We present the first formal verification of the Raft consensus protocol, a critical component of many distributed systems. We proved that our implementation is a linearizable replicated state machine, which required iteratively discovering and proving nSysInv system invariants. Our verified implementation can be extracted and run on real networks.

The primary challenge we faced during the verification process was proof maintenance, since proving one invariant often required strengthening and updating other parts of our proof. To address this challenge, we propose a methodology of planning for change during verification. Our methodology adapts classical information hiding techniques to the context of proof assistants, factors out common invariant strengthening patterns into custom induction principles, proves higher-order lemmas that show any property proved about a particular component imply analogous properties about related components, and makes proofs robust to change using structural tactics. We also discuss how our methodology may be applied to systems verification more broadly.