TCP/IP Tutorial and Technical Overview - IBM Redbooks

The clustering technique dispatches connections to target servers, excluding
failed servers, from a list of target servers that can receive connections. In this
way, the dispatching function avoids routing connections to a server that is not
capable of satisfying such a request.
The clustering technique requires the implementation of equal application
instances running on different machines. If the application, the operating system
with TCP/IP stack, or the machine fails, the dispatching technique immediately
provides a backup.
A user requesting service from a particular server would no longer address an
application in a particular server but now would address a group of servers. The
connection request is now sent to the dispatcher, who decides to which available
application server it is forwarded. Therefore, users are not aware to which
application server (within the group) they are connected.
The clustering technique requires addresses that refer to groups of applications.
This can be solved through virtual IP addresses. A virtual IP address (VIPA) is
the IP address of a group of application servers, for example, a Telnet server.
This VIPA is used for a connection request. The dispatcher is the receiver of the
connection request from the user. It selects from a list of available servers a real
server and forwards the request to this server.
The process of selecting an available application server may be extended by the
dispatcher by using different kind distribution rules. The distribution of connection
requests will be discussed in the load balancing section.
Another aspect of availability to consider is when the dispatcher fails. In this
case, a backup dispatcher has to be implemented with the same IP address so
that users can send their connection requests to the backup dispatcher. A
backup dispatcher also propagates its IP address to the network. Therefore,
routers use the new path that directs the user’s connection requests to the
backup dispatcher.
If dispatchers maintain client/server connections, the backup dispatcher has to
take over the currently running connections. A takeback process must be
implemented to return running connections to the primary dispatcher.
Virtualization is also a technique used to provide availability and scalability.
Virtualization has similarities to the clustering technique with regards to
transparency shown to the users regarding which physical machine is being
used as well as the there being no impact to the users if a machine is to fail. We
discuss virtualization next.
Chapter 24. Availability, scalability, and load balancing 911