07 — Locking: Pessimistic vs Optimistic

A frequent interview topic: pessimistic vs optimistic locking 6:29.

Pessimistic locking

• Transaction A locks the row first; when transaction B tries to access the same row (imagine a seat reservation), it has to wait until the lock is released 6:29.

Optimistic locking

• Usually implemented with versioning 7:02.
• Flow: A commits and the row version goes from v1 to v2; when B tries to commit against the now-stale version, it is rejected and must retry 7:02.
• Rule: if the versions don't match, the write gets rejected 7:02.

sequenceDiagram
    participant A as Transaction A
    participant DB as Row (v1)
    participant B as Transaction B
    Note over A,B: Pessimistic
    A->>DB: lock row
    B->>DB: access row → must wait
    Note over A,B: Optimistic
    A->>DB: commit (v1 → v2)
    B->>DB: commit against v1 → rejected, retry