Singleton Pattern: When to Use vs Alternatives

When Singleton Is Appropriate:

First, Single Logical Resource: When the domain has exactly one instance conceptually (one parking lot, one application configuration, one hardware device driver).

Second, Controlled Access: When you need to enforce constraints on instance creation (prevent multiple loggers writing to the same file, prevent multiple connection pools exhausting resources).

Third, Lazy Expensive Initialization: When the resource is costly to create (loading large configuration files, establishing network connections) and should be created once and reused.

Fourth, Global Coordination: When components across the application need to coordinate through shared state (cache manager, event bus).

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When Singleton Is the Wrong Choice:

First: Testing Challenges
Singletons introduce global state that persists across test cases. If ConfigurationManager.getInstance() loads settings during one test, those settings affect subsequent tests unless explicitly reset. Alternative: Use Dependency Injection (DI) to pass configuration objects as constructor parameters. Tests can provide mock configurations without affecting global state.

Second: Hidden Dependencies
When a class calls Logger.getInstance() internally, that dependency is invisible in the constructor signature. Code reviewers and maintainers cannot see what the class depends on. Alternative: Inject the logger through the constructor: OrderProcessor(logger: Logger). Dependencies become explicit and the class becomes easier to understand and test.

Third: Tight Coupling
Direct calls to Singleton.getInstance() couple your code to the concrete singleton class, making it hard to swap implementations. Alternative: Define an interface (e.g., ILogger) and inject implementations. The singleton can still exist but is accessed through abstraction.

Fourth: Scalability Constraints
In distributed systems or cloud environments, a singleton per process does not scale. Multiple application instances each have their own singleton, leading to inconsistent state. Alternative: Use external shared storage (Redis cache, centralized configuration service) or accept that each instance has its own local manager.

Alternatives to Singleton:

Dependency Injection (DI): Create one instance at application startup and inject it wherever needed. Frameworks like Spring manage instance lifecycle. The object behaves like a singleton (one instance) but without the static accessor method, improving testability.

Monostate Pattern: All instances share the same static state. Multiple objects exist but behave as one. Use when you need instance-level behavior but shared state. However, this is rare and often more confusing than helpful.

Factory Pattern: Factory controls instance creation and can enforce single instance internally without exposing static methods. Clients request instances from the factory, which returns the same object each time.

Service Locator Pattern: Central registry holds single instances of services. Clients query the registry by service type. This decouples clients from concrete singletons but introduces a new dependency on the locator.

Decision Framework:

Use Singleton when: The domain truly has one instance AND you need to prevent accidental creation AND the instance is accessed from many unrelated parts of the codebase.

Use Dependency Injection when: You want testability AND explicit dependencies AND flexibility to swap implementations.

Use neither when: Components can create their own instances locally without coordination.