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Oracle 10g RAC - How To Keep Your Database Operational 24 Hours/Day

So what is RAC?

In a nutshell, Oracle 10g RAC (Real Application Clusters) is an enhancement of the database-clustering technology introduced with Oracle 9i which in turn is an enhancement of the Oracle Parallel Server technology available in Oracle 7 & 8.

Oracle 10g Real Application Clusters make Oracle databases highly scalable and highly available by enabling multiple Oracle instances (the background processes and memory structures) running on separate servers to share the same database (the actual data).

Synchronisation of the data cached by each instance is achieved by Cache Fusion which copies via the high-speed interconnect modified data bocks from the memory of 1 node to the memory of another node as required.

The use of Oracle 10g RAC increases availability because if one node goes down or is out of service, the database is still accessible from the other nodes meaning users and applications can continue to access the database. Failover from one node to another can also take place automatically.

Scalability is increased because new nodes (servers) can be added to the cluster at any time. Therefore as the processingrequirements increase, the processing resources can grow to match them.

All of this is also completely transparent to the applications which do not have to be modified to take advantage of Oracle 10g Real Application Clusters.

Oracle 10g RAC Architecture

Now that we know what it is, let's dig a little deeper and see what Oracle 10g Real Application Clusters actually consist of.

Hardware

In hardware terms a cluster consists of 2 or more tightly-coupled servers (no need for high-end expensive servers, low cost commodity servers are just as good for this) each of which needs a minimum of 2 network cards (including one high-speed network adapter for the interconnect), 1GB of RAM and several GB of disk space for the Oracle software, swap space and temp space.

Obviously for this to work the database files have to be on cluster-aware disk storage and accessible by all nodes. On Linux these devices can be on an NFS mount using network attached storage devices.

Software

The next level up is the operating system: supported options include Winodws, Linux (Red Hat, SUSE, Oracle), Solaris, Tru64, HP-UX and AIX. Each node must run the same operating system.

For the file system you can use ASM (the recommended option), raw partitions, NFS, Oracle Cluster File System (only on Linux and Windows) or one of the certified 3rd party cluster file systems.

Next is the clusterware which binds the servers together so that they act as one unit. This can be Oracle's clusterware, known as Cluster Ready Services (CRS), or a supported 3rd party clusterware.

Oracle's clusterware consists of 3 major components.

The foundation layer is provided by the cluster synchronisation services which maintains the cluster heartbeat, monitors the voting disk for split-brain failures (where 2 or more nodes attempt to control the database simultaneously) and manages cluster membership information (nodes, listeners, instances etc).
Cluster ready services - this manages cluster resources and is responsible for starting and stopping them and switching them to another node in the event of a failure.
Event management - this is a background process that logs events

The top levels are of course, the database and the applications using the database.

See our Oracle resources section for links to more information about rac.

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