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Problem: interrupted operations
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===============================
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Many version control systems tend to have trouble when operations are
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interrupted. This can happen in various ways:
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* program hits a bug and aborts
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* tree is naively copied (e.g. by cp/tar) while an operation is in
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We can reduce the window during which operations can be interrupted:
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most importantly, by receiving everything off the network into a
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staging area, so that network interruptions won't leave a job half
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complete. But it is not possible to totally avoid this, because the
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power can always fail.
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I think we can reasonably rely on flushing to stable storage at
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various points, and trust that such files will be accessible when we
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I think by using this and building from the bottom up there are never
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any broken pointers in the branch metadata: first we add the file
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versions, then the inventory, then the revision and signature, then
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link them into the revision history. The worst that can happen is
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that there will be some orphaned files if this is interrupted at any
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rsync is just impossible in the general case: it reads the files in a
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fairly unpredictable order, so what it copies may not be a tree that
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existed at any particular point in time. If people want to make
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backups or replicate using rsync they need to treat it like any other
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* make a copy which will not be updated, and rsync from that
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* lock the database while rsyncing
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The operating system facilities are not sufficient to protect against
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all of these. We cannot satisfactorily commit a whole atomic
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transaction in one step.
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Operations might be updating either the metadata or the working copy.
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The working copy is in some ways more difficult:
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* Other processes are allowed to modify it from time to time in
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If they modify it while bazaar is working then they will lose, but
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we should at least try to make sure there is no corruption.
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* We can't atomically replace the whole working copy. We can
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(semi) atomically updated particular files.
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* If the working copy files are in a wierd state it is hard to know
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whether that occurred because bzr's work was interrupted or because
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the user changed them.
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(A reasonable user might run ``bzr revert`` if they notice
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something like this has happened, but it would be nice to avoid
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We don't want to leave things in a broken state.
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Solution: write-ahead journaling?
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=================================
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One possibly solution might be write-ahead journaling:
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Before beginning a change, write and flush to disk a description of
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what change will be made.
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Every bzr operation checks this journal; if there are any pending
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operations waiting then they are completed first, before proceeding
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with whatever the user wanted. (Perhaps this should be in a
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separate ``bzr recover``, but I think it's better to just do it,
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perhaps with a warning.)
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The descriptions written into the journal need to be simple enough
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that they can safely be re-run in a totally different context. They
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must not depend on any external resources which might have gone
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If we can do anything without depending on journalling we should.
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It may be that the only case where we cannot get by with just
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ordering is in updating the working copy; the user might get into a
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difficult situation where they have pulled in a change and only half
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the working copy has been updated. One solution would be to remove
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the working copy files, or mark them readonly, while this is in
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progress. We don't want people accidentally writing to a file that
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needs to be overwritten.
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Or perhaps, in this particular case, it is OK to leave them in
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pointing to an old state, and let people revert if they're sure they
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want the new one? Sounds dangerous.
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Aaron points out that this basically sounds like changesets. So
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before updating the history, we first calculate the changeset and
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write it out to stable storage as a single file. We then apply the
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changeset, possibly updating several files. Each command should check
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whether such an application was in progress.
added
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doc/interrupted.txt
