Stale connections
WAS provides a special subclass of java.sql.SQLException when using connection pooling to access a relational database. This com.ibm.websphere.ce.cm.StaleConnectionException subclass exists in both a WebSphere 4.0 data source and in the new data source using the relational resource adapter, and is used to indicate that the connection currently held is no longer valid. This situation can occur for many reasons, including the following:
- The application tries to get a connection and fails, as when the database is not started.
- A connection is no longer usable because of a database failure. When an application tries to use a previously obtained connection, the connection is no longer valid. In this case, all connections currently in use by the application can get this error when they try to use the connection.
- The connection is orphaned (because the application had not used it in at most two times the value of the unused timeout setting) and the application tries to use the orphaned connection. This case applies only to Version 4.0 data sources.
- The application tries to use a JDBC resource, such as a statement, obtained on a stale connection.
- A connection is closed by the Version 4.0 data source auto connection cleanup and is no longer usable. Auto connection cleanup is the standard mode in which connection management operates. This mode indicates that at the end of a transaction, the transaction manager closes all connections enlisted in that transaction. This enables the transaction manager to ensure that connections are not held for excessive periods of time and that the pool does not reach its maximum number of connections prematurely.
One ramification of having the transaction manager close the connections and return the connection to the free pool after a transaction ends, is that an application cannot obtain a connection in one transaction and try to use it in another transaction. If the application tries this, a StaleConnectionException is thrown because the connection is already closed.
In the case of trying to use an orphaned connection or a connection cleaned up by auto connection cleanup, a StaleConnectionException indicates that the application has attempted to use a connection already returned to the connection pool. It does not indicate an actual problem with the connection. However, other cases of a StaleConnectionException indicate that the connection to the database has gone bad, or stale. Once a connection has gone stale, you cannot recover it, and completely close the connection rather than returning it to the pool.
Detecting stale connections
When a connection to the database becomes stale, operations on that connection result in an SQLException from the JDBC driver. Because an SQLException is a rather generic exception, it contains state and error code values that you can use to determine the meaning of the exception. However, the meanings of these states and error codes vary depending on the database vendor. The connection pooling run time maintains a mapping of which SQL state and error codes indicate a StaleConnectionException for each database vendor supported. When the connection pooling run time catches any SQLException, it checks to see if this SQLException is considered a StaleConnectionException for the database server in use.
Recovering from stale connections
Recovering from stale connections is a joint effort between the appserver run time and the application developer. From an application server perspective, the connection pool is purged based on its PurgePolicy setting.
Explicitly catching a StaleConnectionException is not required in an application. Because applications are already required to catch java.sql.SQLException, and StaleConnectionException extends SQLException, StaleConnectionException can be thrown from any method that is declared to throw SQLException, and is caught automatically in the general catch-block. However, explicitly catching StaleConnectionException makes it possible for an application to recover from bad connections. When application code catches StaleConnectionException, it should take explicit steps to handle the exception.
Resource adapter
Data sources
Connection pooling
Handling data access exception - StaleConnectionException
Connection pool settings
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