WebSphere Application Server V7.0: Concepts ... - IBM Redbooks

Local sessions (non-persistent)
If the session data is stored in the application server memory only, the session
data is not available to any other servers. Although this option is the fastest and
the simplest to set up, an application server failure ends the session, because
the session data is lost.
The following settings can help you manage the local session storage:
► Maximum in-memory session count
This setting enables you to define a limit to the number of sessions in
memory. This prevents the sessions from acquiring too much of the JVM heap
and causing out-of-memory errors.
► Allow overflow
This setting permits an unlimited number of sessions. If you choose this
option, monitor the session cache size closely.
Note: Session overflow is enabled by default in WebSphere Application
Server V7.0
► Session time-out
This setting determines when sessions can be removed from cache.
Database persistent sessions
You can store session data in an external database. The administrator must
create the database and configure the session database in WebSphere through
a data source.
The Use multi-row schema setting gives you the option to use multi-row sessions
to support large session objects. With multi-row support, the WebSphere session
manager breaks the session data across multiple rows if the size of the session
object exceeds the size for a row. This also provides a more efficient mechanism
for storing and retrieving session contents when session attributes are large and
few changes are required to the session attributes.
Memory-to-memory replicated persistent sessions
Memory-to-memory replication copies session data across application servers in
a cluster, storing the data in the memory of an application server and providing
session persistence. Using memory-to-memory replication eliminates the effort
of maintaining a production database and eliminates the single point of failure
that can occur with a database. Test to determine which persistence mechanism
is the best one in your environment.
264 WebSphere Application Server V7.0: Concepts, Planning, and Design