Charlotte: Reformulating Blockchains into a Web of Composable Attested Data Structures for Cross-Domain Applications

Cross-domain applications are rapidly adopting blockchain techniques for immutability, availability, integrity, and interoperability. However, for most applications, global consensus is unnecessary and may not even provide sufficient guarantees. We propose a new distributed data structure: Attested Data Structures (ADS), which generalize not only blockchains but also many other structures used by distributed applications. As in blockchains, data in ADSs is immutable and self-authenticating. ADSs go further by supporting application-defined proofs (attestations). Attestations enable applications to plug in their own mechanisms to ensure availability and integrity. We present Charlotte, a framework for composable ADSs. Charlotte deconstructs conventional blockchains into more primitive mechanisms. Charlotte can be used to construct blockchains but does not impose the usual global-ordering overhead. Charlotte offers a flexible foundation for interacting applications that define their own policies for availability and integrity. Unlike traditional distributed systems, Charlotte supports heterogeneous trust: different observers have their own beliefs about who might fail, and how. Nevertheless, each observer has a consistent, available view of data. Charlotte's data structures are interoperable and composable: applications and data structures can operate fully independently or can share data when desired. Charlotte defines a language-independent format for data blocks and a network API for servers. To demonstrate Charlotte's flexibility, we implement several integrity mechanisms, including consensus and proof of work. We explore the power of disentangling availability and integrity mechanisms in prototype applications. The results suggest that Charlotte can be used to build flexible, fast, composable applications with strong guarantees.