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

When introducing redundancy, the need for a load distributor or front-end traffic
handler arises. An IP sprayer can direct Web client requests to the next available
Web server, bypassing any server that is not available. The IP sprayer server can
be configured with a cascade backup (hot standby) that takes over the operation
in case the primary server fails. This configuration eliminates the IP sprayer as a
single point of failure in the architecture.
Improved availability is one of the key benefits of scaling to multiple machines.
Applications that are hosted on multiple machines generally have less down time
and are able to service client requests more consistently.
Hardware redundancy
Eliminate single points of failure in a system by including hardware and
redundancy. Hardware redundancy can be invented at different levels. In the
topology considerations we are considering hardware redundancy at the system
level only.
You can use the following techniques:
► Use horizontal scaling to distribute application servers (and applications)
across multiple physical machines or z/OS images. If a hardware or process
failure occurs, clustered application servers are available to handle client
requests. Additional Web servers and IP sprayers can also be included in
horizontal scaling to provide higher availability.
► Use backup servers for databases, Web servers, IP sprayers, and other
important resources, ensuring that they remain available if a hardware or
process failure occurs.
► Keep the servers (physical machines) within the cluster sprayed in different
secured rooms to prevent site-related problems.
Process redundancy
Provide process redundancy and isolation so that a failing server does not impact
the remaining healthy servers. The following configurations provide some degree
of process isolation:
► Deploy the Web server on a different machine than the application servers.
This configuration ensures that problems with the application servers do not
affect the Web server and vice versa. Separate systems also increase the
security level.
► Use horizontal scaling, placing application servers on different systems.
► The usage of vertical scaling provides process isolation as related to
WebSphere processes only.
Chapter 5. Topologies 123