Re: Exclusive File Access

FROM : Malte Tancred
DATE : Thu Nov 21 13:47:11 2002

On thursday, nov 21, 2002, at 12:28 Europe/Stockholm, mw wrote:
> Here's my idea. Let's say that Person A opens a file (that is all well
> and
> simple, right?), and Person B opens the same file at about the same
> time
> (doesn't really matter). Person A makes some changes. The program
> checks
> either the modification date of the file, or some other
> "change-tracker"
> (such as an MD5 hash, which I believe was suggested previously). It
> sees
> that the file hasn't changed, so it does a normal save (overwriting the
> original file with the new information). Now, let's say that Person B
> makes
> some changes to a completely different part of the file. When he
> saves, the
> program performs the same check, and finds that there have been
> modifications to the file between the time it was opened by Person B
> and the
> current save. So The program reads the whole file into memory. Now, we
> have
> 2 copies of the file in RAM, the original one (modified by Person A,
> though), and the one modified by Person B. So the program starts the
> process
> of comparing the elements of 2 file contents in RAM, and then writes
> the
> verified changes back to file.

What happens if Person C beats Person B to it, before Person B's
program is finished comparing the version saved by Person A with Person
B's own version?

Concurrent versioning is a difficult problem to solve. There are tools
dedicated to handle these situations. Use these tools and keep your own
programs simple.

Cheerio,
Malte
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