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Fault-Tolerant Systems

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Another important distinction that needs to be made is between a high availability (HA) system and a fault tolerant (FT) system. Fault tolerant systems offer higher level of resilience and recovery. They use a high degree of hardware redundancy and specialized software to provide near-instantaneous recovery from any single hardware or software unit failure. This technology is relatively expensive and requires allocation of 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. Whereas, clustered systems are more often characterized as high availability and scalable solutions. HA Clusters may not guarantee non-stop operation, but they provide availability sufficient for most mission critical business applications.

Database Availability

When referring to the availability of databases, the total environment and infrastructure in which a typical database is located need to be examined. Typically, a database is an application hosted on a server. The database application has its own availability features that are unique from the system availability point of view.

There are three situations that need to be considered:

* Database Availability is a function of the system availability, or more appropriately the server availability. If the server is inoperable for any reason, the database is not available or usable.

* The server is up and running and at the same time the database is functioning as expected, but end users or applications are unable to access the database on account of failures in the networking path. In this case, even though the system and/or database are highly available, the data service in not reachable.

* The database is a logical structure for the physical data store that resides on the physical disks, also known as the storage device. If for some reason, the storage unit fails or the server is unable to reach or connect to the storage unit, the database cannot be brought up. As a result, database service is not available for use.

As an application, the database is more viewed as a service. Factors like end-user database path failures, system failures or storage failures equally affect the ability to provide the database access to application users. As a result, all of these factors need to be taken into consideration to design a better database system.  System Availability and Database Availability may not be always the same.

Another important issue relevant for the database is the need to maintain the database consistency. Unlike application servers or other application instances, multiple database instances or copies of database cannot exist. As the database contents change in real-time, multiple copies cannot be maintained in a timely manner.

The real solution is a Parallel Database that has a single copy of the database files, but can be accessed by multiple nodes concurrently through database instances.

Clustered Systems

HA systems are almost synonymous with clustered systems. In order to minimize downtime, two or more servers, called nodes, are linked to form a high-availability cluster system. At the same time, Parallel Databases are another type of the clustered systems, which offers multiple instances or database processing locations.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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