Read committed vs serializable, optimistic locking, advisory locks.

Transactions — ACID, Isolation Levels, and Deadlocks

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.

Interactive Pipeline Flow

UnderstandImplementOptimise

🚀 BEGIN — start transaction

BEGIN starts a transaction. All statements until COMMIT or ROLLBACK are in the same atomic unit. Wrap in try/finally to guarantee ROLLBACK on exceptions.

Architecture Diagrams

Two strategies for handling concurrent updates.

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Key Design Decisions

🔒Use SELECT FOR UPDATE for read-modify-write

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.

⚔️Prevent deadlocks with consistent lock order

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.

⏱️Always set lock_timeout

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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