralized and secure ledger, blockchain can be used to manage digital asset for many kinds of companies (Dutra et al. 2018 ). Decentralized data storage means you do not give the data to a centralized agency but give it instead to people around the world because no one can tamper with the data on the blockchain. Businesses can use blockchain to store data, improve the transparency and security of the data, and prevent the data from being tampered with. At the same time, blockchain also supports data sharing. For instance, all of the key parties in the accounting profession leverage an accountancy blockchain to aggregate and share instances of practitioner misconduct across the country on a nearly real-time basis (Sheldon 2018 ).

Supply chain management

Blockchain technology has the potential to significantly change supply chain management (SCM) (Treiblmaier 2018 ). Recent adoptions of the Internet of Things and blockchain technologies support better supply-chain provenance (Kim and Laskowski 2018 ). When the product goes from the manufacturer to the customer, important data are recorded in the blockchain. Companies can trace products and raw materials to effectively monitor product quality.

Smart trading

Businesses can build smart contracts on blockchain, which is widely used to implement business collaborations in general and inter-organizational business processes in particular. Enterprises can automate transactions based on smart contracts on block chains without manual confirmation. For instance, businesses can file taxes automatically under smart contracts (Vishnevsky and Chekina 2018 ).

Conclusions

This paper reviews 756 articles related to blockchain on the Web of Science Core Collection. It shows that the most common subject area is Computer Science, followed by Engineering, Telecommunications, and Business and Economics. In the research of Business and Economics, several key nodes are identified in the literature, such as the top-cited articles, most productive countries, and most common keywords. After a cluster analysis of the keywords, we identified the five most popular research themes: “economic benefit,” “blockchain technology,” “initial coin offerings,” “fintech revolution,” and “sharing economy.”

As an important emerging technology, blockchain will play a role in many fields. Therefore, we believe that the issues related to commercial applications of blockchain are critical for both academic and social practice. We propose several promising research directions. The first important research direction is understanding the mechanisms through which blockchain influences corporate and market efficiency. The second potential research direction is privacy protection and security issues. The third relates to how to manage digital currencies and how to regulate the cryptocurrency market. The fourth potential research direction is how to deeply integrate blockchain technology and fintech. The final topic is cross-chain technology—if each industry has its own blockchain system, then researchers and developers must discover new ways to exchange data. This is the key to achieving the Internet of Value. Thus, cross-chain technology will become an increasingly important topic as time goes on.

Businesses can benefit considerably from blockchain technology. Therefore, we suggest that the application of blockchain be taken into consideration when businesses have the following requirements: accounting settlement and crowdfunding, data storage and sharing, supply chain management, and smart trading.

Our study has recognized some limitations. First, this paper only analyzes the literature in Web of Science Core Collection databases (WOS), which may lead to the incompleteness of the relevant literature. Second, we filter our literature base on the subject category in WOS. In this process, we may have omitted some relevant research. Third, our recommendations have subjective limitations. We hope to initiate more research and discussions to address these points in the future.

Availability of data and materials

Data used in this paper were collected from Web of Science Core Collection.

Abbreviations

Initial coin offering

Web of Science Core Collection

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Acknowledgements

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This research is supported by grants from National Natural Science Foundation of China (Nos. 71701168 and 71701034).

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Min Xu, Xingtong Chen & Gang Kou

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Three generations of blockchain

The scope of blockchain applications has increased from virtual currencies to financial applications to the entire social realm. Based on its applications, blockchain is delimited to Blockchain 1.0, 2.0, and 3.0.

Blockchain 1.0

Blockchain 1.0 was related to virtual currencies, such as bitcoin, which was not only the first and most widely used digital currency but it was also the first application of blockchain technology (Mainelli and Smith 2015 ). Digital currencies can reduce many of the costs associated with traditional physical currencies, such as the costs of circulation. Blockchain 1.0 produced a great many applications, one of which was Bitcoin. Most of these applications were digital currencies and tended to be used commercially for small-value payments, foreign exchange, gambling, and money laundering. At this stage, blockchain technology was generally used as a cryptocurrency and for payment systems that relied on cryptocurrency ecosystems.

Blockchain 2.0

Broadly speaking, Blockchain 2.0 includes Bitcoin 2.0, smart-contracts, smart-property, decentralized applications (Dapps), decentralized autonomous organizations (DAOs), and decentralized autonomous corporations (DACs) (Swan 2015 ). However, most people understand Blockchain 2.0 as applications in other areas of finance, where it is mainly used in securities trading, supply chain finance, banking instruments, payment clearing, anti-counterfeiting, establishing credit systems, and mutual insurance. The financial sector requires high levels of security and data integrity, and thus blockchain applications have some inherent advantages. The greatest contribution of Blockchain 2.0 was the idea of using smart-contracts to disrupt traditional currency and payment systems. Recently, the integration of blockchain and smart contract technology has become a popular research topic in problem resolution. For example, Ethereum, Codius, and Hyperledger have established programmable contract language and executable infrastructure to implement smart contracts.

Blockchain 3.0

In ‘Blockchain: Blueprint for a New Economy’, Blockchain 3.0 is described as the application of blockchain in areas other than currency and finance, such as in government, health, science, culture, and the arts (Swan 2015 ). Blockchain 3.0 aims to popularize the technology, and it focuses on the regulation and governance of its decentralization in society. The scope of this type of blockchain and its potential applications suggests that blockchain technology is a moving target (Crosby et al. 2016 ). Blockchain 3.0 envisions a more advanced form of “smart contracts” to establish a distributed organizational unit that makes and is subject to its own laws and which operates with a high degree of autonomy (Pieroni et al. 2018 ).

The integration of blockchain with tokens is an important combination of Blockchain 3.0. Tokens are proofs of digital rights, and blockchain tokens are widely recognized thanks to Ethereum and its ERC20 standard. Based on this standard, anyone can issue a custom token on Ethereum and this token can represent any right or value. Tokens refer to economic activities generated through the creation of encrypted tokens, which are principally but not exclusively based on the ERC20 standard. Tokens can serve as a form of validation of any right, including personal identity, academic diplomas, currency, receipts, keys, event tickets, rebate points, coupons, stocks, and bonds. Consequently, tokens can validate virtually any right that exists within a society. Blockchain is the back-end technology of the new era, while tokens are its front-end economic face. The combination of the two will bring about major societal transformation. Meanwhile, Blockchain 3.0 and its token economy continue to evolve.

