Sun Java Communications Suite 5 Deployment Planning Guide

Understanding Remote Site Failover

Remote site failover is the ability to bring up a service at a site that is WAN connected to the primary site in the event of a catastrophic failure to the primary site. There are several forms of remote site failover and they come at different costs.

For all cases of remote site failover, you need additional servers and storage capable of running all or part of the users’ load for the service installed and configured at the remote site. By all or part, this means that some customers might have priority users and non-priority users. Such a situation exists for both ISPs and enterprises. ISPs might have premium subscribers, who pay more for this feature. Enterprises might have divisions that provide email to all of their employees but deem this level of support too expensive for some portion of those users. For example, an enterprise might choose to have remote site failover for mail for those users that are directly involved in customer support but not provide remote site failover for people who work the manufacturing line. Thus, the remote hardware must be capable of handling the load of the users that are allowed to access remote failover mail servers.

While restricting the usage to only a portion of the user base reduces the amount of redundant server and storage hardware needed, it also complicates configuration and management of fail back. Such a policy can also have other unexpected impacts on users in the long term. For instance, if a domain mail router is unavailable for 48 hours, the other MTA routers on the Internet will hold the mail destined for that domain. At some point, the mail will be delivered when the server comes back online. Further, if you do not configure all users in a failover remote site, then the MTA will be up and give permanent failures (bounces) for the users not configured. Lastly, if you configure mail for all users to be accepted, then you have to fail back all users or set up the MTA router to hold mail for the nonfunctional accounts while the failover is active and stream it back out once a failback has occurred.

Potential remote site failover solutions include:

There are a variety of costs to these solutions, from hardware and software, to administrative, power, heat, and networking costs. These are all fairly straightforward to account for and calculate. Nevertheless, it is difficult to account for some costs: the cost of mistakes when putting a rarely practiced set of procedures in place, the inherent cost of downtime, the cost of data loss, and so forth. There are no fixed answers to these types of costs. For some customers, downtime and data loss are extremely expensive or totally unacceptable. For others, it is probably no more than an annoyance.

In doing remote site failover, you also need to ensure that the remote directory is at least as up to date as the messaging data you are planning to recover. If you are using a restore method for the remote site, the directory restore needs to be completed before beginning the message restore. Also, it is imperative that when users are removed from the system that they are only tagged as disabled in the directory. Do not remove users from the directory for at least as long as the messaging backup tapes that will be used might contain those users’ data.

Questions for Remote Site Failover

Use the following questions to assist you in planning for remote site failover:

Note –

Do not use both Sun Cluster for local HA and Veritas software for remote site failover. Sun Cluster does not support remote site failover at this time.

Also, do not allow the software to automatically failover from the primary site to the backup site. The possibility for false positive detection of failure of the primary site from the secondary site is too high. Instead, configure the software to monitor the primary site and alert you when it detects a failure. Then, confirm that the failure has happened before beginning the automated process of failing over to the backup site.