7 High Availability Architectures and Solutions

7.1.1 Oracle Database

Oracle Database is a single-instance, standalone (noncluster) database and it is the foundation for all high availability architectures. There are numerous high availability features that you can use in the Oracle Database single-instance database architecture.

Oracle recommends that you use the following Oracle features to make a standalone database on a single computer available for certain failures and planned maintenance activities:

• Fast-Start Fault Recovery bounds and optimizes instance and database recovery times.

• Oracle Restart enhances the availability of Oracle databases, listeners, and Oracle ASM instances in a single-instance environment by monitoring and automatically restarting Oracle processes.

• Oracle Automatic Storage Management (Oracle ASM) and Oracle Automatic Storage Management Cluster File System (Oracle ACFS) tolerate storage failures and optimize storage performance and usage.

• Oracle Flashback Technology optimizes logical failure repair. Oracle recommends that you use automatic undo management with sufficient space to attain your desired undo retention guarantee, enable Oracle Flashback Database, and allocate sufficient space and I/O bandwidth in the fast recovery area.

• Fast Recovery Area manages local recovery-related files.

• Recovery Manager (RMAN) optimizes local repair of data failures. Oracle recommends that you create and store the local backups in the fast recovery area.

• Data Recovery Advisor provides intelligent advice and repair of different data failures

• Oracle Secure Backup provides a centralized tape backup management solution

• Oracle Security Features prevent unauthorized access and changes.

• Corruption Prevention, Detection, and Repair detect and prevent some corruptions and lost writes.

• Online Reorganization and Redefinition allows for dynamic data changes.

• Dynamic Resource Provisioning allows for dynamic system changes.

• Online Patching allows for dynamic database patches for diagnostic and interim patches.

• Online Application Maintenance and Upgrades with Edition-based redefinition allows an application's database objects to be changed without interrupting the application's availability.

• Oracle Enterprise Manager support for patch application simplifies software maintenance

Figure 7-1 shows a basic, single-node Oracle Database that includes an Oracle ASM instance.^Foot 1  This architecture incorporates several high availability features, including Flashback Database, Online Redefinition, Recovery Manager, and Oracle Secure Backup.

Figure 7-1 Single-Node, Nonclustered Oracle Database with an Oracle ASM Instance

Description of "Figure 7-1 Single-Node, Nonclustered Oracle Database with an Oracle ASM Instance"

7.1.2 Oracle Database with Oracle Clusterware (Cold Cluster Failover)

In Oracle Database 11g Release 2 (11.2), Oracle RAC One Node or Oracle RAC is the preferred solution over Oracle Clusterware (Cold Cluster Failover) because it is a more complete and feature-rich solution. See Section 7.1.3, "Oracle Database with Oracle RAC One Node" for more information.

Section 3.4.1 describes how Oracle Clusterware is software that, when installed on servers running the same operating system, enables the servers to be bound together to operate as if they are one server, and manages the availability of user applications and Oracle databases. The servers on which you want to run Oracle Clusterware must be running the same operating system.

Many high availability architectures today use clusters alone to provide some rudimentary node redundancy and automatic node failover. However, when you use Oracle Clusterware, there is no need or advantage to using third-party clusterware.

Oracle Clusterware provides a number of benefits over third-party clusterware. Oracle Clusterware:

• Enables you to use an entire software solution from Oracle, avoiding the cost and complexity of maintaining additional cluster software.

  By reducing the combinations of software that you must coordinate and support, you can increase the manageability and availability of your system software.

• Provides seamless integration with, and migration to, Oracle Real Application Clusters (Oracle RAC) and Oracle Data Guard.

  Section 7.1.8 describes how you can achieve the highest level of availability with Oracle RAC and Oracle Data Guard.

• Includes all of the features required for cluster management, including node membership, group services, global resource management, and high availability functions such as managing third-party applications, event management, and Oracle notification services that enable Oracle clients to reconnect to the new primary database after a failure.

• Uses a private network and voting disk-based communication to detect and resolve split-brain^Foot 2  scenarios.

With Oracle Clusterware, you can provide a cold cluster failover to protect an Oracle Database instance from a system or server failure. The basic function of a cold cluster failover is to monitor a database instance running on a server, and if a failure is detected, to restart the instance on a spare server in the cluster. Network addresses are failed over to the backup node. Clients on the network experience a period of lockout while the failover occurs and are then served by the other database instance after the instance has started. Also, you can use the Oracle Clusterware ability to relocate applications and application resources (using the crsctl relocate resource command) as a way to move the workload to another node so that you can perform planned system maintenance on the production server.