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Xu, M., Chen, X. & Kou, G. A systematic review of blockchain. Financ Innov 5 , 27 (2019). https://doi.org/10.1186/s40854-019-0147-z

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• Systematic literature review
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Blockchain smart contracts: Applications, challenges, and future trends

• Published: 18 April 2021
• Volume 14 , pages 2901–2925, ( 2021 )

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• Shafaq Naheed Khan 1 ,
• Faiza Loukil   ORCID: orcid.org/0000-0003-4753-060X 2 ,
• Chirine Ghedira-Guegan 3 ,
• Elhadj Benkhelifa 4 &
• Anoud Bani-Hani 1  

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In recent years, the rapid development of blockchain technology and cryptocurrencies has influenced the financial industry by creating a new crypto-economy. Then, next-generation decentralized applications without involving a trusted third-party have emerged thanks to the appearance of smart contracts, which are computer protocols designed to facilitate, verify, and enforce automatically the negotiation and agreement among multiple untrustworthy parties. Despite the bright side of smart contracts, several concerns continue to undermine their adoption, such as security threats, vulnerabilities, and legal issues. In this paper, we present a comprehensive survey of blockchain-enabled smart contracts from both technical and usage points of view. To do so, we present a taxonomy of existing blockchain-enabled smart contract solutions, categorize the included research papers, and discuss the existing smart contract-based studies. Based on the findings from the survey, we identify a set of challenges and open issues that need to be addressed in future studies. Finally, we identify future trends.

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1 Introduction

For more than a decade, the blockchain is established as a technology where a distributed database records all the transactions that have happened in a peer-to-peer network. It is regarded as a distributed computing paradigm that successfully overcomes the issue related to the trust of a centralized party. Thus, in a blockchain network, several nodes collaborate among them to secure and maintain a set of shared transaction records in a distributed way without relying on any trusted party. In 2008, Satoshi Nakamoto introduced Bitcoin [ 69 ] that was the first proposed cryptocurrency introducing the blockchain as a distributed infrastructural technology. It allowed users to transfer securely crypto-currencies, known as “bitcoins” without a centralized regulator. Besides, Ethereum [ 16 ], NXT [ 71 ], and Hyperledger Fabric [ 4 ] were also proposed as blockchain-based systems used for the cryptocurrency. Unlike Bitcoin, they can use smart contracts (SC). Blockchain technology overlaps traditional contracts by including the terms of agreements between two or more parties, but surpasses them thanks to smart contracts by automating the execution of agreements in a distributed environment when conditions are met.

Smart contracts are executable codes that run on top of the blockchain to facilitate, execute, and enforce an agreement between untrustworthy parties without the involvement of a trusted third-party [ 16 ]. Smart contracts gave network automation and the ability to convert paper contracts into digital contracts. Compared to traditional contracts, smart contracts enabled users to codify their agreements and trust relations by providing automated transactions without the supervision of a central authority [ 89 ]. In order to prevent contract tampering, smart contracts are copied to each node of the blockchain network. By enabling the execution of the operations by computers and services provided by blockchain platforms, human error could be reduced to avoid disputes regarding such contracts.

Although smart contracts have made progress in recent years, they still face many challenges. For instance, one infamous malicious attack took place in 2016 when the Decentralized Autonomous Organization (DAO) smart contract was manipulated to steal around 2 Million Ether Footnote 1 (50 Million USD on the time) because of its re-entrancy vulnerability [ 103 ]. In addition to the vulnerability problem, smart contracts face several challenges including privacy, legal, and performance issues.

To understand current topics on smart contracts, we conduct a comprehensive survey, with the aim of better identifying and mapping research areas that need further studies. The focus of this survey is studying smart contracts from the technical point of view (e.g., codifying, security, performance issues) and the usage point of view (e.g., smart contract applications in finance, healthcare, etc). The major contributions of this paper are summarized as follows:

We propose a taxonomy of studies based on blockchain-enabled smart contracts including two categories, namely SC improvement and SC usage.

We categorize 200 papers that we have extracted from different digital databases and discuss the existing smart contract-based studies.

Based on the findings from the survey, we identify a set of smart contract challenges and open issues that need to be addressed in future studies. Therefore, this survey provides a helpful reference to the researchers who want to target smart contract improvement or usage in their future studies.

Finally, we discuss future trends of smart contracts and explain how they provide better solutions to the open research challenges.

Considering the above contributions, the remainder of this paper is structured as follows. Section  2 discusses background information about blockchain and smart contracts technologies. Section  3 discusses existing reviews studying smart contract-based approaches. Section  4 describes the adopted survey methodology and the solution taxonomy used to categorize existing smart contract-based solutions. In Sections  5 – 8 , we present existing advances in modeling-driven smart contract improvement, optimization-driven smart contract improvement, resource-driven smart contract usage, and cross-organizational collaboration-driven smart contract usage. Section  9 discusses the study results by introducing challenges and future trends in the studied field. Finally, Section  10 concludes the paper.

2 Background

As aforementioned, blockchain technology has emerged as a distributed computing paradigm that successfully overcomes the problem related to the trust of a centralized party. Thus, in a blockchain network, several nodes collaborate among them to secure and maintain a set of shared transaction records in a distributed way without relying on any trusted party. Specific nodes in the network known as miners are responsible for adding new blocks to a distributed public ledger known as the blockchain.

The first system was Bitcoin [ 69 ], which allowed users to transfer securely the currency (bitcoins) without a centralized regulator. In the blockchain network, miners are responsible for collecting transactions, solving challenging computational puzzles (proof-of-work) in order to reach consensus, and adding the transactions as blocks to the blockchain. Since then, several blockchain-based development platforms have been proposed offering the ability to host/ use smart contracts to execute automatically events and actions., namely NXT [ 71 ], Ethereum [ 16 ], Hyperledger Fabric [ 4 ], etc.

We detail below the smart contract operational process and then discuss some blockchain platforms that support the development of smart contracts.

2.1 Operational process of smart contracts

A smart contract is a common agreement between two or more parties. It stores information, processes inputs, and writes outputs thanks to its pre-defined functions [ 16 ]. For instance, the smart contract can define the constructor function that enables the smart contract creation. Hosting a new smart contract in the blockchain is enabled by invoking the constructor function through a transaction, whose sender becomes the smart contract owner. A self-destruct function is another example of the functions that can be defined in a smart contract. Usually, only the smart contract owner can destruct the contract by invoking this function.

