From Block-based to Blockless: A Paradigm Shift Towards Blockchain Interoperability
Jiang Xiao; Xiaohai Dai; Hai Jin, School of Computer Science and Technology, Huazhong University of Science and Technology, China
IEEE Blockchain Technical Briefs, March 2019
Background of Blockchain Interoperability
Today, a brand-new age of ‘Web 3.0 era’ —sharing and unleashing the Internet of value—has begun. Blockchain technology holds the promise of being the trust machine in ‘Web 3.0 era’. It is foreseeable that there will be a significant number of blockchain systems launched to disrupt all sectors of our life, such as Bitcoin , Ethereum , IOTA , Swarm , etc. In this light, ‘blockchain interoperability’  is perceived as the prominent characteristic of facilitating the interactions amongst different blockchains (i.e., heterogenous or homogeneous) to empower the future ecosystem. A broad range of scenarios for exploring such interoperability include transfer of cross-chain digital assets, recommendation of digital currency, synergy of distributed storage, real-time cross-chain liquidation, identity authentication of users with multiple roles, cross-chain data share, cross-chain certification and so forth.
Mainstream blockchain systems have long faced the challenge of designing a data structure as a fundamental primitive of interoperability. While many systems employ `block-based’ structure to enable cross-chain interaction, the deficiencies of the complex nesting of data, repeated network transmission and redundant data storage   render it unable to achieve high efficiency. To be specific, as the transactions are packaged in the format of blocks, the source chain has to parse a block into multiple transactions before obtaining the required transaction. The process of parsing the blocks introduces extra overhead. Besides, a piece of valid data will be broadcast twice. At the first time, the data is broadcast as a transaction. At the second time, the same data is packaged in a block to be broadcast again, which incurs repeated network transmission. Further still, as blocks in different chains cannot be shared directly in nature, data are stored redundantly. As a consequence, each chain is bound to be an isolated island, which will decrease the vitality of blockchain community, and hinder the further development of blockchain ecosystem.
Design Principles Towards a New Storage Paradigm
It is urgent to design a new storage paradigm that aims to provide a flexible, secure and scalable blockchain infrastructure for interoperability. We hereby introduce the primary principles as follows:
- Unified Transaction Format: The unified transaction format can not only reduce the data storage, but also lower the design difficulty of inter-chain consensus. Furthermore, if different chains are deployed in the same network space, each unified transaction only needs to be broadcast once, thereby reducing the network overhead.
- Parallelizable Extension: The transactions should be appended to the blockchain in parallel, thus promoting both the efficiencies of a single chain and interoperability. For example, a data structure of directed network may be used to replace the original chain. Besides, the process of packaging the transactions into a block is sequential, which contradicts the principle of parallelization extension. Hence, it is desirable to design a new data format instead of the block-base ones.
- Efficient Data Query: Data query is frequent in the scenario of interoperability, such as that a chain needs to check if a transaction has been committed in another chain. In traditional block-based format, the transactions have to be packaged into a block before committed and unpackaged afterwards to be read, which increases the query overhead largely. As a result, the data may be organized in the format of transactions directly, so as to avoid the overhead in packaging and unpackaging the blocks. Furthermore, data in the format of transactions can be parsed into a relational database easily, thus facilitating the data analysis of blockchain.
Overall Design of Vine
We propose Vine (Veritable Interoperability for Next-generation Blockchain Ecosystem), a novel block-less solution to support efficient and secure interactions amongst interdependent blockchains. The intuition is strongly inspired by DAG (Directed Acyclic Graph) that removes the block structure and organizes the data in the format of transactions directly . Compared to block-less blockchain systems, DAG-enabled blockless system can provide higher efficiency. A detailed comparison between block-based and DAG-enabled blockless blockchain systems is concluded in Table 1. Different from the existing DAG-enabled projects , which only focus on a single chain, Vine is the first solution to apply DAG to the scenario of interoperability. Apart from DAG, Vine further unifies the transaction format of different chains and enables different chains to share the underlying network space, thus reducing the network and storage overhead largely.
