Object pool pattern

The object pool pattern is a software creational design pattern that uses a set of initialized objects kept ready to use – a "pool" – rather than allocating and destroying them on demand. A client of the pool will request an object from the pool and perform operations on the returned object. When the client has finished, it returns the object to the pool rather than destroying it; this can be done manually or automatically.

Object pools are primarily used for performance: in some circumstances, object pools significantly improve performance. Object pools complicate object lifetime, as objects obtained from and returned to a pool are not actually created or destroyed at this time, and thus require care in implementation.

When it is necessary to work with a large number of objects that are particularly expensive to instantiate and each object is only needed for a short period of time, the performance of an entire application may be adversely affected. An object pool design pattern may be deemed desirable in cases such as these.

The object pool design pattern creates a set of objects that may be reused. When a new object is needed, it is requested from the pool. If a previously prepared object is available it is returned immediately, avoiding the instantiation cost. If no objects are present in the pool, a new item is created and returned. When the object has been used and is no longer needed, it is returned to the pool, allowing it to be used again in the future without repeating the computationally expensive instantiation process. It is important to note that once an object has been used and returned, existing references will become invalid.

In some object pools the resources are limited so a maximum number of objects is specified. If this number is reached and a new item is requested, an exception may be thrown, or the thread will be blocked until an object is released back into the pool.

The object pool design pattern is used in several places in the standard classes of the .NET framework. One example is the .NET Framework Data Provider for SQL Server. As SQL Server database connections can be slow to create, a pool of connections is maintained. When you close a connection it does not actually relinquish the link to SQL Server. Instead, the connection is held in a pool from which it can be retrieved when requesting a new connection. This substantially increases the speed of making connections.

Object pooling can offer a significant performance boost in situations where the cost of initializing a class instance is high and the rate of instantiation and destruction of a class is high – in this case objects can frequently be reused, and each reuse saves a significant amount of time. Object pooling requires resources – memory and possibly other resources, such as network sockets, and thus it is preferable that the number of instances in use at any one time is low, but this is not required.

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