The Application Server provides HTTP session persistence and failover by writing session data to HADB. You can control the frequency at which the server writes to HADB by specifying the persistence frequency.
Specify the persistence frequency in the Admin Console under Configurations > config-name > Availability Service (Web Container Availability).
All else being equal, time-based persistence frequency provides better performance but less availability than web-method persistence frequency. This is because the session state is written to the persistent store (HADB) at the time interval specified by the reap interval (default is 60 seconds). If the server instance fails within that interval, the session state will lose any updates since the last time the session information was written to HADB.
With web-method persistence frequency, the server writes the HTTP session state to HADB before it responds to each client request. This can have an impact on response time that depends on the size of the data being persisted. Use this mode of persistence frequency for applications where availability is critical and some performance degradation is acceptable.
For more information on web-method persistence frequency, see Configuring Availability for the Web Container in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 High Availability Administration Guide.
With time-based persistence frequency, the server stores session information to the persistence store at a constant interval, called the reap interval. You specify the reap interval under Configurations > config-name > Web Container (Manager Properties), where config-name is the name of the configuration. By default, the reap interval is 60 seconds. Every time the reap interval elapses, a special thread “wakes up,” iterates over all the sessions in memory, and saves the session data.
In general, time-based persistence frequency will yield better performance than web-method, since the server’s responses to clients are not held back by saving session information to the HADB. Use this mode of persistence frequency when performance is more important than availability.