# Optimistic concurrency — ETag / If-Match / Version (#253, #259, #265, #269)

Why the replace-all and config-bearing aggregates carry optimistic concurrency, and how the
contract evolved (including the reversals). The **mechanics** — headers, status codes, the
recipe — live in `docs/api-conventions.md` §7a–§7c; this file preserves the *decision
rationale*, relocated from the append-only `docs/decisions.md` at the v26.9.0 consolidation.

Issue trail: #253 (PR1 infra + PR3 Diff/Scalar fan-out), #259 (stable child identity for
schedule-item replace), #265 (RFC 7232 If-Match semantics), #269 (non-If-Match force-write +
cross-editor ETag rotation). Refs #197.

## Contents

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## Records formerly in this file

Each record below moved to its own file under `records/` (ersatztv#610); the rationale is
unchanged. Resolve by **key** — that is the stable identity. A date-based pointer from
another doc or an old issue comment should land here and then follow the link.

- 2026-07-11 — #253 PR3: Diff + Scalar concurrency fan-out (Collection / Playout×2 / MultiCollection / RerunCollection) — [`concurrency.diff-scalar-fanout`](records/concurrency/diff-scalar-fanout.md)
- 2026-07-11 — Optimistic-concurrency contract for replace-all PUTs (#253 PR1: infra + Block reference) — [`concurrency.replace-all-contract`](records/concurrency/replace-all-contract.md)
- 2026-07-11 — Stable child identity for schedule-item replace (#259, split from #252/#253) — [`concurrency.schedule-item-child-identity`](records/concurrency/schedule-item-child-identity.md)
- 2026-07-12 (#269 — non-If-Match root writers force-write past a concurrent Version bump) — [`concurrency.force-write-non-ifmatch`](records/concurrency/force-write-non-ifmatch.md)
- 2026-07-12 — Cross-editor ETag rotation completed for Collection/Playout config siblings (#269) — [`concurrency.etag-rotation-completion`](records/concurrency/etag-rotation-completion.md)
- 2026-07-12 — If-Match evaluates per RFC 7232: valid-but-non-matching → 412, only grammar violations → 400 (#265) — [`concurrency.ifmatch-rfc7232`](records/concurrency/ifmatch-rfc7232.md)
- 2026-07-18 — Concurrent same-item add is idempotent, not a 500: catch the unique-violation per provider (#308) — [`concurrency.idempotent-concurrent-add`](records/concurrency/idempotent-concurrent-add.md)
