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

5.1 Topology selection criteria
5.1.1 High availability
There are many ways to architect a system that satisfies your need. In Chapter 4,
“Infrastructure” on page 93, we introduced some planning considerations for a
WebSphere Application Server environment. This section provides an overview
of the primary considerations in selecting a topology:
► “High availability” on page 122
► “Disaster recovery” on page 125
► “Security” on page 126
► “Maintainability” on page 126
► “Performance” on page 127
► “Application deployment” on page 129
High availability means that a system can tolerate a certain amount of failure and
errors, before it is no longer working properly. High availability is an important
consideration to take into account when designing your architecture. The
importance comes from the fact that your design for high availability is a key
criteria for the capability of your service to meet your expected service level
agreement (SLA). High availability is a complex topic that affects every single
component required to run your environment. High availability is not only a
design challenge for WebSphere but for every component on every layer of the
environment.
High availability is achieved by introducing redundancy in your architecture to be
fault tolerant. This means you need redundancy at different levels depending on
your availability requirements (for example: power supplies, network cables,
switches, processes, machines and so forth). WebSphere Application Server
Network Deployment provides a vast variety of options to provide a highly
available runtime environment for your applications. In the subsequent sections
we discuss many of WebSphere Application Server’s high availability features
and how you can benefit from them.
Avoid single points of failure
To avoid a single point of failure and to maximize system availability, the topology
needs to have some degree of redundancy. WebSphere Application Server high
availability topologies typically involve horizontal scaling across multiple
machines. Vertical scaling can improve availability by creating multiple
processes, but the machine itself still remains a single point of failure.
122 WebSphere Application Server V7.0: Concepts, Planning, and Design