A smart contract is likely to be a class that includes state variables, functions, function modifiers, events, and structures [ 16 ] which is intended to execute and control relevant events and actions according to the contract terms. Besides, it can even call other smart contracts. Each smart contract includes states and functions. The former are variables that hold some data or the owner’s wallet address (i.e., the address in which the smart contract is deployed). We can distinguish between two state types, namely constant states , which can never be changed, and writable states , which save states in the blockchain. The latter are pieces of code that can read or modify states. We can distinguish between two function types, namely read-only functions , which do not require gas Footnote 2 to run and write functions that require gas because the state transitions must be encoded in a new block of the blockchain. Furthermore, paying currency is required to avoid infinitely smart contract runs.

As aforementioned, a smart contract is hosted in the blockchain by invoking its constructor function through a transaction submitted to the blockchain network, then the constructor function is executed, and the final code of the smart contract is stored on the blockchain. Once deployed, the creator of the smart contract got the returned parameters (e.g., contract address), then users can invoke any available smart contract’s function by sending a transaction.

2.2 Platforms for Smart Contracts

Smart contracts can be developed and deployed in different blockchain platforms (e.g., NXT, Ethereum, and Hyperledger Fabric). Several platforms offer distinctive features for developing smart contracts including contract programming languages, contract code execution, and security levels. Some platforms support high-level programming languages to develop smart contracts.

Bitcoin [ 69 ] is a public blockchain platform that can be used to process cryptocurrency transactions, but with a very limited computing capability. Bitcoin uses a stack-based bytecode scripting language. The ability to create a smart contract with rich logic using the Bitcoin scripting language is very limited. Major changes would need to be made to both the mining functions and the mining incentivization schemes to enable smart contracts proper on Bitcoin’s blockchain [ 52 ].

NXT [ 71 ] is an open-source blockchain platform that relies entirely on a proof-of-stake consensus protocol. It includes a selection of smart contracts that are currently living. However, it is not Turing-complete, meaning only the existing templates can be used and no personalized smart contract can be deployed.

Ethereum [ 16 ] is the first blockchain platform for developing smart contracts. It supports advanced and customized smart contracts with the help of a Turing-complete virtual machine, called the Ethereum virtual machine (EVM). EVM is the runtime environment for smart contracts, and every node in the Ethereum network runs an EVM implementation and executes the same instructions. Solidity, as a high-level programming language, is used to write smart contracts, and the contract code is compiled down to EVM bytecode and deployed on the blockchain for execution. Ethereum is currently the most popular development platform for smart contracts and can be used to design various kinds of decentralized applications (DApps) in several domains.

Rather than the public blockchain, such as Bitcoin and Ethereum that any party can participate in the network, Hyperledger Fabric [ 4 ] is permissioned with only a collection of business-related organizations can join in through a membership service provider, and its network is built up from the peers whose are owned and contributed by those organizations. Hyperledger Fabric is an open-source enterprise-grade distributed ledger technology platform, proposed by IBM and supports smart contracts. It offers modularity and versatility for a broad set of industry use cases. The modular architecture for Hyperledger Fabric accommodates the diversity of enterprise use cases through plug and play components.

Ethereum and Hyperledger Fabric smart contracts differ in multiple aspects. While Solidity is the well-known programming language used to write Ethereum smart contracts, Hyperledger Fabric supports multi-language smart contracts, such as Go, Java, and Javascript [ 4 ]. For contract code execution, the contract code in Ethereum is included in a transaction, which is propagated in the peer-to-peer network, and any miner that receives this transaction can execute it in its local virtual machine [ 16 ]. In Hyperledger Fabric, when a transaction is created by the application, the transaction is only executed and signed by specified peers (endorsing peers). After receiving the application’s transaction proposal, each of these endorsing peers independently executes it by invoking the chain-code to which the transaction refers [ 4 ]. For security, chaincode runs within a container environment (e.g., Docker) for isolation.

These blockchain-based development platforms are used in the existing studies that we detail in the following sections.

3 Related literature reviews/surveys

We provide a brief overview of the existing reviews that have studied blockchain-enabled smart contracts.

While several literature reviews/surveys are published in order to study the blockchain-enabled smart contracts, there are still some ongoing challenges that have not been addressed. Table  1 presents a comparative summary of the existing blockchain-enabled smart contract reviews/surveys according to six criteria, namely proposing a taxonomy, considering several blockchain platforms, considering application domains, covering smart contract improvement tools, identifying research gaps, and scope of literature review. We observe that there is a lack of taxonomy focusing on smart contract improvement (i.e., addressing smart contract security, privacy, and performance issues) and smart contract usage (i.e., addressing domain-specific issues).

To sum up, it can be said that the existing surveys concerning blockchain-enabled smart contracts focus on classifying the papers based on smart contract issues. Our work extends the existing surveys by studying the smart contract application domains, analyzing the smart contract challenges, and introducing some research gaps that need to be addressed in future studies.

4 Research Methodology and Solution Taxonomy

We describe below the adopted research methodology, such as the search strategy, filtering process, and inclusion and exclusion criteria. Besides, we present the solution taxonomy used to categorize the final set of included papers.

4.1 Systematic Literature Review

We used three existing databases, namely ScienceDirect, IEEEXplore, and ACM Digital Library to search for relevant works using the “smart contract” string keyword. In the first phase, we found 523 publications as shown in both Fig.  1a , which depicts the percentage of the acquired research paper per digital database as well as Fig.  1b , which depicts the total number of preliminary studies acquired from each digital database.

Publication trend

To choose the relevant papers to be analyzed in our review, we filtered the primary studies retrieved from the databases. To do so, we defined a set of inclusion and exclusion criteria, which are summarised in Table  2 . Based on the outcomes of the first phase, we applied the set of inclusion and exclusion criteria to exclude the publications considered outside the scope of this review. Thus, we only included studies that satisfy all the inclusion criteria. We excluded duplicate publications, surveys, and literature reviews by filtering studies based on the title, the abstract, and the list of keywords.

As a result of the filtering process, we excluded 323 publications and included 200 relevant publications for this systematic review. Figure  1b depicts also the number of the relevant studies included in this research from each digital database.

4.2 Publication trends and Categorization

To examine the trend of the smart contract field in terms of the publication date, Fig.  2 depicts the number of included studies published each year from 2015 to September 2020. We observe that the total number of published papers in the studied field increases in the past few years, indicating the importance of the topic. Thus, the smart contract field is rapidly growing in recent years.

