When concurrent actors might share mutable state, first ask whether they need the same mutable object. If not, eliminate the sharing. When sharing is real, enforce serialization structurally: lockfiles, sequential phases, exclusive ownership. Instructions and conventions are not concurrency control.
Why: Concurrent writes to shared state create race conditions that are intermittent, hard to reproduce, and expensive to debug.
Pattern:
- Identify shared mutable state (files both read and write, branches both push to, APIs both define and consume).
- Default: eliminate the shared write target. Ask: do these actors need one canonical object, or are they publishing independent facts? Give each actor its own owned file, key, branch, or state directory, and merge only at the read/reporting boundary. Two workers writing their own
lastXfield into onestate.jsonis still shared mutation.indexer-state.json+metrics-state.jsonis not. - Only when one shared write target is a real invariant, serialize access structurally (lockfiles, sequential phases, single-writer actor, or atomic compare-and-swap). Treat “we need a lock” as a design smell to check, not as the default answer.