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

4.3 Sizing the infrastructure
After determining the initial application and infrastructure design and scalability
techniques, you need to determine the system resources required for the project
to keep the service level agreements (SLAs). Whenever making sizing decisions
ensure that all design decisions are still honored.
Application design will evolve over time, and sizing is usually done in the early
stages of design. However, when sizing, it is important that you have a static
version of the application design with which to work. The better view you have of
the application design, the better your sizing estimate will be.
You have to consider which hardware platforms you want to use. This decision
depends on the following factors:
► Scaling capabilities of the platform
► Platforms WebSphere Application Server supports
► Performance, security, and high availability requirements of your environment.
Next, determine your scaling approach:
► Scaling up—vertical scaling
Scaling up is done inside a single system, typically on a multi-processor
machine. Even if this approach might be sufficient from the performance
perspective, the underlying hardware is the single points of failure.
► Scaling out—horizontal scaling
Scaling out means increasing the number of machines. Scaling out is typically
done to improve high availability by limiting single points of failure as well as
for scalability reasons when hardware resources are the limiting factor. You
have to support and maintain multiple machines. If you decide to use the
scale out approach keep in mind that your design must be capable of
processing the workload even if one system fails.
Sizing estimates are based solely on your input, which means the more accurate
the input, the better the results. Sizing work assumes an average standard of
application performance behavior and an average response time is assumed for
each transaction. Calculations based on this are performed to determine the
estimated number of machines and processors your application will require. If
your enterprise has a user experience team, they might have documented
standards for a typical response time that your new project is required to meet.
If you need a more accurate estimation of your hardware requirements and you
already have your application, consider using one of the benchmarking services
discussed in 4.4, “Benchmarking” on page 107.
106 WebSphere Application Server V7.0: Concepts, Planning, and Design