Advisory locks in PostgreSQL

Introduction

Advisory locks let you control access to resources in your database without blocking normal queries. They help avoid conflicts when multiple tasks try to use the same resource.

When you want to prevent two processes from running the same job at the same time.

When you need to coordinate access to a shared resource like a file or cache outside the database.

When you want to lock something without locking database rows or tables.

When you want a lightweight way to signal that a resource is in use.

When you want to avoid deadlocks by managing locks manually.

Syntax

PostgreSQL

SELECT pg_advisory_lock(key);
SELECT pg_advisory_unlock(key);
SELECT pg_try_advisory_lock(key);

The key is usually a big integer that identifies the resource.

pg_advisory_lock waits until the lock is free, pg_try_advisory_lock returns immediately with success or failure.

Examples

This locks the resource identified by 12345. If another session holds it, this waits.

PostgreSQL

SELECT pg_advisory_lock(12345);

This tries to lock resource 12345 but returns immediately with true or false.

PostgreSQL

SELECT pg_try_advisory_lock(12345);

This releases the lock on resource 12345.

PostgreSQL

SELECT pg_advisory_unlock(12345);

Sample Program

This example tries to get an advisory lock on resource 9999. If successful, you can safely do your work. Then it releases the lock.

PostgreSQL

BEGIN;
SELECT pg_try_advisory_lock(9999) AS got_lock;
-- Do work only if got_lock is true
SELECT pg_advisory_unlock(9999);
COMMIT;

Important Notes

Advisory locks are session-based and released automatically if the session ends.

Use the same key in all sessions that need to coordinate on the same resource.

Advisory locks do not block normal database operations like row locks do.

Summary

Advisory locks let you control access to resources using simple integer keys.

You can wait for a lock or try to get it immediately.

Remember to release locks when done to avoid blocking others.

Practice

1. What is the main purpose of advisory locks in PostgreSQL?

easy

A. To control access to resources using user-defined keys

B. To automatically manage table-level locks during transactions

C. To speed up query execution by caching results

D. To backup the database safely without downtime

Solution

1. Step 1: Understand advisory locks concept

   Advisory locks allow applications to coordinate access to resources by using custom keys, not automatic locks on tables or rows.

2. Step 2: Compare options

   The other options describe other database features unrelated to advisory locks.

3. Final Answer:

   To control access to resources using user-defined keys -> Option A

4. Quick Check:

   Advisory locks = user-defined resource control [OK]

Hint: Advisory locks use keys to manage resource access [OK]

Common Mistakes:

• Confusing advisory locks with automatic table locks
• Thinking advisory locks speed up queries
• Assuming advisory locks handle backups

2. Which of the following is the correct syntax to acquire a session-level advisory lock with key 12345?

easy

A. SELECT pg_advisory_lock(12345);

B. LOCK TABLE pg_advisory_lock(12345);

C. SELECT acquire_lock(12345);

D. BEGIN LOCK 12345;

Solution

1. Step 1: Recall advisory lock syntax

   PostgreSQL uses the function pg_advisory_lock(key) to acquire a session-level advisory lock.

2. Step 2: Evaluate options

   SELECT pg_advisory_lock(12345); is the correct function call. The other options use invalid syntax or non-existent functions.

3. Final Answer:

   SELECT pg_advisory_lock(12345); -> Option A

4. Quick Check:

   pg_advisory_lock(key) = correct syntax [OK]

Hint: Use SELECT pg_advisory_lock(key) to lock [OK]

Common Mistakes:

• Using LOCK TABLE instead of function call
• Calling non-existent functions like acquire_lock
• Trying to lock with BEGIN LOCK syntax

3. What will be the result of this query if the advisory lock with key 999 is already held by another session?

SELECT pg_try_advisory_lock(999);

medium

A. true

B. false

C. null

D. error

Solution

1. Step 1: Understand pg_try_advisory_lock behavior

   This function tries to acquire the lock immediately and returns true if successful, false if the lock is held by someone else.

2. Step 2: Analyze the scenario

   Since the lock with key 999 is already held, the function returns false without waiting.

3. Final Answer:

   false -> Option B

4. Quick Check:

   pg_try_advisory_lock returns false if lock busy [OK]

Hint: pg_try_advisory_lock returns false if lock busy [OK]

Common Mistakes:

• Expecting true even if lock is held
• Thinking it returns null or error
• Confusing pg_try_advisory_lock with pg_advisory_lock

4. You wrote this code:

SELECT pg_advisory_unlock(123);

But the lock was never acquired before. What will happen?

medium

A. The function returns true and releases the lock

B. The function blocks until the lock is acquired

C. The function throws an error

D. The function returns false because no lock was held

Solution

1. Step 1: Understand pg_advisory_unlock behavior

   This function releases a lock if held and returns true; if no lock was held, it returns false.

2. Step 2: Analyze the scenario

   Since the lock was never acquired, the function returns false without error or blocking.

3. Final Answer:

   The function returns false because no lock was held -> Option D

4. Quick Check:

   Unlock returns false if lock not held [OK]

Hint: Unlock returns false if no lock held [OK]

Common Mistakes:

• Expecting an error when unlocking unheld lock
• Thinking unlock blocks or waits
• Assuming unlock always returns true

5. You want to ensure two different sessions do not run a critical section simultaneously using advisory locks. Which approach is best?

-- Session 1 and 2 run this code:
SELECT pg_try_advisory_lock(42);
-- If true, run critical section, then
SELECT pg_advisory_unlock(42);

hard

A. Use pg_advisory_unlock before acquiring lock to clear old locks

B. Use pg_try_advisory_lock to attempt lock and skip if busy

C. Use pg_advisory_lock to wait until lock is available before running

D. Use random keys each time to avoid conflicts

Solution

1. Step 1: Understand locking strategies

   pg_try_advisory_lock returns immediately and may skip critical section if lock busy; pg_advisory_lock waits until lock is free.

2. Step 2: Choose best approach for critical section

   To ensure only one session runs critical section at a time, waiting for the lock is safer than skipping it.

3. Step 3: Evaluate other options

   Unlocking before acquiring is unsafe and random keys defeat locking purpose.

4. Final Answer:

   Use pg_advisory_lock to wait until lock is available before running -> Option C

5. Quick Check:

   Waiting lock ensures exclusive access [OK]

Hint: Use pg_advisory_lock to wait for exclusive access [OK]

Common Mistakes:

• Using try lock and skipping critical section silently
• Unlocking before locking without owning lock
• Using random keys causing no real locking