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| 1 | A Blueprint for Interoperable Blockchains (2019) | Research in blockchain systems has mainly focused on improving security and bridging the performance gaps between blockchains and databases. Despite many promising results, we observe a worrying trend that the blockchain landscape is fragmented in which many systems exist in silos. Apart from a handful of general-purpose blockchains, such as Ethereum or Hyperledger Fabric, there are hundreds of others designed for specific applications and typically do not talk to each other. In this paper, we describe our vision of interoperable blockchains. We argue that supporting interaction among different blockchains requires overcoming challenges that go beyond data standardization. The underlying problem is to allow smart contracts running in different blockchains to communicate. We discuss three open problems: access control, general cross-chain transactions, and cross-chain communication. We describe partial solutions to some of these problems in the literature. Finally, we propose a novel design to overcome these challenges. | Motivating Example: Alice owns a coveted football ticket stored on a ticketblockchain (many permissioned blockchains are designed forasset management, and therefore can be used to implementsuch ticket application). Bob and Carol are avid footballfans and both want to buy the ticket. They are users ofthe ticket blockchain, and each of them owns currencies atother coin blockchains (for example Ethereum or XRP [12]).Alice wants to start an auction on her ticket, to which Boband Carol submit their bids. The ticket blockchain does notsupport bid submissions, thus Bob and Carol have to declaretheir bids in their own blockchains. This auction is imple-mented as smart contracts running on the three blockchainsrealizing a distributed workflow. Challenges: In summary, the challenges in supporting cross-chaintransaction are how to provide efficient read transactions,how to extract more concurrency, and how to reduce com-munication for general transactions. One major limitation of current smart contracts is thatthey do not talk to the outside world. They can only accessresources on the blockchain such as the ledger and execu-tion engine. Some external services, such as Ora-clize [8], provide authentic data feeds, such that messagessigned by them are accepted by the contract. Interledger protocol (ILP) [6] and Cosmos’inter-blockchain communication (IBC) [3] are two recentproposals for connecting blockchains. In summary, the challenge is to design a stateless commu-nication protocol that ensures timely and reliable deliveryof messages. The protocol needs to work at the applica-tion layer, which understands and contains optimizationsfor common requirements of cross-chain communication. Fi-nally, the protocol needs to be resilient to network failures. Solutions:
Here, we highlight three orthogonal research direc-tions that may benefit from our work. First, off-chain scalingsystems such as Plasma [40] and sidechains [16] use multipleblockchains to improve overall throughput. Our work canbe applied to make communication between the sub-chainsmore efficient and richer. Second, there is a trend towardsdecentralized systems, of which blockchain is only one ex-ample. Other examples include identity management [34],personal storage [11], and social network [27]. These sys-tems exist in silos, and our work can be applied to buildnovel applications on top of them. Finally, our approachcan help realize general purpose distributed workflows [33]. |
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