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Managing Serviceguard Fifteenth Edition > Appendix C Designing Highly Available Cluster Applications

Minimizing Planned Downtime


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Planned downtime (as opposed to unplanned downtime) is scheduled; examples include backups, systems upgrades to new operating system revisions, or hardware replacements. For planned downtime, application designers should consider:

  • Reducing the time needed for application upgrades/patches.

    Can an administrator install a new version of the application without scheduling downtime? Can different revisions of an application operate within a system? Can different revisions of a client and server operate within a system?

  • Providing for online application reconfiguration.

    Can the configuration information used by the application be changed without bringing down the application?

  • Documenting maintenance operations.

    Does an operator know how to handle maintenance operations?

When discussing highly available systems, unplanned failures are often the main point of discussion. However, if it takes 2 weeks to upgrade a system to a new revision of software, there are bound to be a large number of complaints.

The following sections discuss ways of handling the different types of planned downtime.

Reducing Time Needed for Application Upgrades and Patches

Once a year or so, a new revision of an application is released. How long does it take for the end-user to upgrade to this new revision? This answer is the amount of planned downtime a user must take to upgrade their application. The following guidelines reduce this time.

Provide for Rolling Upgrades

Provide for a “rolling upgrade” in a client/server environment. For a system with many components, the typical scenario is to bring down the entire system, upgrade every node to the new version of the software, and then restart the application on all the affected nodes. For large systems, this could result in a long downtime.

An alternative is to provide for a rolling upgrade. A rolling upgrade rolls out the new software in a phased approach by upgrading only one component at a time. For example, the database server is upgraded on Monday, causing a 15 minute downtime. Then on Tuesday, the application server on two of the nodes is upgraded, which leaves the application servers on the remaining nodes online and causes no downtime. On Wednesday, two more application servers are upgraded, and so on. With this approach, you avoid the problem where everything changes at once, plus you minimize long outages.

The trade-off is that the application software must operate with different revisions of the software. In the above example, the database server might be at revision 5.0 while the some of the application servers are at revision 4.0. The application must be designed to handle this type of situation.

For more information about the rolling upgrades, see Appendix E “Software Upgrades ”, and the Release Notes for your version of Serviceguard at http://docs.hp.com -> High Availability.

Do Not Change the Data Layout Between Releases

Migration of the data to a new format can be very time intensive. It also almost guarantees that rolling upgrade will not be possible. For example, if a database is running on the first node, ideally, the second node could be upgraded to the new revision of the database. When that upgrade is completed, a brief downtime could be scheduled to move the database server from the first node to the newly upgraded second node. The database server would then be restarted, while the first node is idle and ready to be upgraded itself. However, if the new database revision requires a different database layout, the old data will not be readable by the newly updated database. The downtime will be longer as the data is migrated to the new layout.

Providing Online Application Reconfiguration

Most applications have some sort of configuration information that is read when the application is started. If to make a change to the configuration, the application must be halted and a new configuration file read, downtime is incurred.

To avoid this downtime use configuration tools that interact with an application and make dynamic changes online. The ideal solution is to have a configuration tool which interacts with the application. Changes are made online with little or no interruption to the end-user. This tool must be able to do everything online, such as expanding the size of the data, adding new users to the system, adding new users to the application, etc. Every task that an administrator needs to do to the application system can be made available online.

Documenting Maintenance Operations

Standard procedures are important. An application designer should make every effort to make tasks common for both the highly available environment and the normal environment. If an administrator is accustomed to bringing down the entire system after a failure, he or she will continue to do so even if the application has been redesigned to handle a single failure. It is important that application documentation discuss alternatives with regards to high availability for typical maintenance operations.

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