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Ioan Rogers > Prophet > doc/notes-on-merging


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Assumptions about merge sources

    - a source represents a branch 
    - a branch has had many revisions applied
    - revisions we apply to a branch from another branch are always applied UNCHANGED
    - in the event of a conflict, we will apply a "pre-changeset" to smooth the application and a "post-changeset" if the applied changeset doesn't match the desired outcome.

    on pull
        we end up applying every single source changeset this peer has seen since the last time we saw this peer, one by one
        we possibly apply another two changesets for each changeset to smooth the application

        we skip any source changeset we've seen from another source before

    on push,
        we publish each changeset we've made targetly.
            - including "after" fixup changesets
            - not including "before" fixup changesets.

assumptions about merge changesets

we can get (and hash): -original database uuid -original database revno -original database author -all changes


    Add ticket 1 - subject 'foo'; status 'new'


    j pull from a@1

    j@1 - foo;new
    a@1 - foo;new

    j@2  foo->bar
    a@2  foo->baz
    c@1 pull from j@2

        c now has:

    c@2 bar->frotz

    c@3 pull from a@2
        a hands c: a@2:foo->baz     


        to target c applies a pre-fixup:  
        to target c applies a@2:
        to target c applies a conflict resolution

    c@4 push to a@2

        beforehand, a's state: baz
        beforehand, c's state: frotz

        beforehand, c's unpushed transactions: 
            j@2 foo->bar
            c@2 bar->frotz
                    pre-fixup: frotz->foo
                    a@2: foo->baz
                    post-fixup: baz->frotz

            so, what do we push?
                fixup to get a to our earliest state unpushed. that's kind of stupid and results in us replaying everthing all the time.
                compute a single large changeset from a@HEAD to c@HEAD and apply that?

investigate "rumor" peer to peer replication

take 2

    there's a new peer I've not seen before. I need to merge in all their changes:

        if a changeset comes from a replica@rev lower than our last-seen version for replica, skip it

        for each of their changesets, import it, applying fixups as needed


    get the source list of all merge tickets.
    find all target revs which source has never seen.
        - genuine target changesets
        - third-party changesets that the source has no merge ticket for

        - skip "before" fixup changesets 
        - include "after" fixup changesets, since they're merge resolutions

    iterate over these revs the source has never seen.
    when the changeset applies cleanly, just apply it.

    when the changeset does _not_ apply cleanly, 
        - apply a 'before' fix up transaction
        - apply the original changeset
        - apply a merge resolution changeset.
            - TODO: this doesn't feel quite right
            - the resolution should be the same as the equivalent merge resolution transaction on the "pull" variant if it exists.
                            What info do we have here?
                                - record uuid
                                - nearest parent variant
                                - target variant
                                - source variant
                                - information from the 'future (target head)' about the eventual desired outcome 
    - audit stage:
        compare target head and source head. - they should now be identical, since the last transactions we replayed were all target merge fixup changesets. (is that true?)
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