Table 1. Comparison of block-based and DAG-enabled blockless blockchain systems
Figure 1 illustrates the schematic diagram of Vine. Three chains, namely token chain, supply chain and finance chain, are painted in yellow, blue and red colors respectively. To share the transactions among the three chains, data layer and network layer are connected together. Each transaction is represented by a rectangle in the data layer. To mark which chain is interested in a transaction, the transaction is painted with the color of the corresponding chain. For example, the transaction labeled with number ‘1’ only belongs to the token chain, while the transaction labeled with number ‘2’ only belongs to the supply chain. As green is a combination of yellow and blue, transactions painted in green belong to both token chain and supply chain, e.g., transaction labeled with number ‘3’. Similarly, transactions painted in black represents all of the three chains interested in the transactions.
The unified data structure based on DAG can effectively reduce the overheads and raise the efficiency in terms of blockchain interoperability. To be specific, process of block parsing is removed due to blockless structure, so as to avoid the overhead in parsing the blocks. In addition, deploying different chains in the same network space directly can avoid the repeated broadcast of the same transactions. What’s more, as data is stored in the format of transactions, the same transaction in different chains can be shared easily.
Blockchain, the groundbreaking technology behind Bitcoin and Ethereum is gaining increasing attention and experiencing a boost of global adoption. By building upon the novel ‘blockless’ storage paradigm, the massive interdepend applications will be sprout up and promoted in the next-generation blockchain ecosystem.
Figure 1. Schematic diagram of Vine
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Dr. Jiang Xiao is currently an associate professor in the School of Computer Science and Technology at Huazhong University of Science and Technology (HUST), Wuhan, China. Jiang received the BSc degree from HUST in 2009 and the Ph.D. degree from Hong Kong University of Science and Technology (HKUST) in 2014. She has been engaged in research on blockchain, distributed computing, big data analysis and management, and wireless indoor localization. Awards include Hubei Dawnlight Program 2018, CCF-Intel Young Faculty Research Program 2017, and Best Paper Awards from IEEE ICPADS/GLOBECOM 2012.
Xiaohai Dai received the M.S. degree in the School of Computer Science and Technology from Huazhong University of Science and Technology (HUST), Wuhan, China, in 2017. He is currently pursuing the Ph.D. degree in School of Computer Science and Technology from HUST. His current research interests include blockchain and distributed system.
Hai Jin is a Cheung Kung Scholars Chair Professor of computer science and engineering at Huazhong University of Science and Technology (HUST) in China. Jin received his Ph.D. in computer engineering from HUST in 1994. In 1996, he was awarded a German Academic Exchange Service fellowship to visit the Technical University of Chemnitz in Germany. Jin worked at The University of Hong Kong between 1998 and 2000, and as a visiting scholar at the University of Southern California between 1999 and 2000. He was awarded the Excellent Youth Award from the National Science Foundation of China in 2001. Jin is the chief scientist of ChinaGrid, the largest grid computing project in China, and the chief scientist of National 973 Basic Research Program Project of Virtualization Technology of Computing System, and Cloud Security. Jin is a fellow of the IEEE and a member of the ACM. He has co-authored 15 books and published over 700 research papers. His research interests include computer architecture, virtualization technology, cluster computing and cloud computing, peer-to-peer computing, network storage, and network security.
Dr. Ammar Rayes (IEEE Senior Member) is a Distinguished Engineer at Cisco’s Advanced Services Technology Office focusing on Network Analytics, IoT and Machine Learning. He has authored three books, over 100 publications in refereed journals and conferences on advances in software & networking related technologies and over 25 patents. He is the Founding President of www.issip.org , Editor-in-Chief of “Advances of IoT” Journal and served as a Guest Editor of several journals and IEEE Communication Magazines.
He received his BS and MS Degrees in EE from the University of Illinois at Urbana and his Ph.D. degree in EE from Washington University in St. Louis, Missouri.
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