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PostgreSQL advisory locks can keep cooperating workers from entering the same job’s critical section at the same time. Give the task a stable lock key, have each worker call pg_try_advisory_lock, and run the work only if the call returns true. A losing worker can skip rather than wait. This is coordination, not a durable job queue: it does not record jobs, schedule retries, or guarantee exactly-once side effects.
How advisory locks prevent two workers from doing the same work
An advisory lock is a lock on an application-defined key. PostgreSQL tracks it, but the database does not require unrelated code to respect it. Every worker or code path that must coordinate has to use the same key mapping and locking convention.
For a recurring singleton task, such as a daily cleanup, all workers can try to acquire the same key. For work associated with a stable logical resource, such as one account or report, derive a key consistently from that resource. A worker proceeds only when it gets the lock; if acquisition fails, another session currently holds the exclusive lock.
PostgreSQL accepts either one 64-bit key or two 32-bit keys. Those two key spaces do not overlap. Key uniqueness and meaning are your application’s responsibility: document the namespace and mapping, and avoid lossy hashing unless the consequences of collisions are acceptable. See the PostgreSQL advisory-lock documentation.
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Choose the lock function by how long ownership must last
The main choice is whether the protected work fits inside one transaction or must remain exclusive across a longer job run.
| Function | Lock lifetime | When it fits |
|---|---|---|
pg_try_advisory_xact_lock |
Until the current transaction ends, whether it commits or aborts; cannot be manually unlocked. | The entire critical section fits inside one transaction. |
pg_try_advisory_lock |
Until explicitly unlocked or the PostgreSQL session ends; rollback does not release it. | The job spans transactions or external calls and needs a session-level lock. |
Both try functions return immediately: true means the lock was acquired, and false means it was not. Their blocking counterparts wait instead of letting the worker skip. PostgreSQL documents the function variants and their behavior in its advisory-lock function reference.
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Use a transaction-level lock for a transaction-sized job
Acquire pg_try_advisory_xact_lock in the transaction that performs the protected work. If it returns false, do not perform that work. The lock is released automatically at transaction end, including on rollback, so there is no separate unlock step.
Use a session-level lock only while keeping its session
A job that spans multiple transactions or makes external calls may need pg_try_advisory_lock. Keep the same PostgreSQL connection associated with the worker for the lock’s whole lifetime. Release it explicitly after success and on error; if the session ends, PostgreSQL releases the lock automatically. Repeated acquisitions of a session-level lock by the same session stack, so matching unlocks are needed for early release.
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Do not acquire a session-level lock through one pooled connection and assume a later query or unlock on an unrelated connection refers to the same PostgreSQL session. Pin the owning connection for the duration, or use a transaction-level lock if the critical section fits within one transaction. Connection loss also needs an application-level response: stop the work or make it safe to retry rather than assuming the lock still protects it.
What this pattern does—and does not—coordinate
Advisory locks are local to a single database. Workers must coordinate through that same database; a lock in one database is not a cross-database or cross-cluster lock. They are also advisory: code that does not acquire the agreed key can still perform the protected operation.
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The lock represents temporary mutual exclusion, not durable job state. By itself, it does not store a pending job, record completion, implement retry or backoff policy, retain per-job history, or make external side effects exactly once. If a worker crashes after an external action but before recording success elsewhere, another attempt may repeat that action. Design the operation to be idempotent or otherwise safe to retry, and store job state separately when durability matters.
When to use a queue table with SKIP LOCKED instead
Use a persisted queue table when jobs need durable rows, status transitions, retries, history, or concurrent workers claiming different jobs. In a queue-like consumer transaction, SELECT ... FOR UPDATE SKIP LOCKED lets a worker skip rows already locked by another worker. PostgreSQL cautions that this produces an inconsistent view and is intended for queue-like consumers, not general-purpose reads. It is a row-claiming design, distinct from using one advisory lock to exclude work on an application-defined resource. See PostgreSQL’s SELECT locking documentation.
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| Question | Advisory lock | Queue rows with SKIP LOCKED |
|---|---|---|
| What is being coordinated? | One application-defined resource or singleton task. | Persisted job rows, often with different workers claiming different rows. |
| How long does ownership last? | One transaction or, with a session lock, the session’s lock lifetime. | Typically the transaction that claims and processes locked rows; job state must be stored in the queue design. |
| Does the lock itself persist job state or retries? | No. | The queue table can store job state; retry policy remains an application design decision. |
| What happens under contention? | A try function returns false, or a blocking function waits. | Locked rows are skipped so a worker can claim other available rows. |
| Where does coordination apply? | Only within the same database. | Workers consuming the same queue table in the same database. |
Operational checks and failure modes
Inspect locks and remember database scope
PostgreSQL exposes outstanding advisory locks in pg_locks. Its database column matters because advisory locks are database-local. Use that view to investigate locks held by sessions, and check the session’s connection and cleanup path when a session-level lock remains longer than intended. See the pg_locks documentation.
Account for lock-table capacity
Advisory and regular locks share a finite memory pool governed by max_locks_per_transaction and max_connections. PostgreSQL describes typical capacity as tens to hundreds of thousands depending on configuration, not a fixed universal limit. High-cardinality use—such as creating locks for many distinct resources—deserves capacity planning against the deployed configuration. Details are in the official advisory-lock documentation.
Do not assume LIMIT constrains lock calls
When advisory-lock functions appear in a query with LIMIT, expression evaluation order can mean locks are acquired for more rows than the limit suggests. PostgreSQL documents using a subquery to constrain which rows feed the lock call; consult the advisory-lock documentation before relying on a query’s apparent evaluation order.
Quick Recap
Implementation checklist
- Define the work identity. Decide whether the lock protects one singleton task or a stable logical resource. Specify the key shape, namespace, and deterministic mapping used by every worker.
- Choose the lifetime. Use a transaction-level try lock if the entire critical section fits in one transaction; use a session-level try lock only when the same session can stay pinned for the job’s lifetime.
- Handle contention explicitly. Proceed only when the try function returns true. Treat false as “another worker owns this work now,” and decide whether the worker should skip, wait via a blocking function, or seek other work.
- Make recovery safe. For session locks, release explicitly on success and error, while allowing session termination to provide automatic cleanup. Separately define what happens after connection loss or a crash, including how repeated external effects are prevented or tolerated.
- Use durable job records when needed. If jobs need per-job claiming, status, history, or retry tracking, design a queue table rather than treating an advisory lock as the job record.
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