Included articles per year

As a result of an in-depth analysis of the included studies in this review, a comprehensive taxonomy is constructed to provide an additional support for designers to understand the various dimensions that they have to consider when designing a smart contract. The major motivations of this survey are to identify (i) the main publications about smart contracts, (ii) the current state of research in this field, and (iii) possible gaps in the literature that could become research problems to be solved by the scientific community. Through this survey, we aspire not only to define the conceptual background of blockchain-enabled smart contracts, but also to identify research issues to be explored at new studies. Indeed, we categorize existing smart contract research into two major categories, namely smart contract improvement and smart contract usage. The former includes studies aiming at addressing the smart contract challenges, such as functionality verification, performance, vulnerabilities, and lack of trustworthy data feeding. The latter includes studies aiming at addressing domain-specific challenges using smart contracts. Figure  3 depicts the proposed taxonomy of blockchain-enabled smart contracts, including modeling-driven smart contract improvement (see Section  5 ), optimization-driven smart contract improvement (see Section  6 ), resource-driven smart contract usage (see Section  7 ), and cross-organizational collaboration-driven smart contract usage (see Section  8 ).

Taxonomy of blockchain-enabled smart contract based studies

5 Modeling-driven smart contract improvement

Smart contracts have suffered from multiple security vulnerabilities in the past few years [ 8 ], which have resulted in both theft and gigantic financial losses. Such vulnerabilities could have been avoided with the help of formal analysis and verification of such smart contracts before deploying them on the blockchain. Since existing programming languages, such as Solidity are not built for formal verification, several researchers have proposed alternative approaches in order to improve the smart contract functionality verification. In this category, we discussed modeling-driven smart contract improvement solutions, which can be categorized into programming-centric solutions (see Table  3 ) and formal verification-centric solutions (see Table  4 ).

5.1 Programming-centric solutions

The essence of a smart contract is the computer code that can be executed automatically on the computer, so programming smart contracts correctly is an important research direction. Several researchers argued that developing new contract languages is an effective way to write a correct smart contract. Table  3 presents some newly proposed programming languages such as SmaCoNat [ 78 ], Flint [ 83 ], and Scilla [ 85 ]. For instance, Regnath and Steinhorst [ 78 ] proposed a human-readable, security, and executable programming language, called SmaCoNat. The authors converted programming language grammar into natural language sentences in order to improve program readability.

New contract languages promised to address the existing domain-specific language vulnerabilities. However, since they have not been put into practice, they could have their vulnerabilities. Thus, designing and implementing secure smart contracts still require adaptive software engineering technologies and expertise from multiple research domains, such as networking, programming languages, formal methods, and cryptography.

5.2 Formal verification-centric solutions

Typically, formal testing is applied to ensure that a software behaves and performs as expected in its specifications and requirements based on all possible inputs’ conditions. For smart contracts, Truffle [ 93 ], is an example of a development framework for Ethereum that enables writing formal test cases based on certain mathematical logic and rules for smart contracts written in JavaScript or Solidity languages. These test cases can be written in JavaScript and can be executed on a test network to check several properties of smart contracts. As aforementioned, formal testing can only make sure that a smart contract did what it is supposed to do based on its specification, however, it cannot help the smart contract developers to find bugs or vulnerabilities. Therefore, automated formal verification is a promising approach to detect bugs and other errors to guarantee the functional correctness of smart contracts. According to [ 2 ], formal verification can provide the highest level of confidence in the correct behavior of smart contracts. At present, the use of formal methods to verify smart contracts has been widely adopted by several researchers, and significant results have been achieved in practice. Table  4 presents some formal verification-centric solutions. For instance, Amani et al. [ 2 ] extended an existing EVM formalization in Isabelle/HOL by a sound program logic at the level of bytecode. The principle of the method is to organize the bytecode sequences into linear code blocks and create a logic program, where each block is processed as a set of instructions. Each part of the verification is validated in a single trusted logical framework from the perspective of bytecode.

Currently, formal verification tools are still in the experimental stage and have not been widely used. Therefore, the smart contract formalization research direction deserves a lot of attention, thus it provides the highest level of confidence about the correct behavior of smart contracts. Real progress in this research field can improve trust in the smart contract, especially when used to develop critical systems, such as financial, healthcare, and banking systems.

6 Optimization-driven smart contract improvement

Smart contracts have emerged as a new promising solution for developing fully decentralized applications without involving a trusted third-party. Despite the bright side of smart contracts, several concerns continue to undermine their adoption, namely performance issues, security threats, and privacy issues. Indeed, new smart contract applications are more demanding in terms of contract execution time, execution cost, security, and privacy fields. In this category, we discuss optimization-driven smart contract improvement solutions, which can be categorized into performance optimization-centric solutions (see Table  5 ) and security optimization-centric solutions (see Table  6 ).

6.1 Performance optimization-centric solutions

Smart contract performance refers to the ability of smart contract systems to deliver in a reasonable response time and sustain performance when the number of contracts is increasing [ 1 ]. Table  5 presents some examples of performance optimization-centric solutions. Some performance issues in blockchain systems, not limited to, are throughput bottleneck, limited scalability, transactions latency. To overcome performance issues in smart contract systems, some researchers have proposed solutions to execute smart contracts in parallel instead of sequentially [ 26 , 34 ]. For instance, Gao et al. [ 34 ] have proposed a parallel execution scheme that relies on two key techniques, namely a fair contract partition algorithm leveraging integer linear programming to partition a set of smart contracts into multiple subsets, and a random assignment protocol assigning subsets randomly to a subgroup of users. Other studies have been proposed for smart contract optimization by saving gas. In fact, if the smart contract execution exceeds an amount of gas (known as gas limit), an out-of-gas exception is raised, interrupting the current execution. For instance, GasReducer [ 18 ] is a tool for automatically detecting EVM operation sequences that can be replaced with other operations that have the same semantics but need less gas, and then replacing them with efficient code.

6.2 Security optimization-centric solutions

Security of a smart contract refers to its robustness against attacks from malicious users that exploit generally the contract security vulnerabilities to gain profit or the lack of trustworthy data feeding to inject malicious data. Table  6 presents some examples of vulnerability detection tools, transactional privacy models, and trustworthy data feeding solutions.

