Design - Mercurial

(for details from a source code point of view, see WhatGoesWhere)

Data structures

Nodeids are unique ids that represent the contents of a file and its position in the project history. See ["Nodeid"].

A revlog, for example .hg/data/somefile.d, is the most important data structure and represents all versions of a file. See ["Revlog"].

A manifest describes the state of a project by listing each file and its nodeid to specify which version. See ["Manifest"].

A changeset lists all files changed in a checkin along with a change description and metadata like user and date. See ["ChangeSet"].

Putting it all together

We now have enough information to see how this all works together. To look up a given revision of a file:

look up the changeset in the changeset index
reconstruct the changeset data from the revlog
look up the manifest nodeid from the changeset in the manifest index
reconstruct the manifest for that revision
find the nodeid for the file in that revision
look up the revision of that file in the file's index
reconstruct the file revision from the revlog

If we want to go the other way and find the changeset associated with a given file revision, we follow the linkrev.

   .--------linkrev-------------.
   v                            |
.---------.    .--------.    .--------.
|changeset| .->|manifest| .->|file    |---.
|index    | |  |index   | |  |index   |   |--.
`---------' |  `--------' |  `--------'   |  |
    |       |      |      |     | `-------'  |
    V       |      V      |     V    `-------'
.---------. |  .--------. |  .---------.
|changeset|-'  |manifest|-'  |file     |
|data     |    |data    |    |revision |
`---------'    `--------'    `---------'

Tracking Working Directory State

The other piece of Mercurial is the working directory. Mercurial tracks various information about the working directory. See WorkingDirectory.

Merging

See ["Merge"].

References

"Towards a Better SCM: Revlog and Mercurial", Matt Mackall ([attachment:Presentations/ols-mercurial-paper.pdf PDF])
[http://hgbook.red-bean.com/hgbookch4.html#x8-640004 "Behind the scenes"] in [http://hgbook.red-bean.com/hgbook.html "Distributed revision control with Mercurial"], Bryan O’Sullivan