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Chain of Responsibilitybehavioral
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Observerbehavioral
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Singletoncreational
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Criteria	behavioralChain of ResponsibilityView pattern	behavioralObserverView pattern	creationalSingletonView pattern
Basics
Category	behavioral	behavioral	creational
Summary	Pass a request through a sequence of handlers until one handles it or the chain ends.	—	—
Intent	Pass requests along a chain of handlers until one of them handles the request or the request reaches the end of the chain.	Define a subscription mechanism so multiple observers can react to subject events without the subject knowing concrete subscriber details.	Ensure a class has only one instance and provide a global access point to it.
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 notify multiple dependent objects when state changes occur, but hardcoding those dependencies makes the design rigid and tightly coupled.	A system needs exactly one shared instance of a class, and that instance must be globally accessible without allowing uncontrolled duplication.
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.	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	Can make dependencies harder to test if the singleton is used everywhere May introduce hidden global state that complicates debugging and concurrency
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	Web Backend Event-driven systems	Web Backend Mobile UI kits Cross-platform 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.	Observers work well inside a single application boundary for UI events, domain events, and in-process notifications where subscribers change dynamically.	Singleton is useful for centralized services like configuration, logging, caching, and shared application state.