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Aucb.619 fa.editor-p utcsrgv!utzoo!decvax!ucbvax!C70:editor-people Sat Mar 6 09:37:33 1982 File locking with NLS >From KLH@MIT-AI Thu Mar 4 16:48:48 1982

The NLS (now AUGMENT) system on 10X/20X handles the file locking problem rather well. Basically, NLS files are always readable (subject to usual protection bits, etc); if someone else is working on a file that you want to read, you are allowed to look at it, but you are also notified that "File FOO.NLS is being modified by HACKER", and you are not allowed to see the mods that HACKER is making, until that user has explicitly "updated" the file, at which point everything is merged in. From the viewpoint of HACKER, nothing unusual is going on; the file is not considered to be modified until a user actually makes some change to it, and the modifier is always looking at the modified version. One unusual aspect of the implementation is that the "partial copy" (containing mods) can stay around as long as HACKER wants (weeks or months, across logouts/crashes/etc), thus write-locking the original file until the updates are finished.

Note that the mechanism is implemented in a single user program, rather than the operating system. It does depend on shared page mapping.

I don't have time to explain the workings in more detail, so if people have questions I hope that there is another AUGMENT user on this list who will volunteer to answer them.

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