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Laravel Database Advisory Lock

Advisory Locking Features of Postgres/MySQL/MariaDB on Laravel

Requirements

Installing

Basic usage

[!IMPORTANT] The default implementation is provided by ConnectionServiceProvider, however, package discovery is not available. Be careful that you MUST register it in config/app.php by yourself.

Advanced Usage

[!TIP] You can extend Connection classes with AdvisoryLocks trait by yourself.

Implementation Details

Key Hashing Algorithm

• Postgres advisory locking functions only accept integer keys. So the driver converts key strings into 64-bit integers through hashtext() function.
  • An empty string can also be used as a key.
• MySQL advisory locking function accepts string keys but their length are limited within 64 chars. When key strings exceed 64 chars limit, the driver takes first 24 chars from them and appends 40 chars sha1() hashes.
  • MariaDB's limit is actually 192 bytes, unlike MySQL's 64 chars. However, the key hashing algorithm is equivalent.
  • MariaDB accepts an empty string as a key, but does not actually lock anything. MySQL, on the other hand, raises an error for empty string keys.
• With either hashing algorithm, collisions can theoretically occur with very low probability.

Locking Methods

• Session-Level locks can be acquired anywhere.
  • They can be released manually or automatically through a destructor.
  • For Postgres, there was a problem where the automatic lock release algorithm did not work properly, but this has been fixed in version 4.0.0. See #2 for details.
• Transaction-Level locks can be acquired within a transaction.
  • You do not need to and cannot manually release locks that have been acquired.

Timeout Values

• Postgres does not natively support waiting for a finite specific amount of time, but this is emulated by looping through a temporary function.
• MariaDB does not accept infinite timeouts. very large numbers can be used instead.
• Float precision is not supported on MySQL/MariaDB.

Caveats about Transaction Levels

Key Principle

Always avoid nested transactions when using advisory locks to ensure adherence to the S2PL (Strict 2-Phase Locking) principle.

Recommended Approach

When transactions and advisory locks are related, either locking approach can be applied.

[!NOTE] Transaction-Level Locks:
Acquire the lock at the transaction nesting level 1, then rely on automatic release mechanisms.

[!NOTE] Session-Level Locks:
Acquire the lock at the transaction nesting level 0, then proceed to call DB::transaction() call.

[!WARNING] When writing logic like this, DatabaseTruncation must be used instead of RefreshDatabase.

Considerations

[!CAUTION] Transaction-Level Locks:
Don't take transaction-level locks in nested transactions. They are unaware of Laravel's nested transaction emulation.

[!CAUTION] Session-Level Locks:
Don't take session-level locks in the transactions when the content to be committed by the transaction is related to the advisory locks.

What would happen if we released a session-level lock within a transaction? Let's verify this with a timeline chart, assuming a READ COMMITTED isolation level on Postgres. The bank account X is operated from two sessions A and B concurrently.

Session A	Session B
BEGIN
︙	BEGIN
pg_advisory_lock(X)	︙
︙	pg_advisory_lock(X)
Fetch balance of User X (Balance: 1000 USD)	︙
︙	︙
Deduct 800 USD if balance permits (Balance: 1000 USD → 200 USD)	︙
︙	︙
pg_advisory_unlock(X)	︙
︙	Fetch balance of User X (Balance: 1000 USD :heavy_exclamation_mark:)
︙	︙
︙	Deduct 800 USD if balance permits (Balance: 1000 USD → 200 USD :bangbang:)
COMMIT	︙
︙	pg_advisory_unlock(X)
Fetch balance of User X (Balance: 200 USD)	︙
	COMMIT
	︙
	Fetch balance of User X (Balance: -600 USD :interrobang::interrobang::interrobang:)