Optimistic vs. Pessimistic Locking
Choose whether competing writers fail and retry or wait and serialize.
On this page
What problem are we solving?How the options workWhen is each option better?Concrete exampleFailure modesScale implicationsInterview rule of thumbApply itWhat problem are we solving?
Two requests want to change a resource without violating a shared predicate.
How the options work
| Option | Mechanics | Best fit |
|---|---|---|
| Optimistic concurrency | Read version v, then update WHERE version=v and business predicate holds. Zero rows or serialization failure means retry/conflict. | Low contention, disconnected edits and short single-row changes. |
| Pessimistic locking | Lock the relevant rows in a transaction, recheck business state, write, commit and release locks. | Frequent conflicts or small multi-row decisions with stable lock order. |
When is each option better?
Optimism is better when most work succeeds uncontested and retry is cheap. Pessimism can be better when speculative work is often wasted. A single conditional UPDATE itself takes database locks internally: “optimistic” describes the application’s conflict protocol, not the absence of storage locks.
Concrete example
For A7 version 12, Alice’s UPDATE advances to 13. Bob’s version-12 UPDATE loses. For three adjacent seats, lock all three in a fixed order, verify all are free and commit all-or-nothing.
Failure modes
Optimistic retries can livelock under a hot-key storm unless bounded/backed off. Pessimistic transactions can deadlock or hold resources while waiting for external calls. Retry the entire aborted transaction, lock rows in deterministic order, and never keep a lock open while a human or payment provider responds.
Scale implications
Neither method creates more units of a scarce resource. Keep invariant scope narrow; bound transaction duration and admission. Cross-row predicates may need explicit parent/sentinel locks, constraints or Serializable isolation with retries; locking one existing row does not automatically prevent new matching rows.
Interview rule of thumb
“Where is the atomic check?” Start with a conditional write for one row. Use short ordered row locks when coordination across a small known set makes it clearer.
Apply it
Booking, Payments, E-commerce, File storage.
Before choosing, name the required guarantee, one failure window and the metric that would force you to revisit this decision.
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