Overview of High Availability

1.1 What Is High Availability?

Availability is the degree to which an application, service, or function is accessible on demand. Availability is measured by the perception of an application's user. Users experience frustration when their data is unavailable or the computing system is not performing as expected, 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 disruption as the failure of critical components in the architecture. If a user cannot access the system, it is said to be unavailable. Generally, the term downtime is used to refer to periods when a system is unavailable.

Users who want their systems to be always ready to serve them need high availability. A system that is highly available is designed to provide uninterrupted computing services during essential time periods, during most hours of the day, and most days of the week throughout the year; this measurement is often shown as 24x365. Such systems may also need a high availability solution for planned maintenance operations such as upgrading a system's hardware or software.

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 applications—is just as critical to implementing a highly available solution. A related characteristic is resilience. For example, low-cost commodity hardware, combined with software such as Oracle Real Application Clusters (Oracle RAC), can be used to implement a very reliable system. The resilience of an Oracle RAC database allows processing to continue even though individual servers may fail.
Recoverability: There may be many ways to recover from a failure. Therefore, it is important to determine what types of failures may occur in your high availability environment and how to recover from those failures quickly in order to meet 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 essential to recover from the unexpected failure. Although 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 database administrator of a problem.
Continuous operation: Providing 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 in the database or even adding CPUs to your hardware, should be transparent to the user in a high availability architecture.

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

Tolerate failures such that processing continues with minimal or no interruption
Be transparent to—or tolerant of—system, data, or application changes
Provide built-in preventive measures
Provide active monitoring and fast detection of failures
Provide fast recoverability
Automate detection and recovery operations
Protect the data to minimize or prevent data loss
Implement the operational best practices to manage your environment
Achieve the goals set in SLAs (for example, recovery time objectives (RTOs) and recovery point objectives (RPOs)) for the lowest possible total cost of ownership

1.2 Importance of Availability

The importance of high availability varies among applications. 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 now demand service-level agreements from their information technology (IT) departments and solution providers, reflecting the increasing dependence on these solutions. 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 business can be in jeopardy. The business might lose revenue, incur penalties, and receive bad publicity that has a lasting effect on customers and on the company's stock price.

It is important to examine the factors that determine how your data is protected and maximize availability to your users.

1.3 Cost of Downtime

The need to deliver increasing levels of availability continues to accelerate as enterprises reengineer 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.

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 on their solutions to provide service.

Other factors to consider in the cost of downtime are:

The maximum tolerable length of a single unplanned outage

If the event lasts less than 30 seconds, then it may cause very little impact and may be barely perceptible to users. As the length of the outage grows, the effect may grow exponentially and negatively affect the business.
The maximum frequency of allowable incidents

Frequent outages, even if short in duration, may similarly disrupt business operations.

When designing a solution, it is important to recognize the true cost of downtime to understand how the business can benefit from availability improvements.

Oracle provides a range of high availability solutions to 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 data are 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 of 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 as unplanned downtime, especially in global enterprises that support users in multiple time zones.

Table 1-1 describes unplanned outage types and provides examples of each type.

