singleInstance

SingleInstance is a term used in software development to describe a constraint or mechanism that permits only a single active instance of a component, application, or service to exist at runtime. It can refer to two related but distinct usages: (1) a design pattern that enforces a single instance of a class within a process, and (2) run-time enforcement that prevents multiple process instances of an application or component from running concurrently.

In the design context, the singleton pattern ensures a class has exactly one instance and provides a global access point. Typical implementations use a private constructor, a private static field to hold the instance, and a public static method or property to return it. Thread-safety and lazy initialization are common concerns.

In the run-time context, single-instance behavior is implemented with operating-system primitives (such as a named mutex or lock file) or by framework features that detect an existing instance and forward new launches to it. This prevents double execution and can allow the existing instance to handle new work requests, often via inter-process communication.

Advantages include predictable resource usage, a consistent state, and simplified coordination for shared resources. Drawbacks include the risks of global state, testability challenges, and potential bottlenecks or deadlocks in highly concurrent environments. Alternatives include allowing multiple independent instances coordinated via IPC or using a stateless service that can be scaled horizontally.