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Highly Available Databases

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%.

One of the most significant developments of recent years in IT technology is the requirement to keep the systems, especially relational database systems, online 24/7 with as close to 100% availability as possible. This is important because most of the business operations need to access some form of data.

Many business tasks, even though simple in nature and occurring for a short duration, require dealing with persistent data. They may need to capture some data or update some data. The same data may need to be retrieved by another user located at some other business unit. For example, scheduling a visit by an external visitor, arranging a meeting, handling a student application and admission to a university, verification of zip code and last name before allowing a web site visitor access, they all need to retrieve, insert, or capture data.

These seemingly trivial examples bring out another significant point in regards to the nature of databases. Databases need to be available all the time, in other words 24/7. Non-availability of a database that provide crucial data for a business decision process has the potential effect of halting the same business task, however small or large the task may be.

The Need for Highly Available Data

The forces of globalization, 24/7 business operations, and the need for an always-on computing infrastructure means that downtime does not exist in business-critical and mission critical applications. Unplanned downtime and planned downtime are costly, in terms of lost revenue and lost time. In today?s global and internet economy, planned downtime in one time zone has a direct impact on the business hours of another time zone. So if the down time is planned in the New York corporate headquarters, it will affect corporate end users located in, say, London or Sydney.

In the case of internet based business operations, data or system availability becomes much more important than ever before. It is never known which end user, sitting in their family room, would like to make online flight reservation for their next vacation or make an online purchase of a popular DVD or check status of the stock positions. Factors like 24/7 uptime, 7 seconds response, and short or zero maintenance windows bring a lot of challenges for the database administrator and managers.

However, it does not mean that every single database in a business or company is required to be kept on-line. There is always some crucial, must-be-online type data, and at the same time, there is some tolerance for other data. It is necessary, however, for high availability planning for mission-critical databases.

It is important to understand certain key terms and concepts before examining HA systems and databases.


Failure is defined as a departure from expected behavior on an individual computer system or a network system of associated computers and applications. Software, hardware, operator and procedural errors, along with environmental factors can each cause a system failure. Failure of a single component or single computer can directly influence reliability of the overall system. Component failures are unavoidable.


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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