6.2.1 Vulnerability Detection

Discovering potential vulnerabilities in the execution of contracts is important to improve the security and credibility of contracts. Indeed, several studies systematically summarized the contract vulnerabilities and analyzed the security risks [ 8 , 77 , 81 ]. For instance, Atzei et al. [ 8 ] have provided a taxonomy of smart contract vulnerabilities of three levels, namely Solidity, EVM, and Blockchain. In recent years, the most notorious attack is the Decentralized Autonomous Organization (DAO) attack that exploited a re-entrancy vulnerability to steal around 2 Million Ether from a smart contract [ 103 ]. Another attack has happened to the SmartBillions, which presented a fully decentralized and transparent lottery system when an attacker successfully manipulated the block hash of the smart contract’s lottery function twice, and forced the result in his favor to get 400 Ether [ 62 ]. To solve the smart contract vulnerabilities, several vulnerability detection solutions have been proposed. Some studies have given solutions to common vulnerabilities, such as Oyente [ 59 ], SmartInspect [ 15 ], and ContractFuzzer [ 47 ]. Some other work focused on specific vulnerabilities, such as ReGuard [ 54 ] to detect re-entrancy bugs and EthRacer [ 48 ] to detect event-ordering bugs.

6.2.2 Transactional privacy

The privacy issue represents a real challenge for smart contracts to keep critical functions secret, apply cryptography, and avoid disclosing data on the blockchain to the public. The lack of transactional privacy could limit the adoption of smart contracts. To address this issue, Kosba et al. [ 49 ] have proposed Hawk, a decentralized smart contract system. Hawk is a tool allowing smart contract developers to build privacy-preserving contracts without the need for implementing any cryptography. Its compiler automatically generated an efficient cryptographic protocol where contractual parties interact with the blockchain, using cryptographic primitives such as zero-knowledge proofs.

6.2.3 Trustworthy data feeding

The smart contract execution requires some external data about real-world states and events from outside the blockchain. Therefore, trustworthy data feeding mechanisms (known as Oracles) are required to build a bridge between blockchain and the external world (e.g., Web API). For instance, Zhang et al. [ 113 ] have proposed Town Crier, which acted as a link between existing commonly trusted non-blockchain based websites and smart contracts to provide authenticated data to smart contracts while preserving confidentiality with encrypted parameters. However, in case of malicious code or bad data fed to a smart contract, the latter processes the input as is, producing an incorrect and unpredictable outcome. Thus, oracles retain an enormous amount of power over smart contracts in how they are executed because the data they provide determines how the smart contracts execute.

To sum up, research on improving smart contract security and performance has emerged in recent years. While running smart contracts in parallel can speed up contract execution, it faces a challenge in how to execute contracts that depend on each other at the same time. Moreover, optimizing smart contract codes can effectively reduce potential vulnerabilities in contracts and ensure efficient and secure execution of contracts. However, the existing studies are still immature, and unknown vulnerabilities or bugs cannot be detected to be replaced. Thus, the optimization of smart contracts needs further research.

After discussing the smart contract from the technical point of view, we present in the following two sections the existing solutions focusing on smart contract usage in several domains.

7 Resource-driven smart contract usage

As we know, smart contracts are executable code hosted in the blockchain that store information, process inputs, and write outputs thanks to their pre-defined functions. They are used to improve data handling transparency, decentralize resource-constrained device management, and enable changes of the agreement terms at runtime while running on top of a decentralized and transparent network. In this category, we discuss resource-driven smart contract usage solutions, which can be categorized into data management-centric solutions (see Table  7 ), device management-centric solutions (see Table  8 ), and cloud-related solutions (see Table  9 ).

7.1 Data management-centric solutions

In the past, raw data are transferred to a cloud server to be stored and analyzed. However, this centralized solution has caused serious concerns regarding several aspects, such as the necessity to trust the cloud infrastructure security, control loss once data are externalized, and lack of data handling transparency. Consequently, blockchain-based data management emerged as a platform to facilitate transparent data transactions between untrustworthy involved parties on the network. Indeed, peer-to-peer-network-based data management is a more fair system as compared to a system where all transactions are handled by a central server. Table  7 presents some examples of data management-centric solutions concerning data provenance, data access, and data sharing.

7.1.1 Data Provenance

Data provenance refers to a historical record of the data and its origins showing which and how data item is stored, accessed, and processed by whom and for what purpose. Ensuring data provenance can increase data transparency and enforce data integrity. In this regard, a blockchain can offer an immutable storage of records and smart contracts can be used as a responsible for verifying the data origins before storing them. Similar ideas are applied in [ 6 , 44 ], where a blockchain is used as a decentralized and immutable storage for enabling data provenance. For instance, Javaid et al. [ 44 ] have proposed a blockchain-based data provenance and integrity for secure IoT environments framework, called BlockPro. Ethereum and two smart contracts were used to implement it. The first smart contract established data provenance by interacting with the IoT devices and making sure they are legit and the data being uploaded is coming from a known and trusted origin. The second smart contract can only be called by the first one to storing data on and retrieving data from the blockchain.

7.1.2 Data Access

Data access is ensured according to rights given to involved parties in a network to perform some operations on data. These rights are expressed using access control policies, which consist of a set of conditions that are evaluated against the current context to make the access decision each time a request is received [ 40 ]. Recently, for obtaining decentralized self-evaluating policies, access control policies have been codified as executable code and have been managed through a peer-to-peer network while eliminating a central entity. For this purpose, smart contracts can be used to express access control policies to transform the policy evaluation process into a distributed smart contract execution. In this context, several studies [ 36 , 61 , 74 , 88 , 112 , 114 ] have been proposed. For instance, Maesa et al. [ 61 ] proposed to exploit a blockchain to store access control policies and manage attributes, as well as to execute the access decision process. The access control policy is represented through a smart contract that evaluated the stored conditions to make the access decision.

7.1.3 Data Sharing

Data sharing refers to make data available to other parties by the data owner. However, two types of challenges faced data sharing schemes, namely (i) achieving good data sharing while losing the control over the shared data or (ii) remaining poor at sharing in order to keep strong control over the data. To address these challenges, blockchain technology is used because it offers immutable storage of records that improve data handling transparency and can host executable codes (i.e., smart contracts) that authenticate users, verify authorizations, and thereby ensure an efficient and secure data sharing in a peer-to-peer network. Several studies using blockchain-enabled smart contracts have been proposed for data sharing in healthcare [ 22 , 72 , 111 ], cloud environment [ 70 ], and for digital document version control [ 30 ]. In the healthcare context, medical devices and health care applications have been increasingly adopted by patients. However, wireless body sensors collect health records that are sensitive to individuals. Existing electronic health record management systems struggle with balancing data privacy and data access. Blockchain technology is an emerging technology that enables data sharing in a decentralized and transactional fashion. For instance, Dagher et al. [ 22 ] have proposed a blockchain-based framework, called Ancile for secure and efficient access to medical records by patients, providers, and third-parties while preserving the patients’ privacy. Ancile employed smart contracts, data obfuscation techniques, and cryptographic techniques in order to improve privacy and security in the healthcare domain. Recently, Yu et al. [ 111 ] have proposed a blockchain-based medical research support platform, which employed the characteristics of the alliance chain on which hospitals and medical research institutions are treated as nodes. Among them, users such as patients, doctors, and researchers needed to register and authenticate on the alliance chain. Smart contracts are used to upload the pseudonymous addresses of CEMRs to the alliance chain.

