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Principles

Separation of Concerns: Untangle the Request Handler

Separation of concerns means each job the code does, like reading a request, applying a business rule or keeping data, lives in its own place, so a change to one does not touch the others. A request handler that does all three forces every new way in to copy the rule. These lessons take a car rental desk whose web handler prices and saves bookings, move the rule and the storage into their own classes, and then add a partner file import that reuses them.

Pick your language

Separation of concerns: untangle the request handler in Java, step 2: A nightly partner fileJava · ~6 minSeparation of Concerns in JavaSeparation of concerns: untangle the request handler in C#, step 2: A nightly partner fileC# · ~6 minSeparation of Concerns in C#Separation of concerns: untangle the request handler in Python, step 2: A nightly partner filePython · ~6 minSeparation of Concerns in PythonSeparation of concerns: untangle the request handler in PHP, step 2: A nightly partner filePHP · ~6 minSeparation of Concerns in PHPSeparation of concerns: untangle the request handler in TypeScript, step 2: A nightly partner fileTypeScript · ~6 minSeparation of Concerns in TypeScript

When to use it

Use it when

The same rule is needed from more than one way in, like a web request, a file import or a scheduled job.

Skip it when

A small script has one way in and nobody else calls it. Splitting it adds classes to read and nothing to reuse.

Spot it by

A request handler that parses input, holds a business rule and writes storage in one method.