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

7.5.4 Data availability
In a WebSphere Application Server environment, there are multiple places where
data availability is important. Critical areas for data availability are as follows:
► Databases
► HTTP session state
► EJB session state
► EJB persistence
The majority of these requirements can be satisfied using facilities available in
WebSphere Application Server. We discuss these areas in more detail in the
next sections.
Database server availability
A database server is for many systems the largest and most critical single point
of failure in the environment. Depending on the nature of this data, there are
many techniques that you can employ to provide availability for this data:
► For read-only data, multiple copies of the database can be placed behind a
load balancing device that uses a virtual IP. This enables the application to
connect to one copy of the data and to transparently fail over to another
working copy.
► If the data is mostly read-only, consider the option of replication facilities to
keep multiple copies synchronized behind a virtual IP. Most commercial
database management systems offer some form of replication facility to keep
copies of a database synchronized.
► If the data is read/write and there is no prevalence of read-only access,
consider a hardware clustering solution for the database node. This requires
an external shared disk through Storage Area Network (SAN), Network
Attached Storage (NAS), or some other facility that can help to mitigate failure
of a single node.
Session data
WebSphere Application Server can persist session data in two ways:
► Using memory-to-memory replication to create a copy of the session data in
one or more additional servers
► By storing the session data in an external database.
The choice of which to use is really left to the user, and performance results may
vary. External database persistence will survive node failures and application
server restarts, but introduces a new single point of failure that must be mitigated
using an external hardware clustering or high availability solution.
248 WebSphere Application Server V7.0: Concepts, Planning, and Design