Chr2 – consensus for side effects (exactly-once is a lie)
Most consensus libraries (Raft, Paxos) treat the state machine as a pure black box. This is fine until your state machine needs to actually do something, like charge a credit card, fire a webhook, or send an email. If a leader crashes after the side effect but before committing it, you get duplicates. This is my attempt at fixing this problem from first principles ish: build chr2 to make crash-safe side effects first-class citizens. mechanism: Replicated Outbox: Side effects are stored as "pending" in replicated state. Only the leader executes them under a fencing token. Durable Fencing: A…
In plain words
Chr2 is a consensus library that handles side effects safely in distributed systems. It addresses the problem where standard consensus mechanisms like Raft and Paxos can cause duplicate actions—such as charging a credit card twice or sending duplicate emails—when leaders crash. Chr2 uses a replicated outbox to store pending side effects, durable fencing to prevent zombie leaders from re-executing commands, and deterministic context to ensure consistent state transitions. It's designed for developers building systems where external actions must be guaranteed to happen exactly once despite failures.
written from the facts on this page · September 2026
From the sources
In the maker’s words, at launch
Most consensus libraries (Raft, Paxos) treat the state machine as a pure black box. This is fine until your state machine needs to actually do something, like charge a credit card, fire a webhook, or send an email. If a leader crashes after the side effect but before committing it, you get duplicates. This is my attempt at fixing this problem from first principles ish: build chr2 to make crash-safe side effects first-class citizens. mechanism: Replicated Outbox: Side effects are stored as "pending" in replicated state. Only the leader executes them under a fencing token. Durable Fencing: A manifest persists the highest view using atomic tmp+fsync+rename. This ensures a "zombie" leader can't wake up and double-execute stale effects. Deterministic Context: Application code receives a deterministic RNG seed and block_time from the log, ensuring 1:1 state transitions during replay. Strict WAL: Entries are CRC’d and hash chained. it is designed to prefer halting on mid-log corruption over guessing. The Trade-offs: Side effects are intentionally at-least-once; "exactly-once" requires stable effect IDs for sink-side deduplication. It’s a CP system safety over availability. Repo: https://github.com/abokhalill/chr2 if you’ve ever had “exactly once” collapse the first time a leader died mid flight, you know exactly why I built this.
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