Database Perspectives on Blockchains

Modern blockchain systems are a fresh look at the paradigm of distributed computing, applied under new assumptions of large-scale open public networks. They can be used to store and share information without a trusted central party. There has been much theoretical and practical effort to develop blockchain systems for a myriad of uses, ranging from cryptocurrencies to identity control, supply chain management, clearing and settlement, and more. However, none of this work has directly studied the many fundamental database-related issues that arise when using blockchains as the underlying infrastructure to store and manage data. The key difference between using blockchains to store data and centrally controlled databases (of any type) is that transactions are accepted to a blockchain via a consensus mechanism, and not by the controlling central party. Hence, once a user has issued a transaction, she cannot be certain if it will be accepted. Moreover, a yet unaccepted transaction cannot be retracted by the user, and may be appended to the blockchain at any point in the future. This causes difficulties as the user may wish to reissue a transaction, if it was not accepted. Yet this data may then become appended twice to the blockchain. In this paper, we present a database perspective on blockchains by introducing formal theoretical foundations for blockchains as a data storage layer that underlies a database. The main issue that we tackle is the uncertainty in transaction appending that is a direct result of the consensus mechanism. We study two flavors of the transaction appending problem. First, we consider the complexity of determining whether it is possible for a denial constraint to be contradicted, given the current state of the blockchain, pending transactions, and integrity constraints on blockchain data. Second, we determine the complexity of generating transactions that are (or are not) mutually consistent with given subsets of pending transactions. The ability to solve these problems is critical in ensuring that users can issue transactions that are consistent with their intentions. Finally, we chart directions for future work by discussing some of the new and important research challenges that arise when data are stored using blockchains. 2012 ACM Subject Classification H.2 Database Management