- sometimes take a relatively long time to create.
- are always limited in number (most medium- and low-end servers have a maximum of ~100 connections).
- are resources which must be systematically released when no longer in use.
To ensure that database connections are used as efficiently as possible, many applications make use of a connection pool, which can sometimes improve performance in a distributed application. An application will use a connection pool as follows:
- upon startup, the application creates a pool of database connections (or, the application's environment may already include a connection pool).
- during normal operation, the application simply fetches a connection from the pool every time a database operation is performed. This is often done in a "just-in-time" manner. That is, the application does not keep a long-lived reference to its connection objects, but will rather fetch, use, and then close each connection in a single method.
- when the calling code is finished with the connection, it calls a close method, but the underlying connection is not actually closed - rather, it's simply returned to the connection pool.
- when the application shuts down, the connection pool it created upon startup (if any) is shut down as well.
The popular Tomcat tool
implements JNDI, and connection pooling is described in its documentation.
It's not difficult to use.