A high availability cluster is a grouping of HP 9000
series 800 servers having sufficient redundancy of software and
hardware components that a single point of failure will not disrupt
the availability of computer services. High availability clusters
configured with Oracle Real Application Cluster software are known
as RAC clusters. Figure 1-1 “Overview
of Oracle RAC Configuration on HP-UX ” shows a very simple picture of the basic configuration
of a RAC cluster on HP-UX.
In the figure, two loosely coupled HP 9000 series 800 systems
(each one known as a node) are running separate
instances of Oracle software that read data from and write data
to a shared set of disks. Clients connect to one node or the other
via LAN.
RAC on HP-UX lets you maintain a single database image that
is accessed by the HP 9000 servers in parallel, thereby gaining
added processing power without the need to administer separate databases. Further,
when properly configured, Serviceguard Extension for RAC provides a highly available
database that continues to operate even if one hardware component
should fail.
Group
Membership |
 |
Oracle RAC 8.1.x and later systems implement the concept of group membership,
which allows multiple instances of RAC to run on each node. Related
processes are configured into groups. Groups
allow processes in different instances to choose which other processes
to interact with. This allows the support of multiple databases
within one RAC cluster.
A Group Membership Service (GMS) component provides a process monitoring
facility to monitor group membership status. GMS is provided by
the cmgmsd daemon, which is an HP component installed with Serviceguard
Extension for RAC.
Figure 1-2 “Group
Membership Services” shows how group
membership works. Nodes 1 through 4 of the cluster share the Sales
database, but only Nodes 3 and 4 share the HR database. Consequently,
there is one instance of RAC each on Node 1 and Node 2, and there
are two instances of RAC each on Node 3 and Node 4. The RAC processes
accessing the Sales database constitute one group, and the RAC processes
accessing the HR database constitute another group.
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