The cold cluster failover solution with Oracle Clusterware provides these additional advantages over a basic database architecture:

• Automatic recovery of node and instance failures in minutes

• Automatic notification and reconnection of Oracle integrated clients^Foot 3 

• Ability to customize the failure detection mechanism

  For example, you can use your favorite application query in the database check action. Providing application-specific failure detection means Oracle Clusterware can fail over not only during the obvious cases such as when the instance is down, but also in the cases when, for example, an application query is not meeting a particular service level.

• High availability functionality to manage third-party applications

• Rolling release upgrades of Oracle Clusterware

The operation of an Oracle Clusterware cold cluster failover is depicted in Figure 7-2 and Figure 7-3. These figures show how you can use the Oracle Clusterware framework to make both Oracle Database and your custom applications highly available.

Figure 7-2 shows a configuration that uses Oracle Clusterware to extend the basic Oracle Database architecture and provide cold cluster failover. In the figure, the configuration is operating in normal mode in which Node 1 is the active instance connected to Oracle Database that is servicing applications and users. Node 2 is connected to Node 1 and to Oracle Database, but it is currently standby mode.

Figure 7-2 Oracle Database with Oracle Clusterware (Before Cold Cluster Failover)

Description of "Figure 7-2 Oracle Database with Oracle Clusterware (Before Cold Cluster Failover)"

Figure 7-3 shows the Oracle Clusterware configuration after a cold cluster failover has occurred. In the figure, Node 2 is now the active instance connected to the Oracle database and servicing applications and users. Node 1 is connected to Node 2 and to the Oracle database, but Node 1 is currently idle, in standby mode.

To provide this transparent failover capability, Oracle Clusterware requires a virtual IP (VIP) address for each node in the cluster. With Oracle Clusterware, you also define an application VIP so that users can access the application independently of the node in the cluster where the application is running. You can define multiple application VIPs, with generally one application VIP defined for each application running. The application VIP is tied to the application by making it dependent on the application resource defined by Cluster Ready Services (CRS).

Figure 7-3 Oracle Database with Oracle Clusterware (After Cold Cluster Failover)

Description of "Figure 7-3 Oracle Database with Oracle Clusterware (After Cold Cluster Failover)"

7.1.3 Oracle Database with Oracle RAC One Node

Traditionally, Oracle RAC is used in a multinode architecture, with many separate database instances running on separate servers. Oracle RAC One Node allows you to run one instance of an Oracle RAC database on a single node in a cluster. Thus, this feature allows you to consolidate many databases into a single cluster for easier management, while still providing high availability by quickly relocating instances in the event of server failure.

If the node running your Oracle RAC One Node becomes overloaded, you can relocate the instance to another node in the cluster using the online database relocation utility (srvctl relocate database), with no downtime for application users.

You can allocate server resources to multiple instances using Oracle Database Resource Manager Instance Caging. Server scalability is unlimited, and if applications grow to require more resources than a single node can supply, you can perform an online upgrade to a traditional multinode Oracle RAC configuration.

The high availability benefits to using Oracle RAC One Node include the following:

• Offers better database availability than traditional cold failover solutions

• Provides better virtualization for databases than hypervisor-based solutions

• Enables online migration of database instances and online patching and upgrading of operating system and database software (incurring no downtime)

• Delivers a comprehensive, single-vendor solution, with no need to implement third-party products

• Is ready to scale and upgrade to multinode Oracle RAC

• Provides a standardized environment and a common toolset for both single-node and multinode Oracle database deployments

• Is less expensive than cold fail over solutions or a full Oracle RAC deployment

• Fully supports Oracle Data Guard. Any database in a Data Guard configuration, whether a primary or standby database, can be an Oracle One Node database.

For virtualization, Oracle RAC One Node with Oracle VM increases the benefit of Oracle VM with the high availability and scalability of Oracle RAC. If your VM is sized too small, you can migrate the Oracle RAC One instance to another larger Oracle VM node in the cluster (using the online database relocation utility) or move the Oracle RAC One instance to another Oracle VM node, and then resize the Oracle VM. When you move the Oracle RAC One Node instance to the newly resized Oracle VM node, you can dynamically increase any limits programmed with Resource Manager Instance Caging.

