Thread Safety

TL;DR

Explanation

Code is thread-safe if it behaves correctly when multiple threads execute it simultaneously. Strategies: (1) Immutability — objects that cannot be modified are inherently thread-safe. (2) Thread-local storage — each thread has its own copy of mutable state. (3) Atomic operations — hardware-level indivisible read-modify-write. (4) Synchronisation — mutexes/locks. (5) Stateless design — functions with no shared state are thread-safe. In PHP: the language runtime is not thread-safe by default (ZTS — Zend Thread Safety — is a compile option). PHP-FPM uses separate processes (not threads) so most PHP code is safe. Extensions may not be ZTS-compiled. Swoole and ReactPHP introduce real concurrency within one process.

Common Misconception

Why It Matters

Common Mistakes

• Mutable static/global state in classes — shared across requests in Swoole/FrankenPHP.
• Assuming global state is safe in PHP-FPM — each process has its own, but shared resources (DB, Redis) still need protection.
• Not marking PHP extensions as ZTS-safe when using threaded PHP.

Code Examples

// Mutable static — breaks in Swoole/FrankenPHP:
class RequestContext {
    private static ?User $currentUser = null;
    public static function setUser(User $u): void { self::$currentUser = $u; }
}

// Immutable request context — safe in all runtimes:
class RequestContext {
    public function __construct(
        public readonly User $user,
        public readonly string $requestId,
    ) {}
}
// Pass as dependency, not global static

References

• ↗ https://www.php.net/manual/en/pthreads.requirements.php

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