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Donald K. Burleson

Oracle RAC Tips

Overview of the Cluster Technology

What is a cluster? “A cluster is a type of parallel system that consists of interconnected whole computers and is used as a single, unified computing resource.” [Pfister] A “whole computer” is a normal combination of parts that comprise a stand-alone, usable computer. The components include one or more processors (including SMP and NUMA), memory, and I/O facilities. The whole computer is also referred to as a ‘Node’.

Thus, the cluster is a configuration of a group of independent servers, so that they appear on a network as a single machine. This group can be managed as a single system, shares a common namespace, and is designed specifically to tolerate component failures and to support the addition or subtraction of components in a way that's transparent to users.

How Clusters Differ from Distributed Systems

Like a distributed system, the nodes of a cluster retain their own individual identities. However, they may have different architectures and different operating system versions. Regardless of their differences, the nodes in a cluster are presented as a single unified computing resource. This is true even though their individual resources may differ, as one may have a higher-powered processor and the other may be of lower power.

Another difference is the relation between parts. In the case of distributed systems, it is a peer-to-peer relationship. Distributed nodes do not directly access each other’s resources (E.g., file system access). In the case of clusters, file systems are a mutually shared resource.

Clusters are Different from Fault-Tolerant Systems

Fault-tolerant systems offer a higher level of resilience and recovery. They exhibit a high degree of hardware redundancy, as well as specialized software to provide near-instantaneous recovery from any single failure of the hardware or software units. This technology is relatively expensive and requires higher budgets. Fault-tolerant servers are primarily used for applications that support high value, high rate transactions, such as check clearinghouses, automated teller machines, or stock exchanges.

The clustered system solution is more often characterized as highly available and scalable. Clusters may not guarantee non-stop operation, but they do provide availability that is sufficient for most mission-critical applications. Fig 3.1 shows the overall architecture of distributed and clustered systems.

Fig 3.1 Clustered and Distributed Systems

Database Clusters

Clusters supporting relational databases are widely employed by many enterprises. Setting them up on a dual or multi-node cluster often protects databases, being a crucial component in any IT infrastructure. Data consistency and data integrity are key attributes for any database. The database, in addition to its dependence on the server on which it resides, requires a data storage unit for storing the data blocks. It can keep only one copy of the data in the storage unit because of data integrity requirements. In a traditional DB cluster, a node failure causes the surviving node to take over data retrieval from physical storage. Most of the cluster vendors have provided suitable clustering middleware to support the failover functionality of the database.

However, in the case of scalable and parallel database clusters, such as Oracle Real Application Cluster, IBM Sysplex, or IBM UDB Cluster, multiple nodes maintain concurrent access to physical data storage. As a result, the loss of a node does not prevent access to the physical data through another surviving node, and there is no such thing as database failover.

For more information, see the book Oracle Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.  


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