For more information, see the "Administering Oracle RAC One Node" section in the Oracle Real Application Clusters Administration and Deployment Guide.

7.1.4 Oracle Database with Oracle Real Application Clusters (Oracle RAC)

An architecture that combines Oracle Database with Oracle RAC is inherently a highly available system. Unlike a traditional monolithic database server that is expensive and is not flexible to changing capacity and resource demands, Oracle RAC combines the processing power of multiple interconnected computers to provide system redundancy, scalability, and high availability.

The clusters that are typical of Oracle RAC environments can provide continuous service for both planned and unplanned outages. Oracle RAC builds higher levels of availability on top of the standard Oracle Database features. All single-instance high availability features, such as the Flashback technologies and online reorganization, also apply to Oracle RAC. Applications scale in an Oracle RAC environment to meet increasing data processing demands without changing the application code. In addition, allowing maintenance operations to occur on a subset of components in the cluster while the application continues to run on the rest of the cluster can reduce planned downtime.

Oracle RAC exploits the redundancy that is provided by clustering to deliver availability with n - 1 node failures in an n-node cluster. Unlike the cold cluster model where one node is completely idle, all instances and nodes can be active to scale your application. Communication among the nodes is optimized by means of Redundant Interconnect Usage (without requiring the use of bonding or other technologies) to provide stability, reliability, and scalability.

Oracle Database with Oracle RAC architecture provides the following benefits over a traditional monolithic database server and the cold cluster failover model:

• Scalability across database instances

• Flexibility to increase processing capacity using commodity hardware without downtime or changes to the application

• Ability to tolerate and quickly recover from computer and instance failures (measured in seconds)

• Optimized communication in the cluster over redundant network interfaces, without using bonding or other technologies

  Oracle Grid Infrastructure and Oracle RAC make use of Redundant Interconnect Usage that distributes network traffic and ensures optimal communication in the cluster. This functionality is available starting with Oracle Database 11g Release 2 (11.2.0.2). In previous releases, technologies like bonding or trunking were used to make use of redundant networks for the interconnect.

• Rolling upgrades for system and hardware changes

• Rolling patch upgrades for some interim patches, security patches, CPUs, and cluster software

• Fast, automatic, and intelligent connection and service relocation and failover

• Comprehensive manageability integrating database and cluster features with Grid Plug and Play and policy-based cluster and capacity management

• Load balancing advisory and run-time connection load balancing help redirect and balance work across the appropriate resources

• Oracle Quality of Service (QoS) Management for policy-based run-time management of resource allocation to database workloads to ensure service levels are met in order of business need under dynamic conditions. This functionality is available starting with Oracle Database 11g Release 2 (11.2.0.2).

• Oracle Enterprise Management support for Oracle ASM and Oracle ACFS, Grid Plug and Play, Cluster Resource Management, Oracle Clusterware and Oracle RAC Provisioning and patching

Figure 7-4 shows Oracle Database with Oracle RAC architecture. This figure shows Oracle Database with Oracle RAC architecture for a partitioned three-node database. An Oracle RAC database is connected to three instances on different nodes. Each instance is associated with a service: HR, Sales, and Call Center. The instances monitor each other by checking "heartbeats." Oracle Net Services provide client access to the Application/Web server tier at the top of the figure

Figure 7-4 Oracle Database with Oracle RAC Architecture

Description of "Figure 7-4 Oracle Database with Oracle RAC Architecture"

7.1.4.1 Oracle Database with Oracle RAC on Extended Clusters

Oracle Database with Oracle RAC architecture is designed primarily as a scalability and availability solution that resides in a single data center. It is possible, under certain circumstances, to build and deploy an Oracle RAC system where the nodes in the cluster are separated by greater distances. This architecture is referred to as an extended cluster.

An Oracle RAC extended cluster is an architecture that provides extremely fast recovery from a site failure and allows for all nodes, at all sites, to actively process transactions as part of single database cluster. For example, for a business that has a corporate campus, the extended Oracle RAC configuration could consist of individual Oracle RAC nodes located in separate buildings. Oracle RAC on an extended cluster provides greater availability than a local Oracle RAC cluster, but an extended cluster may not completely fulfill the disaster recovery requirements of your organization.

