In which cases is it appropriate to apply this pattern?

The Chain of Responsibility pattern is appropriate when you need to organize flexible, configurable request handling, where several objects could potentially handle it, but it is not known in advance which one.

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#1. When You Need to Split Processing Stages

If a request goes through a series of checks or actions, and each one performs its own piece of work (validation, logging, authorization, data processing).

Example: handling an HTTP request: logging -> authorization check -> validation -> controller.

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#2. When Different Handlers for the Same Request Type Are Possible

If the handling logic depends on conditions, and you don't want to write bulky if/else or switch statements.

Example: different support levels in a helpdesk - agent -> manager -> director.

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#3. When It's Important to Weaken Coupling Between Sender and Handlers

The sender does not know exactly who will handle the request - this allows new handlers to be added easily, without touching existing code.

Example: an event system: a module sends an event, and listeners (handlers) decide whether to react or not.

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#4. When You Need to Allow Interrupting the Processing

If the chain's execution should stop as soon as a suitable handler is found.

Example: an authorization system: token check -> role check -> access granted -> chain interrupted.

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#5. When Configurable or Dynamic Processing Is Required

The chain can be changed at runtime - handlers can be added, removed, or reordered.

Example: middleware in web frameworks (Express.js, Spring, ASP.NET), where each layer decides what to do with the request next.

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#Conclusion

Use Chain of Responsibility when:

• a request can be handled by different objects;
• you need to get rid of rigid conditional constructs;
• it's important to flexibly manage the order and composition of handlers;
• you need the ability to interrupt the chain at the right stage.

Summary: The pattern is ideal for step-by-step, modular request handling, especially in validation, filtering, event handling, and middleware architecture.