Deadlock Diagnoser

A deadlock looks random from the application — a transaction that worked a thousand times suddenly errors out under load — but the database already did the forensics for you. When the engine detects a cycle it picks a victim, rolls it back, and logs exactly who held what and waited on what. This skill reads that report instead of guessing: it pulls the Postgres deadlock log lines (or the SQL Server deadlock graph / innodb status in MySQL), reconstructs the cycle (A holds lock 1 and wants lock 2 while B holds 2 and wants 1), and names the real root cause — which is almost always two code paths acquiring the same rows or tables in different orders. Then it fixes the cause: enforce one consistent lock-acquisition order everywhere, shrink the lock window so the race rarely opens, and add a retry-the-victim safeguard for the deadlocks you can't design away — in that priority, because retries without ordering just trade a deadlock for a rollback storm.

When to use this skill

• The database log shows deadlock detected (Postgres), a deadlock graph / error 1205 (SQL Server), or Deadlock found when trying to get lock (MySQL/InnoDB).
• A transaction intermittently fails or auto-retries only under concurrency — fine in dev, flaky in production at peak.
• Two queries or endpoints mysteriously block each other, or you see processes stuck in a lock wait that times out.
• You're adding a write path that touches multiple rows/tables and want to confirm it locks in the same order as existing code before it ships.
• Lock contention (not a true cycle) is serializing throughput, and you need to tell genuine deadlocks apart from long lock waits.

Instructions

1. Get the engine's deadlock report — don't reconstruct from app logs. In Postgres, read the server log around the error: it prints both processes, their full SQL statements, and the Process N waits for <lockmode> on <relation/tuple>; blocked by process M lines for each side of the cycle (raise log_lock_waits = on and deadlock_timeout context if it's terse). In SQL Server, pull the deadlock graph from the system_health Extended Events session or a trace — it lists each process with its inputbuf (the statement) and the resource-list of locks owned vs. requested. In MySQL/InnoDB, run SHOW ENGINE INNODB STATUS and read the LATEST DETECTED DEADLOCK section. This report is ground truth; the app's stack trace only tells you which transaction lost.
2. Reconstruct the cycle explicitly: who HELD what, who WANTED what. Write it out as a two-column picture — Txn A: holds <lock on resource 1>, waits for <lock on resource 2> / Txn B: holds <lock on resource 2>, waits for <lock on resource 1>. Identify the exact resources (which rows/index ranges/tables) and the lock modes (row FOR UPDATE/exclusive vs. shared, gap locks in InnoDB, intent locks in SQL Server). A real deadlock is a closed cycle of waits; if it's not a cycle, it's lock contention or a lock-wait timeout (step 8), which has a different fix.
3. Find the inconsistent acquisition ORDER — the usual root cause. Grep the codebase for every transaction that touches the resources in the cycle and trace the order each one locks them. The classic bug: one path does UPDATE accounts WHERE id=1 then id=2, another does id=2 then id=1 (or two services lock tables orders then inventory vs. inventory then orders). Watch for ordering that's hidden — a SELECT ... FOR UPDATE with an unordered IN (...) or a join whose row-locking order depends on the plan, an ORM that emits writes in object-graph order, or a foreign-key check that takes a lock on the parent row you didn't write explicitly.
4. Fix the cause first: enforce ONE consistent lock-acquisition order across all transactions. Make every code path acquire the shared resources in the same deterministic order — sort the ids before locking (SELECT ... FOR UPDATE ... ORDER BY id), always lock parent before child, always lock tables in a fixed documented sequence. Consistent ordering makes a cycle impossible: contenders queue instead of deadlocking. This is the only fix that actually removes the deadlock rather than reducing its odds.
5. Shrink the lock window so the race rarely opens. Keep transactions short and narrow: acquire locks as late as possible, commit as early as possible, and lock only the rows you'll write. Never hold a transaction open across a network/RPC/third-party-API call or across user think-time — an external call inside the transaction stretches the lock-hold from milliseconds to seconds and turns rare contention into constant deadlocks. Do the slow work before BEGIN or after COMMIT.
6. Pick a deliberate lock strategy for the access pattern, and right-size isolation. Where the same rows are contended, use pessimistic locking with SELECT ... FOR UPDATE in the consistent order from step 4. Where conflicts are rare, prefer optimistic concurrency — a version/updated_at column checked in the WHERE of the UPDATE and a conflict-retry, which takes no long-held locks. If the engine is over-locking (e.g. Serializable or InnoDB gap locks causing deadlocks on inserts/range scans), drop to the lowest isolation level that's still correct (often Read Committed) to acquire fewer locks.
7. Add the retry-the-victim safeguard — last, not first. A deadlock victim's transaction is rolled back cleanly and is a transient, safe-to-retry error; the app should catch it specifically (Postgres SQLSTATE 40P01, MySQL 1213, SQL Server 1205) and retry the whole transaction with capped exponential backoff and jitter (e.g. 3–5 attempts). Retry the entire transaction from BEGIN — replaying half a rolled-back transaction corrupts state. This handles the deadlocks you can't design away; it does NOT substitute for steps 4–5.
8. Distinguish a true deadlock from plain lock contention before "fixing" the wrong thing. If the report shows a lock-wait timeout rather than a detected cycle, there's no ordering bug — one transaction is simply holding a lock too long (a long-running write, an idle-in-transaction connection, a missing index forcing a wide row/range lock). The fix there is shortening the holder (step 5), adding the index so the lock is narrow (query-plan-analyzer), or killing idle-in-transaction sessions — not reordering locks.

Output

A short report with four parts:

1. The reconstructed cycle — quoted from the engine's deadlock report: Txn A holds <lock on R1>, wants <lock on R2> / Txn B holds <lock on R2>, wants <lock on R1>, with the exact resources, lock modes, and the two offending statements.
2. The root cause — the specific inconsistent lock-acquisition order (or over-long lock scope / over-strict isolation) behind the cycle, naming the two code paths and the resources they lock in conflicting order.
3. The fix — one concrete change: the consistent ordering to enforce (with the exact ORDER BY / lock sequence), or the shortened-transaction change (what to move outside BEGIN), or the isolation-level / locking-strategy change — not a menu.
4. The retry safeguard — the specific deadlock SQLSTATE/error code to catch and the backoff retry of the whole transaction, framed explicitly as the net for residual deadlocks, not the primary fix.