When the two data centers are located relatively close to each other, extended clusters can provide great protection for some disasters, but not all. You should determine if both sites are likely to be affected by the same disaster. For example, if the extended cluster configuration is set up properly, it can protect against disasters such as a local power outage, an airplane crash, or a flooded server room. However, an extended cluster cannot protect against all data corruptions or specific data failures that impact the database, or against comprehensive disasters such as earthquakes, hurricanes, and regional floods that affect a greater geographical area. (For complete disaster recovery and data protection, use the architecture shown in Figure 7-8.)

The advantages to using Oracle RAC on extended clusters include:

• Ability to fully use all system resources without jeopardizing the overall failover times for instance and node failures

• Extremely rapid recovery if one site fails

• All of the Oracle RAC benefits listed in Section 7.1.4

Note:

Although an extended cluster architecture can be effective and has been successfully implemented, you should implement it only in the environments (involving the distance, latency, and degree of protection) recommended in this discussion.

Figure 7-5 shows an Oracle RAC extended cluster for a configuration that has multiple active instances on six nodes at two different locations: three nodes at Site A and three at Site B. The public and private interconnects, and the Storage Area Network (SAN) are all on separate dedicated channels, with each one configured redundantly. For availability reasons, the Oracle database is a single database that is mirrored at both of the sites. Also, to prevent a full cluster outage if either site fails, the configuration includes a third voting disk on an inexpensive, low-end standard network file system (NFS) mounted device.

Figure 7-5 Oracle RAC Extended Cluster

Description of "Figure 7-5 Oracle RAC Extended Cluster"

See Also:

• Oracle Database High Availability Best Practices for information about configuring Oracle Database 11g with Oracle RAC on extended clusters

• White papers about extended (stretch) clusters and about using standard NFS to support a third voting disk on an extended cluster configuration at http://www.oracle.com/technetwork/database/clustering/overview/

7.1.5 Oracle Database with Oracle Data Guard

Oracle Data Guard is a high availability and disaster-recovery solution that provides very fast automatic failover (referred to as fast-start failover) in database failures, node failures, corruption, and media failures. Furthermore, the standby databases can be used for read-only access and subsequently for reader farms, for reporting, and for testing and development.

Although traditional solutions (such as backup and recovery from tape, storage-based remote mirroring, and database log shipping) can deliver some level of high availability, Oracle Data Guard provides the most comprehensive high availability and disaster recovery solution for Oracle databases.

Oracle Data Guard Advantages Over Traditional Solutions

Oracle Data Guard provides a number of advantages over traditional solutions, including the following:

• Fast, automatic or automated database failover for data corruptions, lost writes, and database and site failures

• Automatic corruption repair automatically replaces a corrupted block on the primary or physical standby by copying a good block from a physical standby or primary database

• Most comprehensive protection against data corruptions and lost writes on the primary database

• Reduced downtime for storage, Oracle ASM, Oracle RAC, system migrations and some platform migrations, and changes using Data Guard switchover

• Reduced downtime with Oracle Data Guard rolling upgrade capabilities

• Ability to off-load primary database activities—such as backups, queries, or reporting—without sacrificing the RTO and RPO ability to use the standby database as a read-only resource using the real-time query apply lag capability

• Ability to integrate non-database files using Oracle Database File System (DBFS) as part of the full site failover operations

• No need for instance restart, storage remastering, or application reconnections after site failures

• Transparency to applications

• Transparent and integrated support for application failover

• Effective network utilization

For data resident in Oracle databases, Oracle Data Guard, with its built-in zero-data-loss capability, is more efficient, less expensive, and better optimized for data protection and disaster recovery than traditional remote mirroring solutions. Oracle Data Guard provides a compelling set of technical and business reasons that justify its adoption as the disaster recovery and data protection technology of choice, over traditional remote mirroring solutions.

Oracle Data Guard Advantages Compared to Remote Mirroring Solutions

The following list summarizes the advantages of using Oracle Data Guard compared to using remote mirroring solutions:

• Better network efficiency—With Oracle Data Guard, only the redo data needs to be sent to the remote site and the redo data can be compressed to provide even greater network efficiency. However, if a remote mirroring solution is used for data protection, typically you must mirror the database files, the online redo log, the archived redo logs, and the control file. If the fast recovery area is on the source volume that is remotely mirrored, then you must also remotely mirror the flashback logs. Thus, compared to Oracle Data Guard, a remote mirroring solution must transmit each change many more times to the remote site.

