With a proper HA solution in place, you will know when something goes wrong, and the system will try to react to that without human intervention. Once something does go wrong though, the purpose of the HA software is not to allow you to ignore the problem but rather to maintain stability long enough to give you a chance to diagnose the problem and fix it. Remember, an HA environment coupled with a volume management package is a complement to your normal backups, not a substitute.
HA is a great answer for mission-critical applications and environments that require close to 100 percent uptime. Properly configured and administered, an HA solution can be valuable to solve even disaster recovery situations in a timely manner.
On the negative side, a particular HA system may not provide the necessary solution for a particular application. As far as I know, there is no HA implementation that can perform application migration and duplicate an application state at failure time. If such a feature is an absolute requirement, it may be necessary to turn to fault-tolerant systems, which are briefly introduced in an earlier section.
Cost also may be a concern. HA packages are pricey and may not necessarily target a particular environment properly. Remember, HA requires a functional duplicate of your primary server, plus additional hardware and software to complete the equation. Study your options carefully and completely before making any decision.
Typically, a base configuration includes a one-to-one fail over with no databases or anything special (such as a module to support a third-party application) on a low-end box. If you install the software yourself, the cost runs around $10,000. A midrange configuration with vendor installation of a fairly powerful server with one or two applications (with a module) runs around $45,000. Finally, at the high end of the spectrum are high-end systems such as a clustered 64-processor system with multiple instances of a database. With multiple modules and top-of-the-line support and installation, the price is around $150,000. This may seem expensive, but the cost is worth it in the long run.
The price of a HA software package obviously varies from vendor to vendor and changes over time, but this gives you a rough estimate to work with. These numbers are valid at the time this book was printed.
Modules are supplied by most HA vendors to take care of monitoring and failing-over popular applications. Modules have been developed to work with databases, web servers, firewalls, network monitoring applications, and certain other products. If you have an application for which there is no module, you can write your own simple script. Think about what is required to run the application: network connectivity, access to a certain port on the box, assurance that the processes are running, and on and on. You can do this yourself, or you can contact your HA vendor to see if an appropriate module is in the works or if they could write one for you.
This may sound like a good idea, but it is not a smart one. Building your own HA solution by coding it in-house can result in a poorly tested and supported product that doesn’t have the advantage of regular vendor updates and certified modules. And in the end, it may not be cost effective. An alternative is to purchase a base HA product, and then tweak a module (or even write your own with scripts) to support a given application.
Setting up an HA environment has taken your organization from a reactive posture to a proactive one. Ensuring it is done properly and combining it with other building blocks will be a never-ending process. Do not turn your back on your HA system and assume that you are done, because you are not. Monitoring the system continuously (manually or automatically) is necessary. Pay attention to new technology, as there may be another building block in the near future. And, as always, do your backups.