notes.txt
author mpm@selenic.com
Thu, 30 Jun 2005 09:22:11 -0800
changeset 533 b8404005d6d5
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child 1308 2073e5a71008
permissions -rw-r--r--
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Some notes about Mercurial's design
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Revlogs:
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The fundamental storage type in Mercurial is a "revlog". A revlog is
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the set of all revisions to a file. Each revision is either stored
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compressed in its entirety or as a compressed binary delta against the
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previous version. The decision of when to store a full version is made
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based on how much data would be needed to reconstruct the file. This
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lets us ensure that we never need to read huge amounts of data to
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reconstruct a file, regardless of how many revisions of it we store.
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In fact, we should always be able to do it with a single read,
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provided we know when and where to read. This is where the index comes
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in. Each revlog has an index containing a special hash (nodeid) of the
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text, hashes for its parents, and where and how much of the revlog
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data we need to read to reconstruct it. Thus, with one read of the
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index and one read of the data, we can reconstruct any version in time
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proportional to the file size.
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Similarly, revlogs and their indices are append-only. This means that
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adding a new version is also O(1) seeks.
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Generally revlogs are used to represent revisions of files, but they
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also are used to represent manifests and changesets.
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Manifests:
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A manifest is simply a list of all files in a given revision of a
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project along with the nodeids of the corresponding file revisions. So
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grabbing a given version of the project means simply looking up its
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manifest and reconstruction all the file revisions pointed to by it.
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Changesets:
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A changeset is a list of all files changed in a check-in along with a
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change description and some metadata like user and date. It also
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contains a nodeid to the relevent revision of the manifest. Changesets
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and manifests are one-to-one, but contain different data for
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convenience.
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Nodeids:
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Nodeids are unique ids that are used to represent the contents of a
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file AND its position in the project history. That is, if you change a
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file and then change it back, the result will have a different nodeid
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because it has different history. This is accomplished by including
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the parents of a given revision's nodeids with the revision's text
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when calculating the hash.
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Graph merging:
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Nodeids are implemented as they are to simplify merging. Merging a
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pair of directed acyclic graphs (aka "the family tree" of the file
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history) requires some method of determining if nodes in different
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graphs correspond. Simply comparing the contents of the node (by
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comparing text of given revisions or their hashes) can get confused by
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identical revisions in the tree.
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The nodeid approach makes it trivial - the hash uniquely describes a
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revision's contents and its graph position relative to the root, so
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merge is simply checking whether each nodeid in graph A is in the hash
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table of graph B. If not, we pull them in, adding them sequentially to
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the revlog.
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Branching and merging:
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Everything in Mercurial is potentially a branch and every user
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effectively works in their own branch. When you do a checkout,
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Mercurial remembers what the parent changeset was and uses it for the
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next check in.
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To do a merge of branches in Mercurial, you check out the heads of the
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two branches into the same working directory which causes a merge to
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be performed, and then check in the result once you're happy with it.
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The resulting checkin will have two parents.
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It decides when a merge is necessary by first determining if there are
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any uncommitted changes in the working directory. This effectively
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makes the working directory a branch off the checked in version it's
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based on. Then it also determines if the working directory is a direct
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ancestor or descendent of the second version we're attempting to
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checkout. If neither is true, we simply replace the working directory
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version with the new version. Otherwise we perform a merge between the
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two versions.
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Merging files and manifests:
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We begin by comparing two versions manifests and deciding which files
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need to be added, deleted, and merged.
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Then for each file, we perform a graph merge and resolve as above.
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It's important to merge files using per-file DAGs rather than just
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changeset level DAGs as this diagram illustrates: 
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M   M1   M2
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AB
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 |`-------v     M2 clones M
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aB       AB     file A is change in mainline
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 |`---v  AB'    file B is changed in M2
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 |   aB / |     M1 clones M
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 |   ab/  |     M1 changes B
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 |   ab'  |     M1 merges from M2, changes to B conflict
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 |    |  A'B'   M2 changes A
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  `---+--.|
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      |  a'B'   M2 merges from mainline, changes to A conflict
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      `--.|
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         ???    depending on which ancestor we choose, we will have
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	        to redo A hand-merge, B hand-merge, or both
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                but if we look at the files independently, everything
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		is fine
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The result is a merged version in the working directory, waiting for
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check-in.
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Rollback:
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When performing a commit or a merge, we order things so that the
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changeset entry gets added last. We keep a transaction log of the name
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of each file touched and its length prior to the transaction. On
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abort, we simply truncate each file to its prior length. This is one
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of the nice properties of the append-only structure of the revlogs.
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We can also reuse this journal for "undo".
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Merging between repositories:
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One of the key features of Mercurial is the ability to merge between
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independent repositories in a decentralized fashion. Each repository
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can act as a read-only server or a client. Clients operating by
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pulling all branches that it hasn't seen from the server and adding
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them into its graph. This is done in two steps: searching for new
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"roots" and pulling a "changegroup"
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Searching for new "roots" begins by finding all new heads and
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searching backwards from those heads to the first unknown nodes in
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their respective branches. These nodes are the 'roots' that are used
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to calculate the 'changegroup': the set of all changesets starting at
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those roots. Mercurial takes pains to make this search efficient in
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both bandwidth and round-trips.
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Once the roots are found, the changegroup can be transferred as a
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single streaming transfer. This is organized as an ordered set of
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deltas for changesets, manifests, and files. Large chunks of deltas
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can be directly added to the repository without unpacking so it's
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fairly fast.