Table 1-1 Causes of Unplanned Downtime

Type	Description	Examples
Site failure	A site failure may affect all processing at a data center, or a subset of applications supported by a data center.	Extended sitewide power failure Sitewide network failure Natural disaster makes a data center inoperable Terrorist or malicious attack on operations or the site
Clusterwide failure	The whole cluster hosting an Oracle RAC database is unavailable or fails. This includes: Failures of nodes in the cluster Failure of any other components that result in the cluster being unavailable and the Oracle database and instances on the site being unavailable	The last surviving node on the Oracle RAC cluster fails and the node or database cannot be restarted Both redundant cluster interconnections fail or clusterware failure Database corruption so severe that continuity is not possible on the current database server Disk storage failure
Computer failure	A computer failure outage occurs when the system running the database becomes unavailable because it has failed or is no longer available. When the database uses Oracle RAC then a computer failure represents a subset of the system (while retaining full access to the data).	Database system hardware failure Operating system failure Oracle instance failure
Network failure	A network failure outage occurs when a network device stops or reduces network traffic and communication from your application to database, database to storage, or any system to system that is critical to your application service processing.	Network switch failure Network interface failure Network cable failures
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 available.	Disk drive failure Disk controller failure Storage array failure
Data corruption	A corrupt block is a block that was changed so that it differs from what Oracle Database expects to find. Block corruptions can be categorized as physical or logical: In a physical block corruption, which is also called a media corruption, the database does not recognize the block at all: the checksum is invalid or the block contains all zeros. An example of a more sophisticated block corruption is when the block header and footer do not match. In a logical block corruption, the contents of the block are physically sound and pass the physical block checks; however, the block can be logically inconsistent. Examples of logical block corruption include incorrect block type, incorrect data or redo block sequence number, corruption of a row piece or index entry, or data dictionary corruptions. Block corruptions can also be divided into interblock corruption and intrablock corruption: In intrablock corruption, the corruption occurs in the block itself and can be either a physical or a logical block corruption. In an interblock corruption, the corruption occurs between blocks and can only be a logical block corruption. A data corruption outage occurs when a hardware, software, or network component causes corrupt data to be read or written. The service-level impact of a data corruption outage may vary, from a small portion of the application or database (down to a single database block) to a large portion of the application or database (making it essentially unusable).	Operating system or storage device driver failure Faulty host bus adapter Disk controller failure Volume manager error causing a bad disk read or write Software or hardware defects
Human error	A human error outage occurs when unintentional or other actions are committed that cause data in the database to become incorrect 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.	File deletion (at the file system level) Dropped database object Inadvertent data changes Malicious data changes
Lost or stray writes	A lost or stray write is another form of data corruption, but it is much more difficult to detect and repair quickly. A data block stray or lost write occurs when: For a lost write, an I/O subsystem acknowledges the completion of the block write even though the write I/O did not occur in the persistent storage. On a subsequent block read on the primary database, the I/O subsystem returns the stale version of the data block, which might be used to update other blocks of the database, thereby corrupting it. For a stray write, the write I/O completed but it was written somewhere else, and a subsequent read operation returns the stale value. For an Oracle RAC system, a read I/O from one cluster node returns stale data after a write I/O is completed from another node (lost write). For example, this occurs if a network file system (NFS) is mounted in Oracle RAC without disabling attribute caching (for example, without using the `noac` option). In this case, the write I/O from one node is not immediately visible to another node because it is cached. Block corruptions caused by stray writes or lost writes can cause havoc to your database availability. The data block may be physically or logically correct but subsequent disk reads will show blocks that are stale or with an incorrect Oracle Database block address.	Operating system or storage device driver failure Faulty host bus adapter Disk controller failure Volume manager error Other application software Lack of network file systems (NFS) write visibility across a cluster
Delay or slowdown	A delay or slowdown occurs when the database or the application cannot process transactions because of a resource or lock contention. A perceived delay can be caused by lack of system resources.	Database or application deadlocks Runaway processes that consume system resources Logon storms or system faults Combination of application peaks with lack of system or database resources. This can occur with one application or many applications in a consolidated database environment without proper resource management. Archived redo log destination or fast recovery area destination becomes full

Table 1-2 describes planned outage types and provides examples of each type.

Table 1-2 Causes of Planned Downtime

Type	Description	Examples
System and database 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 of the organizational data structure in 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 before implementing the change, and the technologies and features in place to minimize the impact.	Adding or removing processors to or from an SMP server Adding or removing nodes to or from a cluster Adding or removing disks drives or storage arrays Replacing any Field Replaceable Unit (FRU) Changing configuration parameters Upgrading or patching system hardware and software Upgrading or patching Oracle software Upgrading or patching application software System platform migration Database relocation Moving from 32 bits to 64 bits Migrating to cluster architecture Migrating to new storage
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
Application changes	Planned application changes can include data changes and schema and programmatic changes. The primary objective of these changes is to improve performance, manageability, and functionality.	Application upgrades

Type

Description

Examples

System and database 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 of the organizational data structure in 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 before implementing the change, and the technologies and features in place to minimize the impact.

