Historically, organizations had to shut down business applications when upgrading hardware and/or software. That wasn't a problem when business hours were Monday to Friday, nine to five, holidays excluded. Upgrades could be done on long weekends. That's not the case for most companies today. Many businesses now need to serve their Internet and international operations 24x7. High availability (HA) software, such as iTera HA from Vision Solutions, provides a solution.

HA software maintains an up-to-date replica of all System i data and system objects on a backup system that is available to take over operations when the primary server is unavailable. This capability provides an opportunity to perform upgrades with virtually no downtime.

Consider the following scenario: During normal operations, server "A" runs the business' operations and server "B" is a backup replica kept current by HA software. To perform a hardware or software upgrade on server "A" without downtime, you perform a "role swap" that switches users to server "B" and makes it the primary server. This results in only momentary downtime that may not even be noticeable to users. Server "A", which is now offline, can then be upgraded without affecting normal operations.

Viewed from a high level, HA software performs three ongoing tasks:

• Capture changes made to the primary server.
• Transmit the changes to the backup using remote journaling.
  
• Apply the changes to the backup server.

When server "A" is offline for an upgrade, the HA software holds the changes applied to server "B" while it is acting as the primary server. When the upgrade of server "A" is complete and it is back online, the HA software transmits the held changes and resynchronizes the two servers. Another role swap then reverts server "A" to its role as the operational system.

If you perform testing on server "A" before declaring the upgrade process successful, you may want to create or change some data and/or system objects that you don't want reflected on your production system. iTera HA provides a solution that can revert the changes to their original state. Its "healing" process can detect out-of-synch data and resynchronize on a record-by-record basis. Thus, when you bring server "A" back online after an upgrade, iTera HA will automatically detect what has been added, changed, or deleted during testing and heal the data and system objects on server "A" so that it is again an exact replica of server "B," which had been acting as the primary server during the upgrade process.

Other HA software offers synch-check and resynchronization functionality, but they operate on the object, file, or library levels. Because some objects can be very large, possibly even terabytes, resynchronization takes too long to be practical in this situation. In contrast, because the iTera HA healing process operates at the record level, significantly less data needs to be resynchronized. Healing can, therefore, be accomplished very rapidly. Customers have said that, when using this method, upgrades that consumed two to three days in the past took only two to three hours and incurred virtually no user downtime.

When a software upgrade also requires a database conversion, iTera HA's healing process may also help you refresh the databases without any downtime, provided that you have sufficient disk space on the backup server to hold both the old and the new versions of your databases. (The length limitation of this article precludes a full discussion of this process here. Please contact Vision Solutions if you require more information.)

One requisite function of HA software is to create an initial replica as a starting point for the HA processes. This function can be useful if you are moving to a new System i primary or backup server. In this case, the HA software can automatically create a complete replica of all System i data and system objects on the new system when it is brought online. You can also take advantage of the parallel testing features as discussed previously.

The above discussion describes how to upgrade just the primary server. The job of upgrading both the primary and backup servers is little different. You start by performing the above procedure on either server and repeat it on the other server later, using the HA software's roll swap facility as required to place the server that is to be upgraded in the backup roll.

See the MC Press Buyer's Guide for more information about iTera and Vision Solutions.

Rick Ayres is vice president of business development at Vision Solutions. Contact him at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it or 706.864.8654.