1 Overview of High Availability

This chapter contains the following sections:

1.1 Introduction to High Availability

Databases and the Internet have enabled worldwide collaboration and information sharing by extending the reach of database applications throughout organizations and communities. This reach emphasizes the importance of high availability in data management solutions. Both small businesses and global enterprises have users all over the world who require access to data 24 hours a day. Without this data access, operations can stop, and revenue is lost. Users, who have become more dependent upon their solutions, now demand service-level agreements from their Information Technology (IT) departments and solutions providers. Increasingly, availability is measured in dollars, euros, and yen, not just in time and convenience.

Enterprises have used their IT infrastructure to provide a competitive advantage, increase productivity, and empower users to make faster and more informed decisions. However, with these benefits has come an increasing dependence on that infrastructure. If a critical application becomes unavailable, then the entire business can be in jeopardy. Revenue and customers can be lost, penalties can be owed, and bad publicity can have a lasting effect on customers and a company's stock price. It is critical to examine the factors that determine how your data is protected and maximize the availability to your users.

1.2 What is Availability?

Availability is the degree to which an application, service, or functionality is available upon user demand. Availability is measured by the perception of an application's end user. End users experience frustration when their data is unavailable, and they do not understand or care to differentiate between the complex components of an overall solution. Performance failures due to higher than expected usage create the same havoc as the failure of critical components in the solution.

Reliability, recoverability, timely error detection, and continuous operations are primary characteristics of a highly available solution:

  • Reliability: Reliable hardware is one component of a high availability solution. Reliable software—including the database, Web servers, and application—is just as critical to implementing a highly available solution.

  • Recoverability: There may be many choices in recovering from a failure if one occurs. It is important to determine what types of failures may occur in your high availability environment, and how to recover from those failures in the time that meets your business requirements. For example, if a critical table is accidentally deleted from the database, what action should you take to recover it? Does your architecture provide the ability to recover in the time specified in a service level agreement (SLA)?

  • Timely error detection: If a component in your architecture fails, then fast detection is another essential component in recovering from a possible unexpected failure. While you may be able to recover quickly from an outage, if it takes an additional 90 minutes to discover the problem, then you may not meet your SLA. Monitoring the health of your environment requires reliable software to view it quickly and the ability to notify the DBA of a problem.

  • Continuous operations: Continuous access to your data is essential when very little or no downtime is acceptable to perform maintenance activities. Activities such as moving a table to another location within the database, or even adding additional CPUs to your hardware, should be transparent to the end user in a high availability architecture.

More specifically, a high availability architecture should have the following traits:

  • Be transparent to most failures

  • Provide built-in preventative measures

  • Provide proactive monitoring and fast detection of failures

  • Provide fast recoverability

  • Automate the recovery operation

  • Protect the data so that there is minimal or no data loss

  • Implement the operational best practices to manage your environment

  • Provide the high availability solution to meet your SLA

1.3 Importance of Availability

The importance of high availability varies among applications. However, the need to deliver increasing levels of availability continues to accelerate as enterprises re-engineer their solutions to gain competitive advantage. Most often, these new solutions rely on immediate access to critical business data. When data is not available, the operation can cease to function. Downtime can lead to lost productivity, lost revenue, damaged customer relationships, bad publicity, and lawsuits.

If a mission-critical application becomes unavailable, then the enterprise is placed in jeopardy. It is not always easy to place a direct cost on downtime. Angry customers, idle employees, and bad publicity are all costly, but not directly measured in currency. On the other hand, lost revenue and legal penalties incurred because SLA objectives are not met can easily be quantified. The cost of downtime can quickly grow in industries that are dependent upon their solutions to provide service.

Other factors to consider in the cost of downtime are the maximum tolerable length of a single unplanned outage, and the maximum frequency of allowable incidents. If the event lasts less than 30 seconds, then it may cause very little impact and may be barely perceptible to end users. As the length of the outage grows, the effect may grow exponentially and result in a negative impact on the business. When designing a solution, it is important to take into account these issues and to determine the true cost of downtime and the cost of added availability. An organization should then weigh the cost of downtime and balance it with the expected availability improvement. High availability solutions are effective insurance policies.

