Intent: Used when an object is to be instantiated many times to the point that it causes significant overhead

Structure

Object Pooling. Image taken from GeeksforGeeks

Applicability

You want to allocate and deallocate many object instances.
When you want to have a limited number of said objects at a time (i.e., an extended version of a Singleton).

It reduces the overhead for allocating the objects since they are simply fetched from the pool when needed and returned to the pool when done.
The pooled object is obtained in a predictable time.
In some cases, the memory overhead of storing pre-allocated resources may be quite high.
Garbage Collectors may render them redundant or even an anti-pattern.

In the case when the pool is empty we have three options
- Throw an error (useful when only a bounded amount of the resource may exist)
- The pool can simply create a new instance of that object.
- The pool may block other clients (in a Multiprogramming context) until a thread frees its object.
The client must request an object from the pool via its factory method.
When the object is returned to the pool, it must be reset to an original state in some way.