Member File Access When members of a fault tolerance group depend on file access, it is crucial that each member use a private copy of every relevant file. In particular, consider these two guidelines. • Do not share files. Ensure that two members never modify the same file. Sharing a file can corrupt the data in that file (even when the members do not write to it simultaneously). Using local files prevents this kind of corruption. • Store files on the local host computer, not across the network on a file server—and especially not on a separate network or segment. In situations where network disconnect causes a member to activate, that same disconnect can separate the activating backup member from network-mounted files. Using local files keeps the data where it is needed. Copying Context Files In some programs, an activating member must establish its operating context by reading files left by a previously active member that has failed. When using this technique, be sure to make a local, private copy of the file. When the content of the context files changes frequently, inactive members can periodically copy the context files, so that a network disconnect does not preclude access when it is needed. Upgrading Versions When upgrading a program to a new version, it is sometimes important that old and new versions run simultaneously. Be sure that the two process instances do not reference the same file. Instead, make a local copy of the file for the new version to use.
