Singleton Pattern

Kind: pattern

Layer: Design Patterns Core

Aliases: Singleton

Record: architecture:singleton-pattern

Severity: discouraged

Scope: object_creation, lifetime, composition root

Canonical: Ontology

A design pattern that restricts a class to one instance, reached through a global access point.

Listed in Architecture principles, after Prototype Pattern and before Adapter Pattern.

Requires

• Single-Instance Need

Reinforces

• Controlled Instantiation

Enables

• Shared Resource Access

Conflicts with

• Global Mutable State

In tension with

• Testability
• Dependency Injection

Tensions

• Singleton Pattern / Testability
• Singleton Pattern / Dependency Injection

Violated by

• Global Mutable State

Refactored by

• Composition Root
• Dependency Injection

Severity

• discouraged

Category

• Creational Patterns

Force

• Object creation

Linked from

• Correctness / Determinism / Verification
• Plugin / Extensibility / IoC
• Core Vocabulary
• Creational Patterns
• Severity levels
• The forces
• The resolutions