Observer Pattern

Kind: pattern

Layer: Design Patterns Core

Record: architecture:observer-pattern

Severity: recommended

Scope: event notification, runtime

Canonical: Ontology

A design pattern in which a subject publishes events to subscribers it does not know by name.

Listed in Architecture principles, after Template Method Pattern and before Mediator Pattern.

Requires

• Subject/Subscriber Contract

Reinforces

• Event-Driven Architecture

Enables

• Decoupled Notification

Conflicts with

• Direct Callback Coupling

In tension with

• Ordering
• Debuggability

Tensions

• Observer Pattern / Ordering
• Observer Pattern / Debuggability

Violated by

• Direct Callback Coupling

Severity

• recommended

Category

• Behavioral Patterns

Linked from

• Execution joins the halves
• Event / Messaging / Asynchronous Architecture
• Behavioral Patterns
• Quality Attributes
• Severity levels
• The resolutions

in this codebase

• emitEvent
• subscribeEvent
• core/buses