Connection pooling:

When accessing any database, the initial database connection is an expensive operation. Connection pooling enables administrators to establish a pool of database connections that applications can share on an appserver. When connection pooling capabilities are used, performance improvements up to 20 times the normal results are realized.

Each time a resource attempts to access a backend store (such as a database), the resource must connect to that data store. A connection requires resources to create, maintain, and then release the connection when it is no longer required.

The total data store overhead for an application is particularly high for Web-based applications because Web users connect and disconnect more frequently. In addition, user interactions are typically shorter. Often, more effort is spent connecting and disconnecting than is spent during the interactions. Also, because Internet requests can arrive from virtually anywhere, you can find usage volumes large and difficult to predict.

To help lessen these overhead problems, WAS enables administrators to establish a pool of backend connections that applications can share on an appserver. Connection pooling spreads the connection overhead across several user requests, thereby conserving resources for future requests.

WAS supports JDBC 2.0 Standard Extension APIs to provide support for connection pooling and connection reuse. The connection pool is used to direct JDBC calls within the application, as well as for enterprise beans using the database.

Each entity EJB transaction requires an additional connection to the database specifically to handle the transaction. Take this into account when calculating the number of data source connections.

On UNIX platforms, a separate DB2 process is created for each connection and these processes quickly affect performance on systems with low memory and cause errors.

If clones are used, one data pool exists for each clone. This is important when configuring the database maximum connections.

Benefits of connection pooling

Connection pooling can improve the response time of any application that requires connections, especially Web-based applications. When a user makes a request over the Web to a resource, the resource accesses a data source. With connection pooling, most user requests do not incur the overhead of creating a new connection because the data source can locate and use an existing connection from the pool of connections. When the request is satisfied and the response is returned to the user, the resource returns the connection to the connection pool for reuse. The overhead of a disconnect is avoided. Each user request incurs a fraction of the cost for connecting or disconnecting. After the initial resources are used to produce the connections in the pool, additional overhead is insignificant because the existing connections are reused.

When to use connection pooling

Use WebSphere connection pooling in an application that meets any of the following criteria:

How connections are pooled together

Whenever you configure a unique data source or connection factory you are required to give it a unique JNDI name. Use this name, along with its configuration information, to create a connection pool. A separate connection pool exists for each configured data source or connection factory.

A separate instance of a given configured connection pool is created on each appserver that uses that data source or connection factory. For example, if you run a three server cluster in which all of the servers use myDataSource, and myDataSource has a maximum connections setting of 10, then you can generate up to 30 connections (three servers times 10 connections). Be sure to consider this fact when determining how many connections to your backend resource you can support.

It is also important to note that when using connection sharing, it is only possible to share connections obtained from the same connection pool.

Avoiding a deadlock

Deadlock can occur if the application requires more than one concurrent connection per thread, and the database connection pool is not large enough for the number of threads. Suppose each of the application threads requires two concurrent database connections and the number of threads is equal to the maximum connection pool size. Deadlock can occur when both of the following are true:

To prevent the deadlock in this case, the value set for the database connection pool must be at least one higher, one of the waiting threads to complete its second database connection and free up to allow database connections.

To avoid deadlock, code the application to use, at most, one connection per thread. If the application is coded to require C concurrent database connections per thread, the connection pool must support at least the following number of connections, where T is the maximum number of threads.

The connection pool settings are directly related to the number of connections that the database server is configured to support. If the maximum number of connections in the pool is raised, and the corresponding settings in the database are not raised, the application fails and SQL exception errors are displayed in the stderr.log file.

WebSphere is a trademark of the IBM Corporation in the United States, other countries, or both.

IBM is a trademark of the IBM Corporation in the United States, other countries, or both.