What is the purpose of the Chain of Responsibility pattern?

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The Chain of Responsibility pattern is a behavioral design pattern that allows you to create a chain of loosely coupled objects that can handle a request. Each object in the chain can either process the request or pass it to the next object. This way, you can avoid coupling the sender of the request to a specific receiver, and increase the flexibility and maintainability of your code.

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Decoupling request and receiver:

The Chain of Responsibility pattern lets you pass a request along a chain of handlers until one is found that can deal with it. It’s like a game of hot potato where each player has a unique skill to contribute.
Sequential processing:

By using the Chain of Responsibility, you can set up a sequence where each link in the chain checks one aspect of a request. It's like an assembly line where each worker verifies one part before passing it on.

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Rizwan R.

Founder, Software Engineer - Solsvc
Arthur H.

1 Benefits of the pattern

The Chain of Responsibility pattern has several benefits for software development. First, it promotes the principle of single responsibility, as each object in the chain has only one role to play in handling the request. Second, it reduces the complexity and dependencies of the code, as you can easily add, remove, or reorder the objects in the chain without affecting the rest of the system. Third, it allows you to vary the handling logic dynamically at runtime, as you can modify the chain according to the context or the type of the request.

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Mojtaba Tavakoli

C#| .Net | .Net Core | Microservices| Azure | SQL |Angular| Agile
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In this pattern, each object in the chain represents a potential handler of the request. When a request is made, it is passed through the chain until it reaches an object that can handle it. This object then performs the necessary operations and decides whether to pass the request further down the chain or stop the propagation. This gives the flexibility to add, remove, or rearrange the handlers without affecting the client code. The Chain of Responsibility pattern is particularly useful in scenarios where the system needs to handle a request in multiple ways, and the specific handler may vary depending on the context or runtime conditions. It promotes the "don't ask, just do" principle.

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2 Example of the pattern

A common example of the Chain of Responsibility pattern is the middleware in web applications. Middleware are functions that execute before the main handler of a web request, and can perform tasks such as authentication, validation, logging, or error handling. Each middleware can either pass the request to the next middleware in the chain, or terminate the chain and send a response. For example, in Express.js, a popular web framework for Node.js, you can use the app.use method to register middleware functions in a chain.

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Arthur H.
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This pattern can be helpful, for example, in business scenarios where multiple validation or processing steps are required. Consider an online school that processes course enrollments. Each enrollment request must undergo several checks: prerequisites validation, payment status, seat availability, etc. In this model, we can set up a chain where each handler is responsible for one specific type of validation. The prerequisites validator would either approve the enrollment for the next step or reject it. If approved, it's moved to the payment status checker, and so on. This helps keep the code cohesive and easier to maintain.

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Himanshu Singh

Staff Software Engineer at TomTom
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An example of the Chain of Responsibility Design in Java Language is "Exception Handling". Each Catch block is a handler and there is a sequence of handling the exceptions because the catch block for an Exception Class can't be written before a SuperType Exception. Sometimes each handler is used to process the input and sometimes only a few of them are used, but there will be a sequence in which these handlers will be invoked. In case of Exception handling at max one catch block will be used for exception handling based on the actual exception raised by the try block. There can be a default handler to process every input. Like in the case of Java Exception Handling you can write a finally block to be invoked always at the end. I

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3 Implementation of the pattern

To implement the Chain of Responsibility pattern, you need to define an interface or an abstract class that represents the handler object. The handler object should have a method to handle the request, and a property to store a reference to the next handler in the chain. The handler object should also have a method to set the next handler in the chain. Then, you need to create concrete subclasses or implementations of the handler object that provide the specific handling logic for different types of requests. Finally, you need to create and link the handler objects in a chain.

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Rizwan R.

Founder, Software Engineer - Solsvc
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The Chain of Responsibility pattern is like assembling a team of specialists for problem-solving. Each expert (handler) can delegate the task (request) to the next, ensuring a flexible and maintainable approach to tackling complex issues. It's akin to having a group of doctors with different specialties, where a patient is directed to the right specialist for treatment based on their condition.

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4 Use cases of the pattern

The Chain of Responsibility Pattern is a beneficial tool when multiple objects can handle a request, but it is not known in advance which one should do so. Additionally, it is useful for decoupling the sender and the receiver of the request and avoiding hard-coding the handling logic in one place. Its common use cases include logging, validation, and authorization. For example, when logging, you can create a chain of loggers that write messages to different destinations based on the level or category of the message. In validation, you can create a chain of validators that check input data for different rules such as format, length, or uniqueness. Lastly, when authorizing, you can create a chain of authorizers that check user permissions for different actions like reading, writing, or deleting data.

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Himanshu Singh

Staff Software Engineer at TomTom
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A real-life example of this Design Pattern is the boarding process at the Airports in India. - At the entry gate the security person will check your travel ticket, Government ID, And Passport, and Visa for foreign travel. - After entry, you go for baggage check-in, where your luggage will be checked for weight as per the policy. - Then you go for a security check for your carry-on bag. - Next is a security check for individuals and there you will be sent to different queues based on your gender. - During boarding the crew member scans your ticket for counting and to ensure that you are boarding to correct flight. each security person, crew member -> Handler job done by each person -> process traveler and luggage -> input

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What is the purpose of the Chain of Responsibility pattern?

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1 Benefits of the pattern

2 Example of the pattern

3 Implementation of the pattern

4 Use cases of the pattern

5 Here’s what else to consider

Software Development

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What is the purpose of the Chain of Responsibility pattern?

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1 Benefits of the pattern

2 Example of the pattern

3 Implementation of the pattern

4 Use cases of the pattern

5 Here’s what else to consider

Software Development

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