2020-12-23 19:21:33 +00:00
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{- git-annex vector clock utilities
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-
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- Copyright 2017-2020 Joey Hess <id@joeyh.name>
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-
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- Licensed under the GNU AGPL version 3 or higher.
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-}
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module Annex.VectorClock.Utility where
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import Data.Time.Clock.POSIX
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import Types.VectorClock
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import Utility.Env
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import Utility.TimeStamp
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deal better with clock skew situations, using vector clocks
* Deal with clock skew, both forwards and backwards, when logging
information to the git-annex branch.
* GIT_ANNEX_VECTOR_CLOCK can now be set to a fixed value (eg 1)
rather than needing to be advanced each time a new change is made.
* Misuse of GIT_ANNEX_VECTOR_CLOCK will no longer confuse git-annex.
When changing a file in the git-annex branch, the vector clock to use is now
determined by first looking at the current time (or GIT_ANNEX_VECTOR_CLOCK
when set), and comparing it to the newest vector clock already in use in
that file. If a newer time stamp was already in use, advance it forward by
a second instead.
When the clock is set to a time in the past, this avoids logging with
an old timestamp, which would risk that log line later being ignored in favor
of "newer" line that is really not newer.
When a log entry has been made with a clock that was set far ahead in the
future, this avoids newer information being logged with an older timestamp
and so being ignored in favor of that future-timestamped information.
Once all clocks get fixed, this will result in the vector clocks being
incremented, until finally enough time has passed that time gets back ahead
of the vector clock value, and then it will return to usual operation.
(This latter situation is not ideal, but it seems the best that can be done.
The issue with it is, since all writers will be incrementing the last
vector clock they saw, there's no way to tell when one writer made a write
significantly later in time than another, so the earlier write might
arbitrarily be picked when merging. This problem is why git-annex uses
timestamps in the first place, rather than pure vector clocks.)
Advancing forward by 1 second is somewhat arbitrary. setDead
advances a timestamp by just 1 picosecond, and the vector clock could
too. But then it would interfere with setDead, which wants to be
overrulled by any change. So it could use 2 picoseconds or something,
but that seems weird. It could just as well advance it forward by a
minute or whatever, but then it would be harder for real time to catch
up with the vector clock when forward clock slew had happened.
A complication is that many log files contain several different peices of
information, and it may be best to only use vector clocks for the same peice
of information. For example, a key's location log file contains
InfoPresent/InfoMissing for each UUID, and it only looks at the vector
clocks for the UUID that is being changed, and not other UUIDs.
Although exactly where the dividing line is can be hard to determine.
Consider metadata logs, where a field "tag" can have multiple values set
at different times. Should it advance forward past the last tag?
Probably. What about when a different field is set, should it look at
the clocks of other fields? Perhaps not, but currently it does, and
this does not seems like it will cause any problems.
Another one I'm not entirely sure about is the export log, which is
keyed by (fromuuid, touuid). So if multiple repos are exporting to the
same remote, different vector clocks can be used for that remote.
It looks like that's probably ok, because it does not try to determine
what order things occurred when there was an export conflict.
Sponsored-by: Jochen Bartl on Patreon
2021-08-03 20:45:20 +00:00
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startVectorClock :: IO (IO CandidateVectorClock)
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2020-12-23 19:21:33 +00:00
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startVectorClock = go =<< getEnv "GIT_ANNEX_VECTOR_CLOCK"
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where
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go Nothing = timebased
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go (Just s) = case parsePOSIXTime s of
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deal better with clock skew situations, using vector clocks
* Deal with clock skew, both forwards and backwards, when logging
information to the git-annex branch.
* GIT_ANNEX_VECTOR_CLOCK can now be set to a fixed value (eg 1)
rather than needing to be advanced each time a new change is made.
* Misuse of GIT_ANNEX_VECTOR_CLOCK will no longer confuse git-annex.
When changing a file in the git-annex branch, the vector clock to use is now
determined by first looking at the current time (or GIT_ANNEX_VECTOR_CLOCK
when set), and comparing it to the newest vector clock already in use in
that file. If a newer time stamp was already in use, advance it forward by
a second instead.
