Tuning connection pools

Using connection pools helps to both alleviate excessive connection management and decrease development tasks for data access. Each time an application attempts to access a backend store (such as a database), it requires resources to create, maintain, and release a connection to that data store. Tuning connection pools can mitigate the strain on overall application resources. Administrators can establish a pool of backend connections that applications can share on an application server. Connection pooling spreads the connection processing across several user requests and conserves application resources for future requests.

• Prevent a connection 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 conditions 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 prevent the deadlock in this case, increase the maximum connections value for the database connection pool by at least one. This ensures that at least one of the waiting threads obtains a second database connection and avoids a deadlock scenario.

  For general prevention of connection deadlock, code your applications to use only one connection per thread. If you code the application 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) + 1

  The connection pool settings are directly related to the number of connections that the database server is configured to support. If you increase the maximum number of connections in the pool and the corresponding settings in the database are not increased accordingly, the application might fail. The resulting SQL exception errors are displayed in the following locations:

  • the stderr.log file
  A common cause of connection deadlock occurs when the same connection pool is used by both servlets and by Enterprise JavaBeans (EJBs). The deadlock occurs when the servlet directly or indirectly invokes the bean. For example, when a servlet obtains a JMS connection from the connection pool and sends a message to a message-driven bean (MDB) and waits for a reply. The MDB is configured to use the same connection pool as the servlet. Therefore, another connection from the pool is needed for the MDB to send a reply to the servlet. Servlets and enterprise beans do not use the same connection pool. This illustration is a classic case of concurrent (C) threads, where

  C=2

  and T is the maximum size of the servlet and EJB thread pools.
• Disable connection pooling.
  • For relational resource adapters (RRAs), add the disableWASConnectionPooling custom property for your data sources.
    1. Click JDBC > Data sources.
    2. Click on the name of the data source that you want to configure.
    3. Click Custom properties under the Additional Properties heading.
    4. Click New.
    5. Complete the required fields with the following information:
       • Name: disableWASConnectionPooling
       • Value: true
  • For other resource adapters, consult with the binding specifications for that resource adapter to configure your applications to disable connection pooling.
    1. Programmatically disable connection pooling through the resource adapter.
    2. The application server leverages the following code to detect the javax.resource.NotSupportedException exception and disable connection pooling:

       _managedFactory.matchManagedConnections(s,subject,cri); // 169059 174269     } 
        catch(javax.resource.NotSupportedException e){

• Enable deferred enlistment.

  In the application server environment, deferred enlistment refers to the technique in which the application server waits until the connection is used before the connection is enlisted in the application's unit of work (UOW) scope.

  Consider the following illustration of deferred enlistment:

  • An application component that uses deferred enlistment calls the getConnection method from within a global transaction.
  • The application component does not immediately use the connection.
  • When the application issues the call for initial use of the connection, the transaction manager intercepts the call.
  • The transaction manager enlists the XA resource for the connection and calls the XAResource.start method.
  • The connection manager associated with the XA resource sends the call to the database.

  Given the same scenario, but the application component does not use deferred enlistment, the component container immediately enlists the connection in the transaction. Thus the application server incurs, for no purpose, an additional load of all of the overhead associated with that transaction. For XA connections, this overhead includes the two phase commit (2PC) protocol to the resource manager.

  Deferred enlistment offers better performance in the case where a connection is obtained, but not used, within the UOW scope. The technique saves the cost of transaction participation until the UOW in which participation must occur.

  Check with your resource adapter provider if you need to know if the resource adapter provides this functionality. The application server relational resource adapter automatically supports deferred enlistment.

  Incorporating deferred enlistment in your code:

  The Java™ Platform, Enterprise Edition (Java EE) Connector Architecture (JCA) Version 1.5 and later specification calls the deferred enlistment technique lazy transaction enlistment optimization. This support comes through a marker interface (LazyEnlistableManagedConnection) and a method on the connection manager (LazyEnlistableConnectionManager()):

  package javax.resource.spi; import javax.resource.ResourceException; import 
   javax.transaction.xa.Xid;  interface LazyEnlistableConnectionManager { // application server     void 
   lazyEnlist(ManagedConnection) throws ResourceException; }  interface LazyEnlistableManagedConnection 
  { // resource adapter } 
   

