When MySQL was first released publicly, it was at version 3.11.1. It's currently in the version 4.0.x series for stable releases and the 4.1.x series for development releases. Updates appear fairly often and are issued for both the stable and development series. The pace of ongoing development raises the question for the MySQL administrator as to whether you should upgrade your existing MySQL installation when new releases appear. This section provides some guidelines to help you make this decision. The first thing you should do when a new release appears is to find out how it differs from previous releases. To make sure you're aware of new releases, subscribe to the announce@lists.mysql.com mailing list. Each announcement includes the new change notes, so this is a good way to remain apprised of new developments. (Alternatively, check the "Change Notes" appendix in the MySQL Reference Manual to familiarize yourself with what's new.) Then ask yourself the following questions:
If the answer to all of these questions is No, you don't have any compelling reason to upgrade. If the answer to any of them is Yes, you may want to go ahead. At this point, it's often useful to wait a few days and watch the MySQL mailing list to see what other people report about the release. Was the upgrade helpful? Were bugs or other problems found? Some other factors to consider that may help you make your decision are as follows:
If you're still not sure whether to upgrade, you can always test the new server independently of your current server. You can do this either by running it in parallel with your production server or by installing it on a different machine. It's easier to maintain independence between servers if you use a different machine because you have greater freedom to configure it as you want. If you elect to run the new server in parallel with an existing server on the same host, be sure to configure it with unique values for parameters such as the installation location, the data directory, and the network port and socket on which the server listens for connections. See the "Running Multiple Servers" section earlier in this chapter. In either case, you'll probably want to test the new server using a copy of the data in your existing databases. See the "Backing Up and Copying Databases" section earlier in this chapter for instructions on copying databases. If you do decide to upgrade, see if there are any remarks in the "Change Notes" appendix in the MySQL Reference Manual about any special steps you must take when upgrading. Usually there aren't any, but check anyway. This is particularly important if the new release introduces a behavior that is incompatible with earlier releases. If you upgrade to a version that is not backward-compatible with older versions and then decide to revert to the earlier version, it may not be so easy to downgrade. For example, if you upgrade from MySQL 4.0.x to 4.1.x and convert your tables to 4.1 format, they'll be incompatible with 4.0.x servers. Downgrading can be difficult in this case.
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