On backend side, we sometimes need asynchronous tasks to be completed by the server. Those tasks generally don't return a response at that moment. They are also used for periodic jobs.

Example: We need to save some different sizes photos that user has uploaded. We can just receive original ...

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In Delegation pattern, we delegate an operation to a class instead of a method. Instead of we do operation inside a class, we just instantiate another class and set our property to that instance of the class we created.

Wikipedia: In software engineering, the delegation pattern is a design pattern ...

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Sometimes we need one global object and use it in our application. Creating multiple instances for this object may create errors. especially errors about atomicity, integrity. So on those cases, we use Singleton pattern. We create the object once, initialize it than use it.

Wikipedia: In software engineering, the singleton ...

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Sometimes you need to watch for a variable for changes and take actions according to value. Observer pattern is often used for those cases. It also helps us to separate responsibilities between modules.

Wikipedia: The observer pattern is a software design pattern in which an object, called the subject, maintains ...

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