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data types and serialization for metadata

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A very haskell commit! Just data types, instances to serialize the metadata
to a nice format, and QuickCheck tests.

This commit was sponsored by Andreas Leha.
This commit is contained in:
Joey Hess 2014-02-12 17:54:28 -04:00
parent 655549af91
commit 1b79d18a40
4 changed files with 221 additions and 1 deletions
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doc/design/metadata.mdwn
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@ -126,6 +126,39 @@ Note that any of these filenames can in theory conflict. May need to use
`.variant-*` like sync does on conflict to allow 2 files with same name in
same filtered branch.
## union merge properties
While the storage could just list all the current values of a field on a
line with a timestamp, that's not good enough. Two disconnected
repositories can make changes to the values of a field (setting and
unsetting tags for example) and when this is union merged back together,
the changes need to be able to be replayed in order to determine which
values we end up with.
To make that work, we log not only when a field is set to a value,
but when a value is unset as well.
For example, here two different remotes added tags, and then later
a tag was removed:
1287290776.765152s tag +foo +bar
1287290991.152124s tag +baz
1291237510.141453s tag -bar
The end result is that tags foo and baz are set. This can be simplified:
1291237510.141453s tag +foo +baz -bar
Note the reuse of the most recent timestamp in the simplified version,
rather than putting in the timestamp when the simplification was done.
This ensures that is some other repo is making changes, they won't get
trampled over. For example:
1291237510.141453s tag +foo +baz -bar
1291239999.000000s tag +bar -foo
Now tags bar and baz are set.
# efficient metadata lookup
Looking up metadata for filtering so far requires traversing all keys in