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

Memory-to-memory replication can reduce the impact of failure, but if a node
fails, the data held on that node is lost.
In contrary to the HTTP session persistence, stateful session EJB availability is
handled using memory-to-memory replication only. Using the EJB container
properties, you can specify a replication domain for the EJB container and enable
the stateful session bean failover using memory-to-memory replication. When
enabled, all stateful session beans in the container are able to fail over to another
instance of the bean and still maintain the session state.
EJB persistence
When designing applications that use the EJB 2.1 (and later) specifications, the
ability to persist these beans becomes available. If the beans participate in a
clustered container, bean persistence is available for all members of the cluster.
Using access intent policies, you can govern the data access for the persisted
bean. This EJB persistence API should not be confused with entity EJBs.
7.5.5 Clustering and failover technique
Clustering is the concept of creating highly available system processes across
multiple servers. On distributed platforms it is usually deployed in a manner that
only one of the servers is actively running the system resource.
Hardware-based clustering
Clustering is achieved by using an external clustering software, such as IBM
HACMP on AIX systems or using operating system cluster capabilities like the
Parallel Sysplex on the z/OS platform, to create a cluster of servers. Each server
is generally attached to a shared disk pool through NAS, a SAN, or simply by
chaining SCSI connections to an external disk array. Each system has the base
software image installed. The servers stay in constant communication with each
other over several connections through the use of heartbeats. Multiple paths
should be configured for these heartbeats so that the loss of a switch or network
interface does not necessarily cause a failover.
Note: Too few paths can create problems with both servers believing they
should be the active node. This situation is called cluster isolation and is a
serious thread to data consistency. Too many paths can create unnecessary
load associated with heartbeat management.
Chapter 7. Performance, scalability, and high availability 249