ACID transactions guarantee Atomicity (all-or-nothing), Consistency (constraint preservation), Isolation (concurrent transactions behave as serial), and Durability (committed data survives crashes). Isolation levels trade anomaly protection for concurrency. Read Committed prevents dirty reads. Repeatable Read prevents fuzzy reads. Serializable prevents phantom reads and write skew. Optimistic locking (version columns) avoids lock contention. Deadlocks occur when two transactions hold locks the other needs — always acquire locks in consistent order.
Each stage in order — click any step to read what it does.
SQL isolation levels and the anomalies they prevent.
The trade-offs worth knowing before you build this.
If you read a value and then update it based on the read, use SELECT ... FOR UPDATE. This prevents another transaction from modifying the row between your read and write.
Always acquire locks in the same order across all transactions (e.g., by primary key ascending). This eliminates circular wait — the only cause of deadlocks.
SET lock_timeout = '5s'. Without it, a transaction holding a lock indefinitely will cause all conflicting transactions to queue forever, cascading into an outage.
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