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Chain of Responsibilitybehavioral
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Decoratorstructural
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Observerbehavioral
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Criteria	behavioralChain of ResponsibilityView pattern	structuralDecoratorView pattern	behavioralObserverView pattern
Basics
Category	behavioral	structural	behavioral
Summary	Pass a request through a sequence of handlers until one handles it or the chain ends.	Wrap an object with other objects that implement the same contract so responsibilities can be layered dynamically.	—
Intent	Pass requests along a chain of handlers until one of them handles the request or the request reaches the end of the chain.	Wrap an object with other objects that implement the same contract so responsibilities can be layered dynamically.	Define a subscription mechanism so multiple observers can react to subject events without the subject knowing concrete subscriber details.
Decision factors
Problem	A request must pass through multiple possible handlers, but the sender should not need to know which handler will process it.	A system needs to add optional behaviors to an object without creating many subclasses for every feature combination.	A system needs to notify multiple dependent objects when state changes occur, but hardcoding those dependencies makes the design rigid and tightly coupled.
Tradeoffs	Requests can become harder to trace because control flows through multiple handlers. A long chain may add overhead if many handlers are checked on every request. It keeps handler logic isolated and easier to extend.	Can introduce many small wrapper classes that are harder to trace in debugging. Ordering of decorators matters, so composition needs to be deliberate. Keeps responsibilities modular and easy to combine.	Notification flows can become hard to trace when many observers react to the same event Observers may receive updates they do not need unless subscriptions are carefully designed
Practical fit
Languages	typescript java python angular react react-native csharp dotnet	typescript java python angular react react-native csharp dotnet	typescript java python angular react react-native csharp dotnet
Platforms	frontend backend integration	frontend backend services	Web Backend Event-driven systems
Integration notes	Chain of Responsibility is useful for validation pipelines, support routing, approval flows, and any system where requests may be processed by different handlers in sequence.	Decorators work well for cross-cutting concerns such as logging, retry, caching, compression, and telemetry because each layer keeps the same interface while adding one concern.	Observers work well inside a single application boundary for UI events, domain events, and in-process notifications where subscribers change dynamically.