Oracle provides a range of high availability solutions that fit every organization regardless of size. Small workgroups and global enterprises alike are able to extend the reach of their critical business applications. With Oracle and the Internet, applications and their data are now reliably accessible everywhere, at any time.

1.4 Causes of Downtime

One of the challenges in designing a high availability solution is examining and addressing all the possible causes of downtime. It is important to consider causes of both unplanned and planned downtime when designing a fault tolerant and resilient IT infrastructure. Planned downtime can be just as disruptive to operations, especially in global enterprises that support users in multiple time zones.

Table 1-1 describes the outage categories and provides examples of each outage type.

Table 1-1 Causes of Downtime

Category Outage Type Description Examples


Computer failure

A computer failure outage occurs when the system running the database becomes unavailable because it has shut down or is no longer accessible.

Database system hardware failure
Operating system failure
Oracle instance failure
Network interface failure

Storage failure

A storage failure outage occurs when the storage holding some or all of the database contents becomes unavailable because it has shut down or is no longer accessible.

Disk drive failure
Disk controller failure
Storage array failure

Human error

A human error outage occurs when unintentional or malicious actions are committed that cause data within the database to become logically corrupt or unusable. The service level impact of a human error outage can vary significantly depending on the amount and critical nature of the affected data.

Dropped database object
Inadvertent data changes
Malicious data changes

Data corruption

A data corruption outage occurs when a hardware or software component causes corrupt data to be read or written to the database. The service level impact of a data corruption outage may vary, from a small portion of the database (down to a single database block) to a large portion of the database (making it essentially unusable).

Operating system or storage device driver, host bus adapter, disk controller, or volume manager error causing bad disk read or writes
Stray writes by operating system or other application software

Site failure

A site failure outage occurs when an event causes all or a significant portion of an application to stop processing or slow to an unusable service level. A site failure may affect all processing at a data center, or a subset of applications supported by a data center.

Extended site-wide power failure
Site-wide network failure
Natural disaster making a data center inoperable
Terrorist or malicious attack on operations or the site


System changes

Planned system changes occur when performing routine and periodic maintenance operations and new deployments. Planned system changes include any scheduled changes to the operating environment that occur outside the organizational data structure within the database. The service level impact of a planned system change varies significantly depending on the nature and scope of the planned outage, the testing and validation efforts made prior to implementing the change, and the technologies and features in place to minimize the impact.

Adding/removing processors to/from an SMP server
Adding/removing nodes to/from a cluster
Adding/removing disks drives or storage arrays
Changing configuration parameters
Upgrading/patching system hardware and software
Upgrading/patching Oracle software
Upgrading/patching application software
System platform migration
Database relocation

Data changes

Planned data changes occur when there are changes to the logical structure or physical organization of Oracle database objects. The primary objective of these changes is to improve performance or manageability.

Table definition changes
Adding table partitioning
Creating and rebuilding indexes

Oracle offers high availability solutions to help avoid both unplanned and planned downtime, as well as recover from failures. Chapter 2 discusses each of these high availability solutions in detail.

1.5 What Does This Book Contain?

Choosing and implementing the architecture that best fits your availability requirements can be a daunting task. This architecture must:

  • Encompass redundancy across all components

  • Provide protection from computer failures, storage failures, human errors, data corruption, and site disasters

  • Recover from outages as quickly and transparently as possible

  • Provide solutions to eliminate or reduce planned downtime

  • Provide consistent high performance

  • Be easy to deploy, manage, and scale

To help you select the most suitable architecture for your organization, this book describes several high availability architectures and provides guidelines for choosing the one that best meets your requirements. Knowledge of the Oracle Database server, Oracle Real Application Clusters and Oracle Data Guard terminology is required to understand the configuration and implementation details.

Chief technology officers and information technology architects can benefit from reading the following chapters:

Database administrators and network administrators can find useful information in the following chapters:

Oracle High Availability Best Practice white papers can be downloaded at