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A trigger can modify other rows or tables implicitly during an insert or update. That makes the visible application statement an incomplete description of the transaction and can add locking or recursion. Document and test trigger effects, or move orchestration to an explicit routine/service when hidden behavior harms operability.

Rollback restores code, not every side effect. Expand-and-contract changes preserve compatibility during overlap, and recovery assumptions must be exercised.

A parallel reduction may group values differently, so its combine operation should be associative for equivalent results and have a suitable identity value. Floating-point addition can vary slightly with grouping. Avoid reducers with hidden side effects because execution order may not be stable.

A deadlock detector aborts one transaction so another can progress. Capture lock and query evidence, then retry the whole business transaction only if its effects are safe to repeat. Use bounded attempts with jitter, consistent resource ordering, and idempotency for any external side effects.

map is lazy and can be concise with an existing named function; a comprehension often reads better when transformation and filtering are simple. reduce is appropriate for a well-defined associative fold, but sum, any, all, or an explicit loop often communicates intent better. Avoid hidden side effects because they make ordering and debugging difficult.

The destination rejects generations older than its highest accepted token. A lease does not stop a paused worker, and a fence cannot reject an expired generation it has not yet learned is superseded.