What is the purpose of the Chain of Responsibility pattern?

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.

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

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.

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.

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