Adding or removing processors to or from an SMP server

Adding or removing nodes to or from a cluster

Adding or removing disks drives or storage arrays

Replacing any Field Replaceable Unit (FRU)

Changing configuration parameters

Upgrading or patching system hardware and software

Upgrading or patching Oracle software

Upgrading or patching application software

System platform migration

Database relocation

Moving from 32 bits to 64 bits

Migrating to cluster architecture

Migrating to new storage

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

Application changes

Planned application changes can include data changes and schema and programmatic changes. The primary objective of these changes is to improve performance, manageability, and functionality.

Application upgrades

Oracle offers high availability solutions to help avoid both unplanned and planned downtime, and recover from failures. Oracle Database High Availability Solutions for Unplanned Downtime and Oracle Database High Availability Solutions for Planned Downtime discuss each of these high availability solutions in detail.

1.5 Roadmap to Implementing the Maximum Availability Architecture

Oracle high availability solutions and sound operational practices are key to the successful implementation of an IT infrastructure. However, technology alone is not enough.

Choosing and implementing an architecture that best fits your availability requirements can be a daunting task. Maximum Availability Architecture (MAA) simplifies the process of choosing and implementing a high availability architecture to fit your business requirements. The MAA:

Encompasses redundancy across all components
Provides protection and tolerance from computer failures, storage failures, human errors, data corruption, lost writes, system delays or slowdowns, and site disasters
Recovers from outages as quickly and transparently as possible
Provides solutions to eliminate or reduce planned downtime
Provides consistent high performance and robust security
Provides Oracle Engineered System options to simplify deployment and management and achieve higher scalability, performance, and availability
Achieves SLAs at the lowest possible total cost of ownership
Applies to On-Premise, Oracle Public Cloud, and hybrid architectures consisting of parts on-premise and part in the cloud
Provides special consideration to Container or Oracle Multitenant, Oracle Database In-Memory, and Oracle Sharding architectures

To build, implement, and maintain this type of architecture, you need to:

Analyze your specific high availability requirements, including both the technical and operational aspects of your IT systems and business processes, as described in High Availability and Data Protection – Getting From Requirements to Architecture.
Familiarize yourself with Oracle high availability features, as described in Features for Maximizing Availability.
Understand the availability impact for each MAA tier or various high availability features on businesses and applications, as described in Oracle Database High Availability Solutions for Unplanned Downtime, and Oracle Database High Availability Solutions for Planned Downtime.
Use operational best practices to provide a successful MAA implementation, as described in Operational Prerequisites to Maximizing Availability.
Choose a high availability architecture, as described in High Availability Architectures.
Learn how Oracle's Engineered Systems such Oracle Exadata Database Machine, Oracle SuperCluster, Oracle Database Appliance, and Zero Data Loss Recovery Appliance improve MAA, as described in Oracle Engineered Systems.
Optimize your return on investment (ROI) of any of the high availability architectures and solutions, with Oracle Grid Computing, Oracle Active Data Guard real time reporting and utilization, Oracle Database multitenant architecture using pluggable databases, or Oracle Virtualization and Oracle Data Cloud, as described in Optimizing Return on Investment.
Implement a high availability architecture using the following resources:
- MAA and high availability best practices white papers and other information
  
  Oracle offers various best practices white papers, customer MAA papers with proof of concepts, customer case studies, recorded web casts, demonstrations, and presentations. These resources provide technical details about the MAA various high availability technologies, along with best practice recommendations for configuring and using such technologies.
  
  These MAA resources are available at http://www.oracle.com/goto/maa
- Oracle Database High Availability Best Practices
  
  This book provides the configuration, repair and planned maintenance best practices for any of the MAA reference architectures. It can help you to configure a new high availability environment, or migrate an existing configuration to create a redundant, reliable system without sacrificing simplicity and performance.