• Better performance—Oracle Data Guard only transmits write I/Os to the redo log files of the primary database, whereas remote mirroring solutions must transmit these writes and every write I/O to data files, additional members of online log file groups, archived redo log files, and control files.

  Oracle Data Guard is designed so that it does not affect the Oracle database writer (DBWR) process that writes to data files, because anything that slows down the DBWR process affects database performance. However, remote mirroring solutions affect DBWR process performance because they subject all DBWR process write I/O's to network and disk I/O induced delays inherent to synchronous, zero-data-loss configurations.

  Compared to mirroring, Oracle Data Guard provides better performance and is more efficient, Oracle Data Guard always verifies the state of the standby database and validates the data before applying redo data, and Oracle Data Guard enables you to use the standby database for updates while it protects the primary database.

• Better suited for WANs—Remote mirroring solutions based on storage systems often have a distance limitation due to the underlying communication technology (Fibre Channel or ESCON (Enterprise Systems Connection)) used by the storage systems. In a typical example, the maximum distance between the systems connected in a point-to-point fashion and running synchronously can be only 10 kilometers. By using specialized devices, this distance can be extended to 66 kilometers. However, when the data centers are located more than 66 kilometers apart, you must use a series of repeaters and converters from third-party vendors. These devices convert ESCON or Fibre Channel to the appropriate IP, ATM, or SONET networks.

• Better resilience and data protection—Oracle Data Guard ensures much better data protection and data resilience than remote mirroring solutions. This is because corruptions introduced on the production database probably can be mirrored by remote mirroring solutions to the standby site, but corruptions are eliminated by Oracle Data Guard.

  For example, if a stray write occurs to a disk, or there is a corruption in the file system, or the host bus adaptor corrupts a block as it is written to disk, then a remote mirroring solution may propagate this corruption to the disaster-recovery site. Because Oracle Data Guard only propagates the redo data in the logs, and the log file consistency is checked before it is applied, all such external corruptions are eliminated by Oracle Data Guard. Automatic block repair may be possible, thus eliminating any downtime in an Oracle Data Guard configuration.

• Higher flexibility—Oracle Data Guard is implemented on pure commodity hardware. It requires only a standard TCP/IP-based network link between the two computers. There is no fancy or expensive hardware required. It also allows the storage to be laid out in a different fashion from the primary computer. For example, you can put the files on different disks, volumes, file systems, and so on.

• Better functionality—Oracle Data Guard provides full suite of data protection features that provide a much more comprehensive and effective solution optimized for data protection and disaster recovery than remote mirroring solutions. For example: Active Data Guard, Redo Apply for physical standby databases, and SQL Apply for logical standby databases, multiple protection modes, push-button automated switchover and failover capabilities, automatic gap detection and resolution, GUI-driven management and monitoring framework, cascaded redo log destinations.

• Higher ROI—Businesses must obtain maximum value from their IT investments, and ensure that no IT infrastructure is sitting idle. Oracle Data Guard is designed to allow businesses get something useful out of their expensive investment in a disaster-recovery site. Typically, this is not possible with remote mirroring solutions.

The recommended high availability and disaster-recovery architectures that use Oracle Data Guard are described in the following sections:

• Overview of Single Standby Database Architectures

• Overview of Multiple Standby Database Architectures

• Oracle Data Guard (Standby) Hub

7.1.5.1 Overview of Single Standby Database Architectures

A single standby database architecture consists of the following key traits and recommendations:

• Primary database resides in Site A.

• Standby database resides in Site B. If zero data loss is required with minimum performance impact on the primary database, then the best practice is to locate the secondary site within 200 miles of the primary database. Note, however, that the synchronous redo transport does not impose any physical distance limitation.

• Fast-start failover is recommended to provide automatic failover without user intervention and bounded recovery time. If the primary database uses the asynchronous redo transport, configure your maximum data loss tolerance or the Oracle Data Guard broker's FastStartFailoverLagLimit property to meet your business requirements. The observer (thin client watchdog) resides in the application tier and monitors the availability of the primary database. See Oracle Data Guard Broker for a detailed description of the observer.

• Use a physical standby database if read-only access is sufficient.

• Evaluate logical standby databases if additional indexes are required for reporting purposes and if your application only uses data types supported by logical standby database and SQL Apply.