• Control connection pool sharing.
  You can use the defaultConnectionTypeOverride or globaldefaultConnectionTypeOverride connection pool custom property for a particular connection factory or data source to control connection sharing:

  • The defaultConnectionTypeOverride property changes the default sharing value for a connection pool. This property enables you to control connection sharing for direct queries. If resource references are configured for this data source or connection factory the resource reference's configurations take precedence over the defaultConnectionTypeOverride property settings. For example, if an application is doing direct queries and unshared connections are needed, set thedefaultConnectionTypeOverride property to unshared.
  • The value specified for the globaldefaultConnectionTypeOverride custom property takes precedence over all of the other connection sharing settings. For example if you set this property to unshared, all connection requests are unshared for both direct queries and resource reference lookups. This property provides you with a quick way to test the consequences of moving all connections for a particular data source or connection factory to unshared or shared without changing any resource reference setting.

  Important: If you specify values for both the defaultConnectionTypeOverride and the globaldefaultConnectionTypeOverride properties, only the values specified for the globaldefaultConnectionTypeOverride property are used to determine connection sharing type.

  To add these new custom properties to the settings for a data source or connection factory connection pool, a new connection pool custom property must be created.

  • To add one of these properties to a data source, use the administrative console. Click Resources > JDBC > Data sources. Select your data source from the list. Then under Additional Properties, click Connection pool properties > Connection pool custom properties > New. Now specify defaultConnectionTypeOverride or globalConnectionTypeOverride in the Name field and shared or unshared in the Value field.
  • For other J2C or JMS connection factories, navigate to the connection factory definition in the administrative console. Then under Additional Properties, click Connection pool > Connection pool custom properties > New. Now specify defaultConnectionTypeOverride or globalConnectionTypeOverride in the Name field and shared or unshared in the Value field.

  Important: The properties must be set in the Connection pool custom properties and NOT the general Custom properties on the data source or connection factory.
• Discard connections.

  Reap time and unused timeout settings do not cause the idle or unused connections to be discarded if the servant region is idle. This situation might cause some DB2® connections to be held longer than is necessary.

  You can add the nondeferredreaper custom property to your JDBC driver provider data source settings. This addition has the connections discarded at the time that is specified by a combination of reaper time and unused timeout settings. Adding the nondeferredreaper custom property can cause an idle servant region to become active again.

  To add this custom property to your JDBC driver provider data source settings, in the administrative console, click Resources > JDBC providers > DB2 Universal JDBC Driver Provider > Data sources > data_source > Custom properties > New. Then specify nondeferredreaper in the Name field, true in the Value field, and java.lang.Boolean in the Type field. This new setting does not go into effect until you restart the server that is using this data source.

  Avoid trouble: Activating an idle servant region for the sole purpose of discarding unused connection, might cause additional and sometimes undesirable CPU usage. Also, the following warning message might be logged and should be ignored:

  DSRA8200W: DataSource Configuration:
  DSRA8020E: Warning: The property 'nondeferredreaper' does not exist on the DataSource 
  classcom.ibm.db2.jcc.DB2ConnectionPoolDataSource.

• Purge connection pools based on the purge policy.

  When the connection pool error detection model is configured to exception mapping, the stale connection exception indicates that the connection is no longer valid.

  Typically, a connection error event is fired when a stale connection exception (StaleConnectionException) results from the exception-mapping process. Thus, the connection pool gets purged. However, in rare cases during exception mapping, the stale connection exception is mapped and created, but the connection error event is not fired. Thus, the connection pool is not purged. If you encounter this rare situation, resolve the problem by setting the fireCEEventOnSCE data source custom property. This custom property can help fire the connection error event and purge the connection pool.

  To add this custom property to your JDBC driver provider data source settings, in the administrative console, click Resources > JDBC providers > DB2 Universal JDBC Driver Provider > Data sources > data_source > Custom properties > New. Then specify fireCEEventOnSCE in the Name field, true in the Value field, and java.lang.Boolean in the Type field. This new setting does not go into effect until you restart the server that is using this data source.

  The WebSphere® RRA code has been changed so that the pool is purged properly upon a StaleConnectionException when using ExceptionMapping as the errorDetection model.