TCP/IP Tutorial and Technical Overview - IBM Redbooks

24.1 Availability
24.2 Scalability
Application instances, network interfaces, and machines can fail (planned for
maintenance or unplanned due to application or system error). In these cases,
users must not lose their service. Recovering from application instance failure is
fairly straightforward in that the application is simply restarted. Network interface
failures can also be tolerated by making use of a virtual IP address, which is not
tied to any particular physical interface and thus will never fail.
A virtual IP address can be given to a device that has one or more network
interfaces. This allows the users’ machines to pick up a specific IP associated
with a specific machine or device. However, the IP address given is not tied to
the physical IP address of the device’s interfaces. Therefore, if one of the
interfaces fails, the users are unaware of the failure.
Machine failure, however, is a bit more complex. Users must be able to
immediately reconnect to the service without knowing that they now are using an
alternate image of the application on another system. Users also must not be
aware that the path to the other system has been automatically changed. The
use of virtualization can be very advantageous with regards to increasing the
availability of a system. Virtualization is discussed further later.
Scalability means to provide a solution for a growing business that requires
additional system capacity. When workload capacity becomes smaller due to
many more new connection requests from clients or business partners, a
nondisruptive growth of the current system environment must be made available.
In a traditional single system environment (no clustered systems), a
nondisruptive upgrade of systems is relatively limited. In order to raise capacity,
these systems have to be taken down to install new features. Therefore, they are
not available for a certain time.
The implementation of clustered systems is a better approach (discussed in
more depth later). Adding a new system to the cluster running equal applications
instances does not impact the other systems in the cluster. This solution adds
seamless capacity for a growing business. Compared to traditional systems, the
user is not bound to a given system in a clustered server environment.
Therefore, the management of user connections to servers is more flexible.
When a new system comes online, new connections are directed to that machine
taking over a new workload.
Chapter 24. Availability, scalability, and load balancing 909