Figure 7-6 shows the relationships between the primary database, target standby database, and the observer before, during, and after a fast-start failover. The figure shows the same Oracle Data Guard configuration in three different frames, as described in the following list:

1. The leftmost frame shows the configuration before fast-start failover occurs. Oracle Data Guard is operating in a steady state, with the primary database transmitting redo data to the target standby database and the observer monitoring the state of the entire configuration.

2. The center frame shows the configuration during fast-start failover. Disaster strikes the primary database, and its network connections to both the observer and the target standby database are lost. Upon detecting the break in communication, the observer attempts to reestablish a connection with the primary database for the amount of time defined by the FastStartFailoverThreshold property before initiating a fast-start failover. If the observer is unable to regain a connection to the primary database within the specified time, and the target standby database is ready for fast-start failover, then fast-start failover ensues.

3. The rightmost frame shows the configuration after fast-start failover has occurred. The fast-start failover has completed and the target standby database is running in the primary database role. After the former primary database has been repaired, the observer reestablishes its connection to that database and reinstates it as a new standby database. The new primary database starts transmitting redo data to the new standby database.

Figure 7-6 Primary and Standby Databases and the Observer During Fast-Start Failover

Description of "Figure 7-6 Primary and Standby Databases and the Observer During Fast-Start Failover"

The following list describes examples of Oracle Data Guard configurations using single standby databases:

• A national energy company uses a standby database located in a separate facility 10 miles away from its primary data center. Outages or data loss that could affect customer service and safety are avoided by using Oracle Data Guard synchronous transport and automatic failover (fast-start failover).

• An infrastructure services provider to the telecommunication industry uses a single standby database located over 400 miles away from the primary database configured for synchronous redo transport, enabling zero-data-loss failover for maximum data protection and high availability.

• A telecommunications provider uses asynchronous redo transport to synchronize a primary database on the West Cost of the United States, with a standby database on the East Coast, over 3,000 miles away. This scenario enables the provider to use existing data centers that are geographically isolated, offering a unique level of high availability.

• A global manufacturing company used Oracle Data Guard to replace storage-based remote mirroring and maintain a standby database at its recovery site 50 miles away from the primary site. Oracle Data Guard provides more comprehensive data protection and its more efficient network usage allows plenty of room to grow without the expense of upgrading its network.

7.1.5.2 Overview of Multiple Standby Database Architectures

This architecture is identical to the single-standby database architecture that was described in Section 7.1.5.1, except that there are multiple standby databases in the same Oracle Data Guard configuration. The following list describes some implementations for a multiple standby database architecture:

• Continuous and transparent disaster or high availability protection if an outage occurs at the primary database or the targeted standby database

• Reader farms or lookup databases

• Reporting databases

• Regional reporting or reader databases for better response time

• Synchronous redo transport that transmits to a more local standby database, and asynchronous redo transport that transmits to a more remote standby database for optimum levels of performance and data protection

• Transient logical standby databases (described in Section 3.6.3) for minimal downtime rolling upgrades

• Test and development clones using snapshot standby databases (described in Section 3.6.4)

• Scaling the configuration by creating additional logical standby databases or snapshot standby databases

Figure 7-7 shows the production database at the primary site and multiple standby databases at secondary sites. The figure shows Oracle Database with Oracle Data Guard architecture.

The production database is connected over the network to the physical standby database site and the logical standby database site (the standby databases may be at the same or different sites). The Oracle Data Guard broker communicates with the production database, the physical standby database, and the logical standby database.

The production database transmits redo data (either synchronously or asynchronously) to redo log files at the physical standby database. Then, the redo data is applied from the logs to the physical standby database, which backs up the redo data to physical media.

At the logical standby database, the redo data is transformed into SQL statements, which are applied to the logical standby database. The logical standby database may contain additional indexes and materialized views. Clients are connected to the logical standby database and can work with its data.

At the snapshot standby database redo data is received, but it is not applied until the snapshot standby database is reconverted to a physical standby database. The figure shows users making local updates to the snapshot standby database. These updates are discarded when the snapshot database is reconverted to a physical standby database.

Also, see Figure 5-2 for another example of a multiple standby database environment.

