Extending Existing Blockchains with Virtualchain

Public blockchains are becoming a ubiquitous network service. However, it's hard to make consensus-breaking changes to production blockchain networks. To overcome this, we created Virtualchain, a logical layer for implementing arbitrary fork*-consistent replicated state machines (RSMs) on top of already-running blockchains. Blockchains provide a totally-ordered, tamper-resistant journal of state transition events. New applications can store a log of all state changes in a public blockchain, such as Bitcoin [14], Litecoin [11], or Ethereum [6]. By using the blockchain as a shared ground truth, these applications can then bootstrap global state in a secure, decentralized manner, since every application node subscribed to the blockchain can independently construct the same state as all other application nodes. However, there are two key challenges to using blockchains in this manner. First, a blockchain can fail—it can go offline, or its consensus mechanism can become " centralized " by falling under the de facto control of a single entity. To tolerate failures, application journals must be efficiently migrateable across blockchains. Cross-chain migration is already needed by production systems; for example, we migrated a production system from Namecoin [12] (which became centralized) to Bitcoin [17]. The second challenge is that the application's journal can be forked and corrupted by the underlying blockchain. If the blockchain forks, nodes on different forks will write and read different events, forking the journal. The blockchain may drop and reorder transactions when its forks join, causing bootstrapping nodes to construct different state than already-running nodes. Applications must be able to recover from these failures. To do so, we created Virtualchain. Virtualchain is a logical layer for multiplexing multiple fork*-consistent [10] state transition journals on a blockchain. Application nodes replay their journal to achieve application-level consensus at each block b, such that two nodes will agree on a block if and only if the application transactions in that block leave the nodes in an identical state. If their resulting state after executing the operations in b are identical, they generate what we term a consensus hash for that block. Consensus hashes enable nodes to independently audit and efficiently query their journals, as well as migrate them between blockchains and detect journal forks. Virtualchain is implemented in a software library available and ready for production-use. We have used it to implement a global naming and storage system called Block-stack [1], which is one of the largest applications on the Bitcoin blockchain today [13]. …