Race Condition

TL;DR

Explanation

Race conditions arise when multiple threads/processes access shared mutable state without synchronisation. Classic example: two threads both read a counter (value: 5), both increment locally, both write (both write 6 instead of 7). The TOCTOU (Time-Of-Check-To-Time-Of-Use) variant: check a condition then act on it, but state changes between check and act. In PHP: race conditions occur in file-based counters (read→increment→write without locks), cache stampede (multiple processes regenerating expired cache simultaneously), and database reads without transactions. Prevention: atomic operations, mutexes/locks, database transactions, optimistic locking.

Common Misconception

Why It Matters

Common Mistakes

• File-based counters without flock() — classic race condition.
• Cache aside pattern without stampede protection.
• Check-then-act on database records without SELECT FOR UPDATE.
• Assuming single-threaded PHP is race-condition-free.

Code Examples

// Race condition — read/increment/write without lock:
$count = (int)file_get_contents('counter.txt');
$count++;
file_put_contents('counter.txt', $count);
// Two simultaneous requests both read 5, both write 6

// File lock prevents race:
$fp = fopen('counter.txt', 'c+');
if (flock($fp, LOCK_EX)) {
    $count = (int)stream_get_contents($fp);
    $count++;
    ftruncate($fp, 0); rewind($fp);
    fwrite($fp, $count);
    flock($fp, LOCK_UN);
}
fclose($fp);

// Better: use atomic DB increment:
$pdo->exec('UPDATE counters SET value = value + 1 WHERE id = 1');

References

• ↗ https://en.wikipedia.org/wiki/Race_condition

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