Postgres Impl Advisory Locks

postgres-impl-advisory-locks

Quick Reference :

An advisory lock is a lock with an application-defined meaning. PostgreSQL does NOT enforce it: it locks an integer key, not a row or table, and it is up to the application to agree on what that key represents. The server only guarantees that two sessions cannot hold the same exclusive key at once.

Use advisory locks for coordination that does not map onto a row: "only one worker runs this cron job", "only one instance rebuilds this cache", "serialize this external API call". They are faster than a flag column, cause no table bloat, and are cleaned up automatically at session end.

The behaviour is identical across PostgreSQL 15, 16, and 17.

need a lock?
  ├─ protecting a specific table ROW         → SELECT ... FOR UPDATE  (NOT advisory)
  ├─ coordinating something with no row      → advisory lock
  │     ├─ release should be automatic       → pg_advisory_xact_lock*   (transaction)
  │     └─ must outlive a transaction        → pg_advisory_lock*        (session)
  └─ must not wait if already locked         → pg_try_advisory_*  (returns boolean)

The single most important rule : PREFER transaction-level locks (pg_advisory_xact_lock). They auto-release at transaction end, including on crash or rollback, so they cannot leak. Session-level locks need a disciplined unlock and leak until the connection closes if you forget.

When To Use This Skill :

ALWAYS use this skill when :

• A scheduled job must run at most once across multiple application instances
• Building a distributed mutex or critical section coordinated through PostgreSQL
• Coordinating queue workers so two workers never process the same logical unit
• Serializing an expensive or external operation that has no natural row to lock
• A pg_advisory_lock appears stuck, leaked, or behaves wrongly behind a pooler

NEVER use this skill for :

• Locking an actual table row before update (use SELECT ... FOR UPDATE)
• Deadlock diagnosis on regular locks (use postgres-errors-deadlocks)
• Reading or interpreting EXPLAIN plans and lock waits (use postgres-impl-query-performance-toolkit)
• Scheduling the job itself (use the pg_cron skill or an external scheduler)

Decision Trees :

Session-level vs transaction-level :

how should the lock be released?
  ├─ at the end of the current transaction, automatically
  │     → pg_advisory_xact_lock(key)        no unlock call, cannot leak
  │
  └─ it must survive across transactions, released by explicit call
        → pg_advisory_lock(key)             MUST pair with pg_advisory_unlock(key)
        → risk: forget the unlock = lock held until the session disconnects

default choice: transaction-level. Pick session-level only when the protected
work genuinely spans multiple transactions.

Blocking vs non-blocking :

what if the lock is already held by someone else?
  ├─ wait for it (block until acquired)
  │     → pg_advisory_lock(key) / pg_advisory_xact_lock(key)
  │
  └─ do not wait, decide immediately from the result
        → pg_try_advisory_lock(key) / pg_try_advisory_xact_lock(key)
        → returns true  = you got it, proceed
        → returns false = someone else has it, skip / retry later

Cron-at-most-once and "skip if busy" ALWAYS use the try variant: a blocking lock would just queue every instance and run the job N times in sequence.

Single key vs two-key namespace :

is there any chance another subsystem picks the same integer?
  ├─ no, the key space is fully yours    → pg_advisory_lock(bigint_key)
  └─ yes, multiple subsystems share a DB  → pg_advisory_lock(classid, objid)
        classid = a constant per subsystem (e.g. 42 = "nightly-jobs")
        objid   = the specific resource id within that subsystem

the single-bigint key space and the two-int4 key space NEVER overlap, so a
two-key lock can never collide with a one-key lock.

Advisory lock vs row lock :

is there a real table row that represents the thing you are protecting?
  ├─ YES → SELECT ... FOR UPDATE on that row. The lock follows the data,
  │        survives correctly, and is visible to anyone reading the row.
  └─ NO  → advisory lock. There is nothing else to lock onto.

NEVER use an advisory lock as a substitute for a row lock: the advisory key is
invisible to code that only looks at the row, so the two guards do not see
each other.

Patterns :

Pattern 1 : Cron job at most once across instances

ALWAYS guard a scheduled job with pg_try_advisory_lock (or the xact variant) so that only one of N application instances actually runs it. NEVER use a blocking pg_advisory_lock here: every instance would queue and the job would run N times back to back.

-- each instance runs this; only the first to arrive gets true
SELECT pg_try_advisory_lock(42, 1001) AS got_lock;
-- got_lock = true  -> run the job, then pg_advisory_unlock(42, 1001)
-- got_lock = false -> another instance is running it, skip this tick

WHY : pg_try_advisory_lock returns immediately with a boolean. The instance that gets true runs the job; the others see false and skip. The job runs exactly once per tick regardless of how many instances are scheduled.

Pattern 2 : Distributed mutex with automatic release

ALWAYS prefer the transaction-level lock for a critical section that fits inside one transaction; it releases automatically even if the transaction aborts. NEVER leave a session-level mutex unreleased on an error path.

