4
Problem: interrupted operations
5
===============================
7
Many version control systems tend to have trouble when operations are
8
interrupted. This can happen in various ways:
12
* program hits a bug and aborts
18
* tree is naively copied (e.g. by cp/tar) while an operation is in
21
We can reduce the window during which operations can be interrupted:
22
most importantly, by receiving everything off the network into a
23
staging area, so that network interruptions won't leave a job half
24
complete. But it is not possible to totally avoid this, because the
25
power can always fail.
27
I think we can reasonably rely on flushing to stable storage at
28
various points, and trust that such files will be accessible when we
31
I think by using this and building from the bottom up there are never
32
any broken pointers in the branch metadata: first we add the file
33
versions, then the inventory, then the revision and signature, then
34
link them into the revision history. The worst that can happen is
35
that there will be some orphaned files if this is interrupted at any
38
rsync is just impossible in the general case: it reads the files in a
39
fairly unpredictable order, so what it copies may not be a tree that
40
existed at any particular point in time. If people want to make
41
backups or replicate using rsync they need to treat it like any other
44
* make a copy which will not be updated, and rsync from that
46
* lock the database while rsyncing
48
The operating system facilities are not sufficient to protect against
49
all of these. We cannot satisfactorily commit a whole atomic
50
transaction in one step.
52
Operations might be updating either the metadata or the working copy.
54
The working copy is in some ways more difficult:
56
* Other processes are allowed to modify it from time to time in
59
If they modify it while bazaar is working then they will lose, but
60
we should at least try to make sure there is no corruption.
62
* We can't atomically replace the whole working copy. We can
63
(semi) atomically updated particular files.
65
* If the working copy files are in a weird state it is hard to know
66
whether that occurred because bzr's work was interrupted or because
67
the user changed them.
69
(A reasonable user might run ``bzr revert`` if they notice
70
something like this has happened, but it would be nice to avoid
73
We don't want to leave things in a broken state.
76
Solution: write-ahead journaling?
77
=================================
79
One possibly solution might be write-ahead journaling:
81
Before beginning a change, write and flush to disk a description of
82
what change will be made.
84
Every bzr operation checks this journal; if there are any pending
85
operations waiting then they are completed first, before proceeding
86
with whatever the user wanted. (Perhaps this should be in a
87
separate ``bzr recover``, but I think it's better to just do it,
88
perhaps with a warning.)
90
The descriptions written into the journal need to be simple enough
91
that they can safely be re-run in a totally different context. They
92
must not depend on any external resources which might have gone
95
If we can do anything without depending on journalling we should.
97
It may be that the only case where we cannot get by with just
98
ordering is in updating the working copy; the user might get into a
99
difficult situation where they have pulled in a change and only half
100
the working copy has been updated. One solution would be to remove
101
the working copy files, or mark them readonly, while this is in
102
progress. We don't want people accidentally writing to a file that
103
needs to be overwritten.
105
Or perhaps, in this particular case, it is OK to leave them in
106
pointing to an old state, and let people revert if they're sure they
107
want the new one? Sounds dangerous.
109
Aaron points out that this basically sounds like changesets. So
110
before updating the history, we first calculate the changeset and
111
write it out to stable storage as a single file. We then apply the
112
changeset, possibly updating several files. Each command should check
113
whether such an application was in progress.
added
removed
doc/interrupted.txt