7.2 Device management-centric solutions

One of the technical challenges of having billions of devices deployed worldwide is the ability to manage and synchronize them. Using the current model of the server-client system may have some limitations for device management thus, several researchers are studying the benefits of the blockchain use in this field. Specifically, smart contracts are chosen to guarantee authentication, synchronization, and data integrity while running on top of a decentralized and transparent network. Table  8 presents some newly proposed device management-centric solutions [ 29 , 45 , 58 , 90 , 96 , 104 , 116 ]. For instance, Ellul and Pace [ 29 ] have proposed a split-virtual machine architecture to enable the integration of resource-constrained devices with blockchain systems, called AlkylVM. Each blockchain-connected device would run an instance of AlkylVM, which allows communication between blockchain and IoT devices using the Aryl blockchain node. The latter is responsible for monitoring smart contract transactions and events that would require interaction with IoT devices.

7.3 Cloud-related solutions

In cloud computing, both service requester and service provider agree on a set of requirements, obligations, and rights that is valid for the whole contract life-cycle. Recently, blockchain-enabled smart contracts have been used to enable changes in the agreement terms at runtime through the definition of conditions and actions. Table  9 presents some proposed cloud-related solutions [ 42 , 84 , 98 , 118 ]. For instance, Zhou et al. [ 118 ] have proposed a witness model for enforcing cloud Service Level Agreement (SLA) using smart contracts. The game theory is leveraged to analyze that the witness has to offer honest monitoring service in order to maximize its revenue. The service provider needs to prepay fees to the smart contract for hiring witnesses. The service customer then decides whether to accept the SLA. If yes, it also needs to prepay fees including the service fee and its part of the hiring fee for witnesses. However, a small bug or attack on smart contracts can result in significant issues like privacy leakage or system logic modifications. Some of critical security vulnerabilities can include timestamp dependencies, mishandled exceptions, re-entrancy attacks on smart contracts in cloud-related solutions.

Although smart contracts fulfill many conditions related to data/device management, they have some drawbacks, based on basic design principles of blockchain technology. First, the data stored on smart contracts are publicly readable through public transactions with no read access restrictions. Thus, it is required to avoid storing private or device keys on smart contracts to the public availability of the information. For solving transparency problems related to blockchain, future research might investigate deploying complex cryptographic solutions for securing data stored on smart contracts without boosting cost. Second, the cost of storing data on the blockchain is very high. Therefore, creating hybrid solutions is required to benefit from the traceability of data transactions that are offered by blockchain networks and the efficient and private access and storage of data provided by external data repositories.

After discussing the resource-driven smart contract usage solutions, we present in the following section the existing solutions focusing on cross-organizational collaboration-driven smart contract usage.

8 Cross-organizational collaboration-driven smart contract usage

Smart contracts help to record an agreement between several untrustworthy parties in the form of code that cannot be altered or changed once deployed on the blockchain. Thus, smart contract development allows substituting traditional contracts and develops business growth in several industries, namely supply chain management, logistics and shipping, insurance, and charity. In this category, we discuss cross-organizational collaboration-driven smart contract usage solutions, which can be categorized into profit-centric solutions (see Tables  10 ,  11 , and  12 ) and non-profit-centric solutions (see Table  13 ).

8.1 Profit-centric solutions

The smart contract protocol aims at making contracts more secure, executed in real-time, and more transparent, which are the exact challenges with the existing profit-centric cross-organizational collaboration. Profit-centric solutions aim at increasing the profit by reducing real-time tracking costs, improving cross-border payments, and enhancing distributed problem-solving transparency. Tables  10 ,  11 , and  12 present some examples of profit-centric solutions concerning tracking-based solutions, digital asset-based solutions, and crowdsourcing-related solutions, respectively.

8.1.1 Tracking-based solutions

Although business processes may operate well within a centralized mechanism managing internal activities with individual local databases, there still exists a demand for transparency across processes and trust relationships among involved parties. Indeed, real-time tracking may reduce the unnecessary wait for the confirmation of information. Thus, using a distributed system can enhance the transparency and performance of business processes. Smart contracts can be used to automate the transfer of various types of ownership of assets, property, and value and therefore, lead to more visible and less-intermediated working processes. In this context, several studies using smart contracts have been proposed for supply chain management of foods [ 11 , 17 , 53 ], manufactured products [ 24 , 43 , 50 , 102 ], shipped items [ 39 ], bio-drugs [ 105 ], and imported products [ 108 ]. For instance, Casado-Vara et al. [ 17 ] have proposed a model for agriculture tracking involving blockchain, smart contracts, and a multi-agent system. The blockchain is used to store all transaction information in the supply chain. Besides, the multi-agent system used smart contracts to manage the entire supply chain process more efficiently while removing intermediaries. Furthermore, according to industry estimations, the global halal food market will reach USD 2.55 trillion by 2024 [ 92 ]. Thus, several companies are using blockchain to improve traceability in the halal food supply chain. For instance, a UK based company has partnered with a blockchain platform provider in order to track livestock and fresh food from farm to table through the halal food chain using the blockchain technology [ 92 ].

