Information about the changeset branch integration

Between versions 0.14 and 0.15 monotone underwent a major reorganization of its fundamental data structures. This means pre-0.14 and post-0.15 versions of monotone are strictly incompatible:

  • databases are incompatible
  • network service is incompatible

Unfortunately we were simply not able to make transparent interoperation possible. Too much changed. However, it is possible to migrate from the pre-0.14 representation to the post-0.15 representation. The migration is slightly lossy, but it is the best we are able to provide.

What changed

The change added two new objects to the vocabulary of monotone:

  • changesets
  • revisions

They are both text objects, like manifests, which monotone reads and writes for its own benefit. Revisions are, like manifests, also hashed with SHA1 to produce revision IDs. Changesets are not hashed, because they generally do not appear outside of revisions.

These two new objects replace three previous objects: patch sets, rename certs, and ancestry certs. Now information about renaming and patch application is stored in changesets, and information about ancestry is stored in revisions. Obviously these objects store the information slightly differently than their predecessors. See the next section for motivation.

The observable affect for the end user is moderate: most monotone commands work "the way they used to", except that now they take revision IDs where they used to take manifest IDs. Don't worry about this too much: every revision has a reference to a specific manifest as well, revision IDs still always refer to a specific "tree state", they just happen to refer to a bit more than that as well, as we'll see.

Why it changed

The old objects were buggy. There were a bunch of subtle and difficult-to-fix problems with the way we were representing ancestry and renames before:

  • renames of directories had to be inferred, and this could fail.

  • nontrivial forms of rename chaining didn't work, and it wasn't clear that they could be made to work treating renames as we did:

    a->b and b->a
    a->b and c->a
    a/b->a/c and a->d (depending on order)
  • you could have multiple irreconcilable rename certs attached to the same manifest edge, and this would cause monotone to crash.

  • ancestry could form cycles, if you ever returned to a previous manifest state.

  • ancestry could accidentally jump from one "historical thread" to another if you happened to pass through the same manifest state; two branches acquiring the same state would always look like a merge even when it was a coincidence.

  • it wasn't clear what monotone should do if you assigned different trust to existant branch, ancestry, and rename certs.

These problems were with us for quite a while, even if you didn't encounter them. The changes at 0.15 fix the whole family of bugs by removing the troubling abstractions and replacing them with more robust ones. In particular:

  • Unlike ancestry certs, revisions are chained. Each revision contains as part of its content the revision IDs of its predecessor revisions, so there can never be a loop in the graph. Each new point in the graph gets a new, previously-unseen ID.

  • Revisions are not subject to trust evaluation, they are statements of fact. They happen to be stating a fact which describes an entire historical chain of events, but there is only one trust evaluation: branch membership. The evaluation of branch membership covers the entire history of events described by a revision.

  • Changesets have much stronger reasoning facilities about overlapping renames, directory renames, etc. They actually break down into a "simultaneous rearrangement of inodes" abstraction, in memory, including directories as first class objects. This abstraction has a clear algebra for concatenation, inversion and parallel merger.

How do I migrate?

Before you start any of this, make a backup of the database. All the world's warnings about making backups seem to go un-heeded, but seriously, it's just a single "cp" command. How lazy can you get?

$ cp mydatabase.db mydatabase.db.pre-changesets

Please be sure to do so. After backing up, there are three required steps (and one optional step) to the migration. The first step is to dump and reload your database, to get something that the new version of SQLite can read. Note that you need to keep your old version of monotone around at least long enough to perform this step:

$ old-mtn --db=mydatabase.db db dump > mydatabase.dump
$ mtn --db=new-mydatabase.db db load < mydatabase.dump

The next step is the usual "thing you do when the SQL schema changes":

$ mtn --db=new-mydatabase.db db migrate

Then there is the new step: telling monotone to convert its old representation of manifests to changesets. It will synthesize a lot of new changesets based on what it can find in your database. The newly synthesized information is a slightly lossy version of the information already found in your database. It is lossy in the following respects:

  • All existing rename/edit events will be degraded to delete/add events

  • All existing manifest certs will be reissued as revision certs, but signed with your public key. All previous RSA-key-identity information is thereby lost.

Assuming you're prepared to live with this loss, the next step is to run a special, otherwise undocumented "changesetification" command:

$ mtn --db=new-mydatabase.db db changesetify

This should build the new changeset and revision structure and issue all the necessary certs. It will therefore take a while on a large database. It will not, however, delete the old manifest certs: the new version of monotone will simply ignore them, but they continue to exist in your database, should you wish to recover some of their information for forensic purposes in the future. If you truly wish to remove all manifest certs as well (they are all subsumed by revision certs now, anyways) you can issue one further command:

$ mtn --db=new-mydatabase.db db execute "delete from manifest_certs"

This will purge the manifest_certs table of all its entries, but as we said this step is purely optional. No harm will come from old manifest certs remaining in your database.

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