Distributed systems are hard to implement correctly, and subtle bugs can go undetected using traditional testing. Formal verification offers an alternative for proving the correctness of complex distributed systems. Despite previous efforts to automate and facilitate formal verification, it is still hard to integrate formal verification in software development. Programmers need to query the theorem prover repeatedly to find the correct proof of their system. This cycle of going back and forth with the theorem prover involves a lot of human intervention and is a barrier to adopting formal verification in software development. In this paper, we address the challenges of scaling formal verification in practice by reducing the human intervention required to find the correctness proof of a distributed system. We propose ProofSaX, an automated tool that shards large verification tasks and explores the proof search space for each shard independently to synthesize correctness proofs. We use controlled exploration when adding possible proof annotations to handle the search-space explosion problem. We evaluate ProofSaX on a variety of safety proofs for distributed systems. We show that ProofSaX can find the last proof annotation in 86 out of 103 proof-completion tasks, with runtimes ranging from one minute to two hours.

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