8.1.2 Digital asset-based solutions

Because of their resilience to tampering, smart contracts are appealing in many scenarios, especially in those which require transfers of money to respect certain agreed rules like in financial services. Therefore, smart contracts in the finance application domain manage, gather, and/or distribute the money as a preeminent feature. The lack of a centralized authority reduced costs and in theory provided more control and access to the investors [ 46 ]. To this end, some smart contracts are used for cross-border payments without relying on banks. For instance, the blockchain payment provider, called Ripple is a blockchain solution for payments that is proven in the real world by connecting existing bank ledgers to facilitate near real-time cross-border payments. Ripple may also reduce costs and provide additional pricing transparency of real-time cross-border payments [ 3 ]. Table  11 presents other smart contracts that implemented data/good trading service [ 7 , 65 , 106 ], insurance service [ 9 ], rent/exchange good service [ 14 , 28 ], energy trading and demand management service [ 55 , 100 ], social credit system [ 107 ], and mobile payment system [ 110 ]. For instance, smart contracts are exploited in the insurance industry to automate claims processing, verification, and payment, thus to increase the speed of claim processing as well as to prevent fraud and reduce manual mistakes. Recently, a smart contract-based flight insurance system has been proposed to refund automatically the insured passengers in case of a flight delay [ 13 ]. Moreover, blockchain-based systems can provide solutions to the cyber insurance challenges by realizing an automated, real-time, and immutable feedback loop between the insurer, its customer, and potential auditors [ 20 ]. Moreover, blockchain technology can mitigate the problems faced by traditional insurance while complying with religious principles [ 67 ]. Indeed, a smart insurance model based on Islamic insurance, called Takaful is proposed in [ 64 ]. The main difference between Takaful and conventional insurance that in Takaful, insured funds belong to them, the insurance company is just a manager. Thus, by using blockchain and smart contract technologies, insurance companies can be more transparent, which is the highest feature requested by customers. The authors in [ 64 ] have suggested transforming the traditional insurance policies into smart contracts that can be executed automatically in order to refund the policyholders without causing compensations for fake incidents.

8.1.3 Crowdsourcing-related solutions

Crowdsourcing is an online, distributed problem-solving and production model in which individuals or organizations obtain goods and services from a large group of participants. For instance, crowdfunding has become one popular form of collective funding among several categories of crowdsourcing. Crowdfunding is a process, in which small donations or investments, made by groups of people, support the development of new projects in exchange for free products or different types of recognition. Traditional crowdsourcing is based on a central system where requesters post tasks on a central server or platform, however, this centralized model currently faces various challenges such as prohibitive cost, single point of failure, and vulnerability to malicious attacks. To this end, blockchain is considered as a promising technology that aims at addressing the aforementioned challenges by eliminating the single point of failure, enhancing transparency, and enforcing rules using smart contracts. In this context, several studies using blockchain-enabled smart contracts [ 38 , 87 , 97 , 119 ] have been proposed, as shown in Table  12 . For instance, Zichichi et al. [ 119 ] have proposed a smart contract-based social decentralized autonomous organization for crowdfunding, called LikeStarter where social network site users can raise funds for other users through a simple “like”, built on top of the Ethereum blockchain. Smart contracts are used to control and manage funds without the need for a trusted third entity. LikeStarter assigns Likoins (i.e. tokens related to an artist) to users that fund a given project. These tokens can be employed and converted to buy artifacts and they provide users with voting capabilities (i.e. they can contribute to the decision of the price of certain artifacts).

8.2 Non-profit-centric solutions

Blockchain technology is needed in a cross-organizational collaboration area suffering from a decline in trust from involved parties (e.g., volunteers, donors, voters, etc.) who are unable to know how their contributions are spent/handled. Indeed, smart contracts enable “fully auditable” performance data, which is secure and extremely difficult to falsify or hack.

Table  13 presents some examples of non-profit-centric solutions including volunteer system [ 19 ], philanthropic-related systems [ 32 , 82 , 91 , 94 ], e-voting service [ 75 ], system for educational institutions [ 86 ], and copyright protection [ 115 ]. For instance, Cheng et al. [ 19 ] have proposed VOLTimebank, a volunteer time bank system for a mutual pension based on blockchain and smart contracts. VOLTimebank provides a channel for volunteers to serve the elderly and gives volunteers a way to exchange the services they can offer today with the services that they hope to get in the future. In the philanthropy context, the collection processes are not transparent, and due to this, the involved organizations struggle to gain donors’ trust and interest. Thus, some efforts have been made to map the charity collection process on blockchain technology, for instance, Farooq et al. [ 32 ] have proposed a charity collection platform, which is based on blockchain technology, and is transparent for donors and legal authorities to conduct an audit. The design uses smart contracts and digital wallets to transfer money in real-time with complete data security and an auditable trail of every transaction. These smart contracts have been introduced to securely transfer donations to individual beneficiaries, organization, and their associated projects. Zhao and O’Mahony [ 115 ] have proposed BMCProtector, a prototype implementation based on an Ethereum blockchain and smart contract technologies, for effective protection of music copyright and rights of copyright owners. The deployed smart contract is responsible for sharing the copyright parameters.

Despite the benefits of blockchain and smart contracts in reforming operations in a wide variety of industries, namely supply chain, insurance, and charity, certain challenges to their widespread adoption still exist. These challenges include legal issues, lack of standards and protocols, privacy issues, and error intolerance. Arguments that smart contracts are no panacea for all financial use cases doubt the applicability of smart contracts to certain scenarios as far as agreement type and scale.

9 Discussion

We discuss below the study results and present challenges and future development trends in smart contract research.

9.1 Challenges and Open issues

As an emerging technology, smart contracts currently face many challenges, such as legal, reliance on “off-chain” resources, immutability, scalability, and consensus mechanism issues (see Fig.  4 ).

Challenges and open issues

9.1.1 Legal issues

The legal issue of smart contracts is another crucial aspect of smart contract challenges. For example, the European General Data Protection Regulation (GDPR) [ 35 ] stipulates that citizens have a “right to be forgotten” which is inconsistent with the immutable nature of blockchain-enabled smart contracts. Other legal issues can be cited including, (i) each country has its own laws and regulations, hence, it is complicated to ensure compliance will all regulations, (ii) law clauses or conditions are not quantifiable, thus it is still complicated to model these conditions in smart contracts so that they are appropriate and quantifiable for a machine to execute them, and (iii) governments are interested in a regulated and controlled use of the blockchain technology in many applications, however, this means that the untrustworthy network will regress to a third-party trusted network, losing part of its essence [ 79 ].

9.1.2 Reliance on “off-chain” Resources

Several smart contracts require receiving information or parameters from resources that are not on the blockchain itself, so-called off-chain resources. For this purpose, oracles are used as trusted third parties that retrieve off-chain information and then push that information to the blockchain at predetermined times. Although existing oracles are well tested, their use may introduce a potential “point of failure”. For instance, an oracle might be unable to push out the necessary information, provide erroneous data, or go out of business. Therefore, smart contracts will need to account for these eventualities before their adoption can become more widespread [ 51 ].

