Which isolation level should I actually use?

Start with your database's default, Read Committed on Postgres, and write hot-path updates as single atomic SQL statements so lost updates cannot happen. Reserve Serializable for the few transactions enforcing invariants across multiple rows or tables, and give those a retry loop. Raising the global level everywhere buys anomalies you don't have at a throughput cost you will notice.

Why did my transaction fail with a serialization error when nothing seemed wrong?

At Repeatable Read or Serializable, the database aborts transactions whose reads and writes overlap dangerously with concurrent ones, even if your specific run would have been fine. It is the engine being conservative to guarantee correctness. The fix is not lowering the level but retrying the transaction, ideally automatically with a capped backoff.

What is write skew and why doesn't Repeatable Read stop it?

Write skew is two transactions reading overlapping data, then each writing different rows, so that both commit but the combined result breaks an invariant, like two doctors both leaving call because each saw the other on duty. Repeatable Read only guarantees each transaction a stable snapshot; since neither wrote a row the other read, no conflict is detected. Only Serializable tracks those cross-transaction dependencies.

Fundamentalschapter 6 of 6 · 4 lessons

Database Isolation Levels

Control how much concurrent transactions can see each other's intermediate state.

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Two people click buy on the last ticket at the same instant. Your database runs both transactions at once, because running them one after another would be far too slow, and now the two are looking at each other's half-finished work.

Isolation levels are the fine print on how much they are allowed to see. Every level answers one question differently: which strange results will you tolerate in exchange for running more transactions at a time? Get the answer wrong and you sell the same seat twice.

Lessons

4 in this chapter

The Anomalies and the LadderFour named ways concurrent transactions corrupt each other, and the four levels that close them.2 min
Read Committed, the WorkhorseYou only see committed data, and that is the entire promise.2 min
Repeatable Read and SnapshotsOne snapshot for the whole transaction: your reads stay frozen in time.2 min
Serializable, and When to Pay for ItFull correctness, purchased with retries. Spend it on the transactions that deserve it.2 min