If the process that writes a given HTML file takes some noticeable amount of time between start and finish, and you want to make sure that web visitors will only see the complete form of the file, something like the following ought to be all you need:

• open a new file for output with a name that won't be visible to web visitors, or create it with the intended public name but in a different path (not publicly exposed) on the same disk;
• once the output to the file is complete and the file is closed (and you're sure there weren't any errors), rename the file to the intended path/name in the public web directory -- it now becomes instantly visible to the next person who (re)loads the url for the file, and from the visitor's perspective, it is never partial or incomplete.

If the web server is a unix box, "renaming" a file from one disk to another (e.g. from /tmp to /public_html) really means copying it, which will not be instantaneous -- maybe not as slow as the process that writes the file in the first place, but still not as fast as renaming a file so that it stays on the same volume, which just involves moving an inode entry from one directory to another. (This is likely to be true on any OS, even those that don't have things called "inodes".)

As for reading config files (it took me a while to get the connection between the first and second paragraph)... If you're worried that a process reading a config file might get an incomplete or "transient" version of the data -- and if this is a persistent, pernicious concern -- you might consider making up a little table (database or flat file) that stores file names with data checksums. Read the file once, compute its checksum, and if that doesn't match the checksum in the table, treat it as an error condition. (You could try reading it again after a delay, to see if the problem persists, but if it fails twice, you might as will quit.)

This would require a little more infrastructure for managing your config files, to make sure that the checksum table is updated every time a file is intentionally added, deleted or altered.

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