BEGIN;
SELECT pg_advisory_xact_lock(hashtext('rebuild-search-index'));
-- critical section: only one session at a time reaches here
-- ... do the work ...
COMMIT;   -- lock released here, automatically, even on ROLLBACK or crash

WHY : pg_advisory_xact_lock is bound to the transaction. The end of the transaction, by COMMIT, ROLLBACK, or backend crash, releases it. There is no unlock call to forget and therefore no leak path. hashtext(text) turns a readable name into the required integer key.

Pattern 3 : Queue workers with SKIP LOCKED

ALWAYS combine SELECT ... FOR UPDATE SKIP LOCKED for claiming queue rows; reach for an advisory lock only when the unit of work has no row. NEVER spin two guards on the same queue: pick row locks OR advisory locks per unit.

-- claim one job row without blocking on rows other workers hold
BEGIN;
SELECT id, payload FROM job_queue
WHERE state = 'pending'
ORDER BY id
FOR UPDATE SKIP LOCKED
LIMIT 1;
-- process it, UPDATE state = 'done', COMMIT

WHY : SKIP LOCKED lets each worker grab a different unclaimed row with no contention. Use a transaction-level advisory lock instead only when the work is keyed by something with no row, for example "process tenant 1234" where tenants are not a queue table.

Pattern 4 : Two-key namespacing to avoid collisions

ALWAYS use the two-integer form (classid, objid) when more than one subsystem takes advisory locks in the same database. NEVER let two unrelated subsystems pick raw integers from the same single-key space.

-- subsystem A: classid 100 = "billing"
SELECT pg_advisory_xact_lock(100, invoice_id);
-- subsystem B: classid 200 = "reports"
SELECT pg_advisory_xact_lock(200, report_id);
-- invoice_id and report_id may be equal; the classid keeps them distinct

WHY : the first integer is a constant namespace per subsystem, the second is the resource id within it. Two subsystems can use overlapping resource ids without colliding. The two-key space is also fully separate from the single-bigint space.

Pattern 5 : Inspecting held advisory locks

ALWAYS query pg_locks with locktype = 'advisory' to see which keys are held and by which backend when debugging a stuck coordination path.

SELECT pl.pid, pl.classid, pl.objid, pl.objsubid, pl.mode, pl.granted,
       psa.state, psa.application_name, psa.query_start
FROM pg_locks pl
JOIN pg_stat_activity psa ON psa.pid = pl.pid
WHERE pl.locktype = 'advisory';

WHY : every advisory lock is visible in pg_locks. objsubid = 1 means a single-key bigint lock (split across classid/objid), objsubid = 2 means a two-int4 lock. granted = false rows are sessions waiting. Joining pg_stat_activity shows who holds or waits and whether that backend is still doing useful work.

Pattern 6 : Non-blocking acquire with explicit release

ALWAYS pair every session-level pg_advisory_lock / pg_try_advisory_lock with exactly one pg_advisory_unlock on every code path, including error paths. NEVER acquire a session lock inside a SELECT ... LIMIT over multiple rows.

SELECT pg_try_advisory_lock(771) AS got;
-- if got = true:
--   ... do the multi-transaction work ...
SELECT pg_advisory_unlock(771);   -- release on success AND on every failure path

WHY : a session lock is held until unlocked or the connection ends. Acquiring it inside SELECT pg_advisory_lock(id) FROM foo WHERE id > 1 LIMIT 100 is unsafe: LIMIT is not guaranteed to apply before the locking function runs, so locks on unexpected rows are acquired and then dangle. Lock single, known keys; wrap row sets in a subquery.

Anti-Patterns :

(Cause, symptom, and fix for each in references/anti-patterns.md)

• AP-1 : Session-level lock never unlocked. Leaks, held until the connection closes.
• AP-2 : Advisory lock through a transaction-mode pooler. Session locks bind to the wrong backend.
• AP-3 : Key collision between unrelated subsystems sharing the single-key space.
• AP-4 : Advisory lock used where a row lock (SELECT FOR UPDATE) is correct.
• AP-5 : Blocking pg_advisory_lock for cron-at-most-once. Job runs N times in sequence.
• AP-6 : Acquiring locks inside SELECT ... LIMIT. Dangling locks on unexpected rows.
• AP-7 : Assuming ROLLBACK releases a session-level lock. It does not.

Reference Links :

• references/methods.md : Every advisory lock function with signature, return type, and key forms; pg_locks advisory columns
• references/examples.md : Working coordination snippets, version-annotated
• references/anti-patterns.md : Anti-patterns with cause, symptom, and fix-pattern

See Also :

• postgres-errors-deadlocks : when regular locks (not advisory) deadlock, SQLSTATE 40P01
• postgres-impl-query-performance-toolkit : pg_locks + pg_blocking_pids for general lock waits
• postgres-core-configuration : max_locks_per_transaction sizing for the shared lock pool
• Vooronderzoek section : §21 (PROPOSE-ADD advisory-locks)
• Official docs : https://www.postgresql.org/docs/17/explicit-locking.html#ADVISORY-LOCKS