Managing Serviceguard Eighteenth Edition, September 2010

Avoid Node-Specific Information
Typically, when a new system is installed, an IP address must be assigned to each active
network interface. This IP address is always associated with the node and is called a
stationary IP address.
The use of packages containing highly available applications adds the requirement for
an additional set of IP addresses, which are assigned to the applications themselves.
These are known as relocatable application IP addresses. Serviceguard’s network
sensor monitors the node’s access to the subnet on which these relocatable application
IP addresses reside. When packages are configured in Serviceguard, the associated
subnetwork address is specified as a package resource dependency, and a list of nodes
on which the package can run is also provided. When failing a package over to a remote
node, the subnetwork must already be active on the target node.
Each application or package should be given a unique name as well as a relocatable IP
address. Following this rule separates the application from the system on which it runs,
thus removing the need for user knowledge of which system the application runs on.
It also makes it easier to move the application among different systems in a cluster for
for load balancing or other reasons. If two applications share a single IP address, they
must move together. Instead, using independent names and addresses allows them to
move separately.
For external access to the cluster, clients must know how to refer to the application.
One option is to tell the client which relocatable IP address is associated with the
application. Another option is to think of the application name as a host, and configure
a name-to-address mapping in the Domain Name System (DNS). In either case, the
client will ultimately be communicating via the application’s relocatable IP address. If
the application moves to another node, the IP address will move with it, allowing the
client to use the application without knowing its current location. Remember that each
network interface must have a stationary IP address associated with it. This IP address
does not move to a remote system in the event of a network failure.
Obtain Enough IP Addresses
Each application receives a relocatable IP address that is separate from the stationary
IP address assigned to the system itself. Therefore, a single system might have many
IP addresses, one for itself and one for each of the applications that it normally runs.
Therefore, IP addresses in a given subnet range will be consumed faster than without
high availablity. It might be necessary to acquire additional IP addresses.
Multiple IP addresses on the same network interface are supported only if they are on
the same subnetwork.
Allow Multiple Instances on Same System
Applications should be written so that multiple instances, each with its own application
name and IP address, can run on a single system. It might be necessary to invoke the
application with a parameter showing which instance is running. This allows
distributing the users among several systems under normal circumstances, but it also
allows all of the users to be serviced in the case of a failure on a single system.
Designing Applications to Run on Multiple Systems 431