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Diffstat (limited to 'doc')
| -rw-r--r-- | doc/design/metadata.mdwn | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/doc/design/metadata.mdwn b/doc/design/metadata.mdwn index 6b8b5bdc0..40c085cdc 100644 --- a/doc/design/metadata.mdwn +++ b/doc/design/metadata.mdwn @@ -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 |