Strong consistency

Strong consistency (linearizability or serializability) ensures reads reflect the latest committed write. Required when invariants are strict: account balances, inventory counts, seat reservations, permission grants. Implementation uses single leader writes, quorum reads with versioning, or distributed transactions with acceptable latency cost.

Not every entity needs strong consistency — apply it where incorrect stale reads cause financial or safety bugs. Document which APIs are strongly consistent versus eventually consistent.

On interviews: identify two entities in the prompt needing strong consistency and which can be eventual, with storage choice justification.

Common pitfalls: strong consistency everywhere killing latency; reading from async replica for balance checks; distributed transactions without failure analysis.

Checklist:

• List invariants requiring latest state.
• Route critical reads to authoritative source.
• Use transactions or compare-and-set where needed.
• Accept latency cost explicitly.