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Figure 16.2 A simple state transition diagram.
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The fewer states your system can occupy, the easier it is to understand. As you develop a transition diagram, you may find a few states that are essentially the same, or that with minor process modifications they can be made to be the same. For example, in Figure 16.2, there s no real reason to have OK and Primary Network Down be separate states; since both have essentially the same event transitions, they can be merged together. When you reduce the number of possible outcomes, you make it easier to diagnose a system failure. Well-defined states, whether they describe the system as operating or failed, are just fine. It s the unknown or ambiguous states that can be trouble. Are you sure that your system won t end up in the twilight zone if several things fail at the same time Do you know that you ll end up in a state where the FMS kicks in and tries to right the boat, or that you ll be able to begin a manual intervention process that gets administrators in front of consoles quickly If you end up doing neither, you re looking at a system that may be down for an indeterminate period of time. You need to draw the line between automation and manual processes. If you can determine those states in which your FMS will throw up its hands and cry, Uncle! Call the system administrators! you ll know where you need strong escalation and people management processes. (Of course, if you actually hear voices coming from your FMS, it may be time for a nice, long vacation.) Not all issues are necessarily apparent when the first design wave is through. If you haven t proven that the designed system can be operated, or that it meets the operational requirements of your staff, you ll need to walk through a state diagram to show that you have all of the necessary expertise at the right locations.
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Going through this process early on can save you time and money later. A design error that leaves you without a path to recovery may be more complex and more difficult to fix once developers have started. If you need the application team to accommodate certain failure cases, they re going to need to know up front. The other benefit to state transition diagrams is that they let you visually compare the trade-offs of different solutions. Let s say you have a 10-minute automatic failover whenever a network becomes unusable, driven by monitoring scripts that watch network traffic and communicate among themselves to time the failover (we talked about this case more in 9). If the downtime you are trying to shorten down to 10 minutes would replace a 15-minute manual process, you may decide that the extra few minutes of recovery time are acceptable in the one transition that occurs during an infrequent failure, or that the work required to improve failover time by 5 minutes is just not worth it.
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Testing is the castor oil in the design phase and the maintenance process of a highly available system. It s always the first activity that goes away (except perhaps for documentation) at crunch time. In many shops, of course, it s always crunch time, and little or no testing ever takes place. Unfortunately, without adequate testing there is no way to tell if your procedures and safeguards are going to work. Good testing always finds holes and inefficiencies in procedures. Testing catches those quick Oh, I ll put together a more permanent solution next week fixes that always seem to work their way into systems, even in the best-run shops. Since systems are always evolving, with applications and procedures being added and modified all the time, testing must be done on a regular basis. Failovers should be tested at least once a quarter. In order for testing to be thorough, it must include the following:
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All documented procedures must be tested. In an emergency, it is likely that written procedures will be followed blindly. If they don t work exactly right, the result may well be chaos. The only thing worse than no documentation is bad documentation. Since it is virtually impossible to target documentation at inexperienced personnel, the target audience should be experienced system administrators who are not familiar with the particular idiosyncrasies of your local systems and environment. When testing your standards implementation, make sure that systems and applications meet the documented local standards. Know what all test results should be before the test is run. Test your backups by restoring a couple of them on a regular basis. If you cannot restore your backups in a test environment, you won t be able to restore them when a disaster occurs. Test any critical process whose failure may cause downtime. Consider less obvious potential SPOFs, like building access scanners or locked doors.
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Besides periodic procedural testing, it is also important to test applications and how they perform in high-stress situations. Always remember that long service times and failures are indistinguishable to the average user. How does your system perform as load averages increase, especially when they do so over long periods of time What breaks first Fix those things, and move on to second- and third-level failures. Bad assumptions made in buffer or counter sizes will often be the first things exposed. Test the sort of problem that may not bite you for a while. The Y2K problem is an excellent example of this. Programs that were written in the 1970s that
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