9.1.3 Immutability issue

The immutability feature is an important characteristic of smart contracts. Indeed, once a smart contract is deployed, the code cannot be changed by any party. However, the dark side of the immutability concept in smart contracts lies mainly in the fact that in the event of any errors made in the code, the immutability feature of a smart contract prevents it from being rectified. Similarly, if circumstances change (e.g., the parties have mutually agreed to change the parameters of their business deal, or if there is a change in law, etc.), no simple path to amend a smart contract is possible. Therefore, extensive and possibly expensive reviews of the smart contract performed by experts before its deployment in a blockchain are required to address the immutability issue.

Another limitation in the blockchain itself that impacts the smart contracts is the irreversible nature of the blockchain, thus once the smart contracts are deployed, they cannot be changed. Moreover, any blockchain nodes can be hacked or misused to report erroneous data that will be logged on the blockchain in an immutable manner.

9.1.4 Scalability issue

Scalability is the primary concern for many blockchain networks. For instance, the Ethereum blockchain can verify 14 transactions per second, which is slow as compared with Visa that can handle up to 24,000 transactions per second. Indeed, the scalability issue leads to network congestion, increased commission fees for transactions, and an increase in the time required to confirm the transactions [ 80 ]. In order to address the scalability issue, extensive research focusing on increasing the number of transactions per second by smart contract platforms is required in the future. However, the transaction verification depends on the consensus mechanism used by the smart contract platforms. Therefore, scalability depends on consensus mechanisms, which is another issue in smart contracts.

9.1.5 Consensus mechanism issue

The consensus mechanism plays the leading role to maintain security, scalability, and decentralization in the blockchain networks at the same time. There are several existing consensus algorithms, including Proof-of-Work (PoW), Proof-of-Stake (PoS), etc. Although the PoW algorithm enables security in the blockchain, it wastes resources. Thus, many organizations switch from the PoW algorithm to new consensus mechanisms that promise lower fees for transactions as well as lower energy costs for the block production process. Therefore, future studies can use new consensus mechanisms, such as proof-of-activity (PoA) or delegated proof-of-stake (DPoS) in order to test them and eventually improve their quality.

9.2 Future Development Trends

Future development trends of smart contracts are introduced from two aspects namely, Layer 2 protocols, and contract management solutions.

9.2.1 Layer 2 protocols

In order to address the aforementioned challenges faced by smart contracts, a viable solution, called Layer 2 is appeared to tackle the blockchain scalability problem. While Layer 1 is the used term to describe the underlying main blockchain architecture, Layer 2 is an overlaying network that lies on top of the underlying blockchain. Indeed, Layer 2 refers to the multiple solutions or protocols being built on top of an existing blockchain system. The main goal of Layer 2 protocols is to solve the transaction speed and scaling difficulties that are being faced by the major cryptocurrency networks. Therefore, Layer 2 protocols refer to a secondary framework, where blockchain transactions and processes can take place independently of Layer 1 (“main-chain”). Two major examples of Layer 2 solutions are the Bitcoin Lightning Network [ 27 ] and the Ethereum Plasma [ 76 ]. The Lightning Network, which in part developed at the MIT Media Lab’s Digital Currency Initiative, is a lightweight software solution for scaling public blockchains and cryptocurrency interoperability. It aims at greatly reducing cost and time constraints by shifting small transactions to a cryptographically secure “off-chain” environment so that only large netting transactions need to be directly settled into a resource-constrained blockchain [ 27 ]. Ethereum Plasma is a series of smart contracts, which allows for many blockchains within a root blockchain. The root blockchain enforces the state in the Plasma chain. The root chain is the enforcer of all computation globally but is only computed and penalized if there is proof of fraud. Many Plasma blockchains can co-exist with their business logic and smart contract terms. Indeed, Plasma enables persistently operating decentralized applications at a high scale [ 76 ]. To sum up, thanks to Layer 2, a great portion of the work that would be performed by the “main-chain” can be moved to the second layer. So while the “main-chain” provides security, the second layer protocols provides better solutions for the scalability issue by offering high throughput, being able to perform hundreds, or even thousands, of transactions per second.

9.2.2 Contract management solutions

Smart contracts encompass far more than just the benefits of blockchain technology. Rather, the term captures the entire digital life cycle of a contract, from negotiation to control and verification of the fulfillment of contractual obligations. Now, it is already possible to use smart contracts even without blockchain technology. Thus, contract management solutions could overcome both the immutability issue and the irreversible nature of blockchain by handling the contract’s life-cycle while eliminating limitations of the technology itself. In state-of-the-art contract management solutions [ 31 ], all parties to the contract must provide proof of identity and authenticate their access to data in order to ensure the basis of trust. Besides, all documents that are associated with the contract are stored in a revision-secure manner and encrypted form on a cloud-based platform developed and operated in Europe. This ensures transparency and traceability for all events, the actions associated with these events, and the designation of the persons responsible [ 23 ]. For instance, Fabasoft Contracts [ 31 ] is one of the latest contract management solutions that is ready-to-use, cloud-based software to support users throughout the entire contract life cycle: from cross-company contract preparation, efficient handling of review and approval processes, to the revision-secure contract archiving. It enables the modeling of contract rights and obligations, which can be automatically verified and enforced. There are several benefits offered by revision-secure contract management, including providing traceability when monitoring the cold chain of food delivery or proving the authenticity of spare automotive parts, as opposed to counterfeit articles [ 23 ].

10 Conclusion

The decentralization, auto-enforcing ability, and verifiability characteristics of smart contracts enable their encoded business rules to be executed in a peer-to-peer network, where each node is “equal” and none has any special authority without the involvement of a trusted authority or a central server. Thus, smart contracts are expected to revolutionize many traditional industries, such as financial, healthcare, energy, etc. In this paper, we presented a comprehensive survey of blockchain-enabled smart contracts from both technical and usage points of view. Thus, we introduced a taxonomy of existing blockchain-enabled smart contract solutions, categorized the included research papers, and discussed the existing smart contract-based studies. Based on the findings from the survey, both smart contract challenges and open issues are identified to be addressed in further studies. Finally, we discussed future trends of smart contracts. This study provides informational support to stakeholders interested in the research of smart contracts.

Ether (ETH): the cryptocurrency of Ethereum apps that is digital, global money.

gas: a unit that measures the amount of computational effort that it will take to execute certain operations.

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Khan, S.N., Loukil, F., Ghedira-Guegan, C. et al. Blockchain smart contracts: Applications, challenges, and future trends. Peer-to-Peer Netw. Appl. 14 , 2901–2925 (2021). https://doi.org/10.1007/s12083-021-01127-0

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