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. WebSphere Application Server does not support JDBC 3.0.
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, the WebSphere Application Server 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.
WebSphere Application Server 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:
- It cannot tolerate the overhead of obtaining and releasing connections
whenever a connection is used.
- It requires Java Transaction API (JTA) transactions within WebSphere Application
Server.
- It needs to share connections among multiple users within the same transaction.
- It needs to take advantage of product features for managing local transactions
within the appserver.
- It does not manage the pooling of its own connections.
- It does not manage the specifics of creating a connection, such as the database name, user name, or password
How connections are pooled together
Whenever you configure a unique data source or connection factory, you are required to give it a unique Java Naming and Directory Interface (JNDI) name. This JNDI name, along with its configuration information, is used 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:
- Each thread has its first database connection, and all are in use.
- Each thread is waiting for a second database connection, and none would become available since all threads are blocked.
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.
T * (C - 1) + 1The 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.
Purging connection pools
The product provides a new MBean operation (purgePoolContents) on the DataSource MBean that purges the connection pool. The pool purging behavior depends on the string parameter passed on purgePoolContents operation.
The immediate purge pool option causes the pool purge to behave in the following ways:
- No new transactions start on any connections obtained prior to the purgePoolContents()
call. Instead, a StaleConnectionException is thrown.
- No new handles are distributed on any connections obtained prior to the
purgePoolContents() call. Instead, a StaleConnectionException is thrown.
- Existing in-flight transactions continue work, and any new activities
on the purgedConnection cause a StaleConnectionException or an XAER_FAIL exception.
- The close() calls issued on any connections obtained prior to the purgePoolContents()
call are completed asynchronously with no wait time.
- Requests for new connections (not handles to existing connections) are
honored.
- Number of connections are decremented immediately. This action might cause the total number of connections in WebSphere Application Server to be, temporarily, out of sync with the database total number of connections.
The normal purge pool option causes the pool purge to behave in the following ways:
- Existing in-flight transactions continue work.
- Shared connection requests are honored.
- Free connections are cleaned up and destroyed.
- In use connection (connections in transactions) are cleaned up and destroyed
when returned to the connection pool.
- The close() calls issued on any connections obtained prior to the purgePool
call are completed synchronously with a wait for the JDBC driver to come back
before proceeding.
- Requests for new connections (not handles to existing connections) are honored.
The immediate purge pool option should only be used if the database is down. To use the Mbean operation purgePoolContents, use wsadmin commands. The purgePoolContents is a DataSource or J2CConnectionFactory Mbean type .
View the following wsadmin example, assuming that the data source name is mydatasource:
- %WAS_HOME\bin\wsadmin
- set ds [$AdminControl queryNames *,type=DataSource,name="mydatasource"]
- $AdminControl invoke $ds purgePool [normal | immediate]
Connection factory
Unshareable and shareable connections
Data sources
Configuring connection factories for resource adapters within applications
Configuring Java 2 Connector connection factories in the administrative console
Tuning performance parameter index