Figure 7-7 Oracle Database with Oracle Data Guard on Primary and Multiple Standby Sites

Description of "Figure 7-7 Oracle Database with Oracle Data Guard on Primary and Multiple Standby Sites"

See Also:

• Oracle Data Guard Concepts and Administration for more information about the various types of standby databases and to find out what data types are supported by logical standby databases

• Oracle Database High Availability Best Practices for configuration best practices

• The "Managing Data Guard Configurations Having Multiple Standby Databases - Best Practices" white paper, and other Oracle Data Guard white papers at

  http://www.oracle.com/goto/maa

The following list describes examples of Oracle Data Guard configurations using multiple standby databases:

• A world-recognized financial institution uses two remote physical standby databases for continuous data protection after failover. If the primary system should fail, the first standby database becomes the new primary database. The second standby database automatically receives data from the new primary database, insuring that data is protected at all times.

• A nationally recognized insurance provider in the U.S. maintains two standby databases in the same Oracle Data Guard configuration: one physical standby and one logical standby database. Their strategy further mitigates risk by maintaining multiple standby databases, each implemented using a different architectures—Redo Apply and SQL Apply.

• A world-recognized e-commerce site uses multiple standby databases—a mix of both physical and logical databases—both for disaster recovery and to scale out read performance by provisioning multiple logical standby databases using SQL Apply.

• A global provider of information services to legal and financial institutions uses multiple standby databases in the same Oracle Data Guard configuration to minimize downtime during major database upgrades and platform migrations.

Also, for large data centers with a need to support many applications with Oracle Data Guard requirements, you can build an Oracle Data Guard hub to reduce the total cost of ownership.

7.1.6 Oracle Database with Oracle Clusterware and Oracle Data Guard

If your business does not require the scalability and additional high availability benefits provided by Oracle RAC, but you still need all the benefits of Oracle Data Guard and cold cluster failover, then Oracle Database with Oracle Clusterware and Oracle Data Guard is a good compromise architecture. Oracle Clusterware cold cluster failover combined with Oracle Data Guard makes a tightly integrated solution in which failover to the secondary node in the cold cluster failover is transparent and does not require you to reconfigure the Oracle Data Guard environment or perform additional steps.

Figure 7-8 shows an Oracle Clusterware and Oracle Data Guard architecture that consists of a primary and a secondary site. Both the primary and secondary sites contain Oracle Application Servers, two database instances, and an Oracle database.

Figure 7-8 Oracle Clusterware (Cold Cluster Failover) and Oracle Data Guard

Description of "Figure 7-8 Oracle Clusterware (Cold Cluster Failover) and Oracle Data Guard"

In Figure 7-8:

• The application servers on the secondary site are connected to the WAN traffic manager by a dotted line to indicate that they are not actively processing client requests at this time. (The application server on the secondary site can be active and processing client requests such as queries if the standby database is a physical standby database with the Active Data Guard option enabled, or if it is a logical standby database.)

• Oracle Data Guard transmits redo data from the primary database to the secondary site to keep the databases synchronized.

• Oracle Clusterware manages the availability of both the user applications and Oracle databases.

• Oracle Clusterware provides tolerance of node failures, whereas Oracle Data Guard provides additional protection against data corruptions, lost writes, and database and site failures. (See Section 7.1.5 for a complete description.)

• Although cold cluster failover is not shown in Figure 7-8, you can configure it by adding a passive node on the secondary site.

7.1.7 Oracle Database with Oracle RAC One Node and Oracle Data Guard

Oracle RAC One Node provides relocation of Oracle RAC primary and standby databases configured with Oracle Data Guard (This functionality is available starting with Oracle Database 11g Release 2 (11.2.0.2)). Any database in a Data Guard configuration, whether a primary or standby database, can be an Oracle RAC One Node database.

For more information see the MAA white paper "Rapid Oracle RAC One Node Standby Deployment" at

http://www.oracle.com/goto/maa

7.1.8 Oracle Database with Oracle RAC and Oracle Data Guard

You can achieve the highest level of availability when using Oracle RAC and Oracle Data Guard and there is no need to make application changes to use these Oracle Database features. The combination of Oracle RAC and Oracle Data Guard provide the most comprehensive architecture for reducing downtime for scheduled outages and preventing, detecting, and recovering from unscheduled outages. This architecture is the recommended configuration for Maximum Availability Architecture (MAA).

To protect against site failures, the MAA recommends that Oracle RAC and Oracle Data Guard reside on separate systems (clusters) and data centers. Figure 7-9 shows the recommended MAA configuration, with Oracle Database, Oracle RAC, and Oracle Data Guard. Configuring symmetric sites is recommended to ensure that each site can accommodate the performance and scalability requirements of the application after any role transition. Furthermore, operational practices across role transitions are simplified when the sites are symmetric.

