Singleton Pattern

What Is It?

The Singleton Pattern ensures a class has only one instance and provides a global point of access to it. In Laravel, the service container manages singletons.

Why?

Some resources should only exist once: database connections, configuration managers, cache connections. Creating multiple instances wastes resources and causes inconsistencies.

When to Use

For shared resources: database connections, configuration, cache, logging, session management. Laravel uses singletons for many built-in services.

When Not to Use

For classes that maintain per-request state. For classes that need to be tested in isolation (singletons are harder to mock). For classes that should have multiple instances.

Example

<?php
// Service Provider singleton registration
public function register(): void
{
    $this->app->singleton(CacheManager::class, function ($app) {
        return new CacheManager($app);
    });
    
    // Always returns the same instance
    $cache1 = app(CacheManager::class);
    $cache2 = app(CacheManager::class);
    // $cache1 === $cache2 is true
}

// Custom singleton
class DatabaseConnection
{
    private static ?self $instance = null;
    
    private function __construct(private array $config) {}
    
    public static function getInstance(array $config = []): self
    {
        if (self::$instance === null) {
            self::$instance = new self($config);
        }
        return self::$instance;
    }
}

// Laravel\'s built-in singletons
// Config, Cache, Event, Log, Queue are all singletons
\Illuminate\Support\Facades\Cache::singleton('cache.store', function ($app) {
    return $app['cache']->store();
});

Pros

Ensures single instance
Global access point
Lazy initialization
Reduces memory usage
Laravel container manages lifecycle

Cons

Harder to test (can't inject mock easily)
Tight coupling to global state
Violates Single Responsibility Principle
Can mask design problems
Difficult to extend or subclass