Mutex & Locking

TL;DR

Explanation

Mutex: binary lock — locked or unlocked. Only the holder can unlock it. Critical section: code protected by the mutex. In PHP: flock() for file-based mutex, Redis SET NX EX for distributed mutex, database advisory locks (GET_LOCK in MySQL). Key properties: atomicity of lock/unlock, single-owner, blocking (waits) vs non-blocking (returns false if unavailable). Distributed systems need distributed locks (Redis Redlock, ZooKeeper, DB advisory locks) since file locks only work within one server. Deadlock risk: never hold mutex A while acquiring mutex B (or maintain consistent order). Release in finally to ensure unlock even on exception.

Common Misconception

Why It Matters

Common Mistakes

• Not releasing mutex in a finally block — lock held forever on exception.
• Using flock() as a distributed lock across multiple servers — doesn't work.
• Holding a mutex too long — reduces concurrency, increases contention.

Code Examples

$fp = fopen('lock.txt', 'c');
flock($fp, LOCK_EX);
doWork(); // Exception here leaves lock held forever!
flock($fp, LOCK_UN);
fclose($fp);

// Always release in finally:
$fp = fopen('lock.txt', 'c');
try {
    flock($fp, LOCK_EX);
    doWork();
} finally {
    flock($fp, LOCK_UN);
    fclose($fp);
}

// Distributed mutex via Redis:
$acquired = $redis->set('lock:job', 1, ['NX', 'EX' => 30]);
if (!$acquired) return; // Another process holds it
try { doWork(); } finally { $redis->del('lock:job'); }

References

• ↗ https://redis.io/docs/latest/develop/use/patterns/distributed-locks/

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