Figure 7-9 Oracle Database with Oracle RAC and Oracle Data Guard - MAA

Description of "Figure 7-9 Oracle Database with Oracle RAC and Oracle Data Guard - MAA"

7.1.9 Oracle Database with Oracle GoldenGate

Similar to using Oracle Data Guard in SQL Apply mode, Oracle GoldenGate can capture database changes, propagate them to destinations, and apply the changes at these destinations. Oracle GoldenGate is optimized for replicating data. Oracle GoldenGate can capture changes at a source database, and the captured changes can be propagated asynchronously to replica databases. A logical copy configured and maintained using Oracle GoldenGate is called a replica, not a logical standby database, because it provides many capabilities that are beyond the scope of the normal definition of a standby database.

You might choose to use Oracle GoldenGate to configure and maintain a logical copy of your production database. Although using Oracle GoldenGate might require additional work, it offers increased flexibility that might be necessary to meet specific business requirements.

Oracle Database with Oracle GoldenGate provides granularity and control over what is replicated and how it is replicated. It supports bidirectional replication, data transformations, subsetting, custom apply functions, and heterogeneous platforms. It also gives users complete control over the routing of change records from the primary database to a replica database. Oracle GoldenGate can capture data changes at the primary database or downstream at a replica database, thus enabling users to build hub-and-spoke network configurations that can support hundreds of replica databases.

Consider using Oracle Database with Oracle GoldenGate if one or more of the following conditions are true:

• Updates are required on both sites or databases, and the changes must be propagated bidirectionally.

• Site configurations are on heterogeneous platforms.

• Different character sets are required between the primary database and its replicas.

• Fine control of information and data sharing are required.

• More investment and expertise to build and maintain an integrated high availability solution is available.

For more information about constructing multiple-source replication environments, see the Oracle GoldenGate documentation.

You can configure Oracle GoldenGate with Oracle Data Guard to provide protection for the individual databases in the configuration.

7.3.1 Oracle Application Server High Availability Architectures

Oracle Application Server provides high availability and disaster recovery solutions for maximum protection against any kind of failure with flexible installation, deployment, and security options. These solutions are categorized into local high availability solutions that provide high availability in a single data center deployment, and disaster-recovery solutions, which are usually geographically distributed deployments that protect your applications from disasters such as floods or regional network outages.

At a high level, Oracle Application Server local high availability architectures include several active-active and active-passive architectures for the OracleAS middle-tier and the OracleAS Infrastructure. Although both types of solutions provide high availability, active-active solutions generally offer higher scalability and faster failover, although they tend to be more expensive. With either the active-active or the active-passive category, multiple solutions exist that differ in ease of installation, cost, scalability, and security.

Building on top of the local high availability solutions is the Oracle Application Server disaster recovery solution. This unique solution combines the proven Oracle Data Guard technology in Oracle Database with advanced disaster recovery technologies in the application realm to create a comprehensive disaster recovery solution for the entire application system. Disaster recovery solutions typically set up two homogeneous sites, one active and one passive. Each site is a self-contained system. The active site is generally called the production site, and the passive site is called the standby site. During normal operation, the production site services requests; in the event of a site failover or switchover, the standby site takes over the production role and all requests are routed to that site. To maintain the standby site for failover, not only must the standby site contain homogeneous installations and applications, data and configurations must also be synchronized constantly from the production site to the standby site. Oracle Application Server instances can be installed in either site as long as they do not interfere with the instances in the disaster recovery setup. Configurations and data must be synchronized regularly between the two sites to maintain homogeneity.

7.3.2 Redundant Architectures

Oracle Application Server provides redundancy by offering support for multiple instances supporting the same workload. These redundant configurations provide increased availability either through a distributed workload, through a failover setup, or both.

From the entry point to an Oracle Application Server system (content cache) to the back-end layer (data sources), all the tiers that are crossed by a request can be configured in a redundant manner with Oracle Application Server. The configuration can be an active-active configuration using Oracle Application Server Cluster or an active-passive configuration using Oracle Application Server Cold Cluster Failover.

7.3.3 High Availability Services in Oracle Application Server

The Oracle Application Server High Availability Guide describes the following high availability services in Oracle Application Server in detail:

• Process death detection and automatic restart

• Configuration management

• State replication

• Server load balancing and failover

• Backup and recovery

• Disaster recovery