When the clock is set to a time in the past, this avoids logging with
an old timestamp, which would risk that log line later being ignored in favor
of "newer" line that is really not newer.
When a log entry has been made with a clock that was set far ahead in the
future, this avoids newer information being logged with an older timestamp
and so being ignored in favor of that future-timestamped information.
Once all clocks get fixed, this will result in the vector clocks being
incremented, until finally enough time has passed that time gets back ahead
of the vector clock value, and then it will return to usual operation.
(This latter situation is not ideal, but it seems the best that can be done.
The issue with it is, since all writers will be incrementing the last
vector clock they saw, there's no way to tell when one writer made a write
significantly later in time than another, so the earlier write might
arbitrarily be picked when merging. This problem is why git-annex uses
timestamps in the first place, rather than pure vector clocks.)
Advancing forward by 1 second is somewhat arbitrary. setDead
advances a timestamp by just 1 picosecond, and the vector clock could
too. But then it would interfere with setDead, which wants to be
overrulled by any change. So it could use 2 picoseconds or something,
but that seems weird. It could just as well advance it forward by a
minute or whatever, but then it would be harder for real time to catch
up with the vector clock when forward clock slew had happened.
A complication is that many log files contain several different peices of
information, and it may be best to only use vector clocks for the same peice
of information. For example, a key's location log file contains
InfoPresent/InfoMissing for each UUID, and it only looks at the vector
clocks for the UUID that is being changed, and not other UUIDs.
Although exactly where the dividing line is can be hard to determine.
Consider metadata logs, where a field "tag" can have multiple values set
at different times. Should it advance forward past the last tag?
Probably. What about when a different field is set, should it look at
the clocks of other fields? Perhaps not, but currently it does, and
this does not seems like it will cause any problems.
Another one I'm not entirely sure about is the export log, which is
keyed by (fromuuid, touuid). So if multiple repos are exporting to the
same remote, different vector clocks can be used for that remote.
It looks like that's probably ok, because it does not try to determine
what order things occurred when there was an export conflict.
Sponsored-by: Jochen Bartl on Patreon
2021-08-03 20:45:20 +00:00
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Just t -> return (pure (CandidateVectorClock t))
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2020-12-23 19:21:33 +00:00
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Nothing -> timebased
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Lower precision of timestamps in git-annex branch
This can reduce the size of the branch by up to 8%. My test was
running git-annex add 1000 times on one file each.
Lots of different high-resolution timestamps were recorded before
and eliminating those, after packing, the git repo was 8% smaller.
Due to the use of vector clocks, high resolution timestamps are
not necessary to make clear which information is most recent when
eg, a value is changed repeatedly in the same second. In such a
case, the vector clock will be advanced to the next second after
the last modification. For example, running
git-annex numcopies 1; git-annex numcopies 2
The first will record the current second, while the next records
the second after that even if it runs in the same second.
As for conflicting information written to two different clones of the
repository, this will make git-annex sometimes pick information that
was written earlier in a second over information written later in the
same second. Usually git-annex does not write conflicting information,
but there are some cases where it could. Eg, storing an object on a remote
can update the remote state log with some state. If two repos both store the
same object, and end up storing different remote state for some reason,
this can result in one that ran a tiny bit later winning. Such a situation
seems unlikely to be user visible. And a small amount of clock skew could
already result in such things.
The only case I can think of where this might be a user visible change
is if a configuration command like git-annex numcopies is being run
in 2 clones of a repository on the same machine at very
close to the same time. Then the user will know which they ran last,
and git-annex won't.
If that did become a problem, this could be dialed back to eg log
milliseconds with still some space saving.
2023-12-11 19:04:06 +00:00
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-- Avoid using fractional seconds in the CandidateVectorClock.
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-- This reduces the size of the packed git-annex branch by up
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-- to 8%.
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--
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-- Due to the use of vector clocks, high resolution timestamps are
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-- not necessary to make clear which information is most recent when
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-- eg, a value is changed repeatedly in the same second. In such a
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-- case, the vector clock will be advanced to the next second after
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-- the last modification.
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timebased = return $
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CandidateVectorClock . truncateResolution 0 <$> getPOSIXTime
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