Retry safety

retry-safety

Fire the same ticket twice and the kitchen must not charge the table twice. Timeouts, crashes, queue redeliveries, and impatient callers re-run operations constantly. The only question is whether the second run finishes the remainder or doubles the damage.

Core principle: the standard is idempotent read-modify-write at the side-effecting edge. The operation reads current state before writing, so a re-run over work already done completes the remainder and changes nothing else. Anything short of that needs a named reason it is still safe.

Read-only. This is a review lens. Fixing is a separate engagement. When the diff touches no side-effecting surface, say so in one line and stop.

The sweep

For each side-effecting edge in the diff, simulate two failure shapes: the whole operation retried after a timeout, and a crash halfway followed by a re-run.

Surface	The double-run hazard
Database writes	duplicate rows, double-applied increments, constraint violations on the second pass
Migrations	re-running a completed step corrupts or aborts (see the migration section)
File and data writes	appends that double, partial files clobbered, temp files orphaned
Network mutations	the remote applied the first call and the retry applies it again (no idempotency key)
Payments and external calls	double charge, double email, double webhook - the ones users notice
Queues and events	redelivery is at-least-once almost everywhere, and consumers assume exactly-once
Caches and counters	increments and TTL refreshes that drift under replay

A transaction is not an answer by itself: the transaction makes one attempt atomic, and then the caller retries the whole transaction.

Migrations carry a second hazard

Beyond re-run safety, a migration lives through a deploy window where old code runs against the new schema and new code runs against old data, and a partial failure leaves the two inconsistent. Flag a migration that:

• is non-reversible with no stated recovery path
• adds NOT NULL or another constraint to a populated table without a default or backfill
• renames or drops a column or table that instances still running mid-rollout depend on
• swaps or inverts an enum or ID mapping
• breaks foreign-key or cascade integrity

The fix arrow is expand-migrate-contract: add the column nullable, backfill, then add the constraint, so old and new code coexist through the rollout.

Report contract

# retry-safety report: <scope> (<date>)

## Verdict
One paragraph, or the single line "no side-effecting surface in this diff".

## Findings
### Short imperative title
- **Where:** file:line
- **Surface:** which side-effecting edge
- **Second run does:** the concrete double-apply, duplicate, or corruption
- **Fix:** the state check or idempotency key that makes the re-run finish the remainder

Common mistakes

• "It's in a transaction, so it's safe." Atomic per attempt is not idempotent across attempts.
• Assuming exactly-once delivery from a queue that promises at-least-once. Read the broker's contract, not the happy path.
• Checking only the clean retry and skipping the crash-halfway-then-rerun shape. Partial state is where the corruption lives.
• Treating idempotency keys on payments and external mutations as a nice-to-have. They are the fix, not an enhancement.
• Flagging read-only code. No side effects means no finding. One line and out.

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The retry standard and the migration deploy-window lens are adapted from the correctness reviewer in alp-river (MIT, Alper Ortac).