PostgreSQL advisory locks let applications coordinate work using lock identifiers whose meaning the application defines. They can help prevent two cooperating workers from performing the same operation simultaneously. They do not automatically protect a table or stop code that ignores the agreed locking protocol.
A safe design specifies the key, lifetime, participating clients, and behavior when ownership cannot be acquired. This guide explains session and transaction locks, connection pools, and failure tests so cooperative locking remains a clear contract rather than an invisible global assumption.
Define the protected operation
State what must not overlap: one maintenance task, one tenant’s synchronization, or processing of a particular logical resource. Avoid describing the lock merely as a database lock without naming the business operation it coordinates.
Decide whether ordinary row locks or a database constraint would better express the requirement. Advisory locks are useful for application-defined coordination, but they should not replace a uniqueness invariant that belongs in the data model.
List every code path that must cooperate. If a manual job, older application version, or alternate service can perform the operation without acquiring the same lock, the exclusion guarantee is incomplete. The lock works because participants follow the protocol.
Establish a stable key convention
Choose a deterministic mapping from logical resources to the supported advisory-lock key space. Separate namespaces for unrelated operations so one task does not accidentally block another. Document the convention and keep it shared across implementations.
Review collision behavior if the mapping uses hashes. Different logical resources can potentially map to one key, depending on the design. Decide whether that merely reduces concurrency or creates a more serious correctness problem.
Do not use an arbitrary language’s process-randomized hash as a cross-worker identity without verifying its stability. Two processes coordinating the same resource must agree on the key. Version changes and alternate languages should be included in compatibility tests.
Choose session or transaction lifetime
Session-level advisory locks persist until explicitly released or the session ends. Their behavior does not follow transaction rollback in the same way as ordinary transaction-scoped locks. A lock acquired in a transaction can remain held after that transaction rolls back.
Transaction-level advisory locks are released when the transaction ends and do not require an explicit unlock in the same manner. This can simplify ownership for work whose lifetime naturally matches one database transaction.
Choose based on the actual operation, not convenience. Holding a transaction open while a slow external service runs can create other database problems. A session lock avoids that particular transaction lifetime but introduces explicit connection ownership and cleanup requirements.
Prefer bounded acquisition behavior
PostgreSQL provides blocking and try-style advisory-lock functions. A try-style operation can report whether ownership was acquired without waiting indefinitely. The application must inspect the result and define whether to skip, reschedule, or return a controlled conflict.
BEGIN;
SELECT pg_try_advisory_xact_lock(4001, 17);
-- Continue only if the returned value is true.
COMMIT;This example uses fictional key values and illustrates transaction-scoped syntax. The comment represents required application logic, not an automatic SQL condition. Committing immediately also ends that transaction’s lock ownership.
If blocking acquisition is appropriate, set a reviewed wait budget and observe contention. A worker waiting forever for another worker’s abandoned session is not a reliable scheduler.
Account for connection pooling
Session locks belong to database sessions, not abstract application requests. A pool that reuses or changes sessions can alter ownership assumptions. Verify whether the same physical connection is retained for acquisition, work, and release.
Do not return a connection holding a session lock to a general pool. Another borrower can inherit unexpected state, while the original task may no longer control the session needed to release it. Use the pool’s documented cleanup behavior and an explicit ownership design.
Transaction pooling requires particular care with session-level mechanisms. Test through the deployed pool mode rather than a direct administrative connection. A design that works in a local script may not have the same session semantics in production.
Keep the work idempotent where possible
A lock prevents intended overlap among cooperating owners, but it does not make the operation exactly once. A process can fail after part of the work completes, and another owner can retry later. External systems may have their own uncertain outcomes.
Use checkpoints, idempotency, or reconciliation appropriate to the operation. If a worker sends a notification and crashes before recording completion, the next run needs a way to avoid or handle duplication. The database lock alone cannot undo the earlier message.
Keep durable progress separate from transient lock state. A lock disappearing after session termination indicates ownership ended; it does not prove the business operation completed successfully.
Observe locks and failure paths
Use supported lock and activity views to identify ownership and contention. Record the logical operation alongside nonsecret identifiers so an operator can connect a database key to the application task. A numeric lock without context is difficult to investigate.
PostgreSQL stores advisory and ordinary locks within shared lock-management resources. Avoid creating an unbounded number of simultaneous advisory locks. Review capacity and keep the locking design proportionate to the workload.
Test success, acquisition failure, transaction rollback, connection loss, and worker crashes in an approved environment. Verify what releases the lock and what durable state remains. For session locks, explicitly test a rollback to demonstrate that it does not provide the cleanup some developers expect.
A practical synchronization workflow
Suppose two workers can synchronize one tenant’s external catalog. They share a stable tenant-specific lock convention. A worker that acquires ownership performs a bounded, checkpointed synchronization; another records that the task is already owned and follows the approved rescheduling policy.
If the owner crashes after updating half the catalog, session termination releases its lock but does not erase partial business progress. The next run uses checkpoints and idempotent updates to continue safely. This is a stronger design than treating a lock as proof of completed work.
Frequently asked questions
Do advisory locks block code that never acquires them?
No. They are cooperative. Participating paths must follow the same protocol.
Does rollback release a session-level advisory lock?
Not merely because it rolls back the transaction. Session-level ownership has its documented independent lifetime.
Where can I check exact behavior?
Read the PostgreSQL advisory-lock documentation. For session assumptions in pooled applications, see our connection pooling guide.
