The reason for doing this is to avoid a rather
substantial performance hit observed when the client driver is used
together with an appserver that uses connection pooling. There are two
1) The connection pool will compare the isolation level it has
stored for the connection with the value returned from
Connection.getTransactionIsolation() each and every time someone
requests a new connection from the pool.
2) The users of the connection pool (ab)use it to avoid having to keep
track of their current connection. So each time a query needs to be
executed a call to the connection pool's getConnection() method is
made. Getting a connection from the connection pool like this also
means that a new PreparedStatement must be prepared each time.
The net result is that each query results in the following sequence:
getTransactionIsolation() --> roundtrip + lookup in server's statement cache
prepareStatment() --> roundtrip + lookup in server's statement cache
executeQuery() --> roundtrip
Arguably this is a "user error" but when suggesting this I'm kindly
informed that this works "just fine" with other datbases (such as
PostgreSQL and ORACLE).
The reason why it works is that these databases do statement caching
in the driver. I've tried to implement a very (too) simple statement
cache in Derby's client driver and to re-enable caching of the
isolation level (see
https://issues.apache.org/jira/browse/DERBY-1148). With these changes
I observe a marked performance improvement when running with appserver
A proper statment cache cannot be implemented without knowing what the
current schema is. If the current schema has changed since the
statement was prepared, it is no longer valid and must be evicted from
The problem with caching both the isolation level and the current schema in
the driver is that both can change on the server without the client
detecting it (through SQL and XA and possibly stored procedures).
I think this problem can be overcome if we piggy-back the information we would
like to cache on messages going back to the client. This can be done by
utilizing the EXCSQLSET DRDA command. According to the DRDA spec (v4, volume 3,
page 359-360) it is possible to add one or more SQLSTT objects after SQLCARD in the reply,
I think it would be possible to cache additional session information when this becomes relevant. It
would also be possible to use EXCSQLSET to batch session state changes
going from the client to the server.