Eventually Durable State Machines

Abstract

Typically, applications are designed to guarantee durability of the data they store. Durability is achieved by replicating client write requests to multiple machines. This replication adds to the time it takes for the application to respond to client requests. And so, latency-sensitive applications may implement ad-hoc mechanisms to circumvent durability costs, such as responding without replicating writes. Such ad-hoc mechanisms are hard to reason about and may leave the application in an inconsistent state.

We propose Eventually Durable State Machine, a principled approach for applications to respond without waiting for replication to complete. The ED model offers fast response time but leaves applications vulnerable to data loss when failures occur. The ED model presents a strong ordering guarantee and a clear failure semantics to reason about the lost writes.

Further, we develop ED Raft protocol, a derivative of the Raft consensus protocol, to implement the Eventually Durable State Machine. We describe the ED Raft and its key properties, and show that ED Raft supports the ED model.