notes.txt
author mpm@selenic.com
Thu, 12 May 2005 01:20:43 -0800
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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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Graph resolving:
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Mercurial does branching by copying (or COWing) a repository and thus
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keeps everything nice and linear within a repository. However, when a
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merge of repositories (a "pull") is done, we may often have two head
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revisions in a given graph. To keep things simple, Mercurial forces
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the head revisions to be merged.
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It first finds the closest common ancestor of the two heads. If one is
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a child of the other, it becomes the new head. Otherwise, we call out
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to a user-specified 3-way merge tool.
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Merging files, manifests, and changesets:
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We begin by comparing changeset DAGs, pulling all nodes we don't have
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in our DAG from the other repository. As we do so, we collect a list
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of changed files to merge.
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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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After we've merged files, we merge the manifest log DAG and resolve
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additions and deletions. Then we are ready to resolve the changeset
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DAG - if our merge required any changes (the new head is not a
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decendent of our tip), we must create a new changeset describing all
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of the changes needed to merge it into the tip.
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Merge performance:
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The I/O operations for performing a merge are O(changed files), not
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O(total changes) and in many cases, we needn't even unpack the deltas
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to add them to our repository (though this optimization isn't
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necessary).
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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.
0
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Remote access:
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Mercurial currently supports pulling from "serverless" repositories.
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Simply making the repo directory accessibly via the web and pointing
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hg at it can accomplish a pull. This is relatively bandwidth efficient
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but no effort has been spent on pipelining, so it won't work
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especially well over LAN yet.
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It's also quite amenable to rsync, if you don't mind keeping an intact
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copy of the master around locally.
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Also note the append-only and ordering properties of the commit
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guarantee that readers will always see a repository in a consistent
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state and no special locking is necessary. As there is generally only
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one writer to an hg repository, there is in fact no exclusion
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implemented yet. 
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Some comparisons to git:
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Most notably, Mercurial uses delta compression and repositories
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created with it will grow much more slowly over time. This also allows
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it to be much more bandwidth efficient. I expect repos sizes and sync
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speeds to be similar to or better than BK, given the use of binary diffs.
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Mercurial is roughly the same performance as git in some areas and is
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faster in others as it keeps around more metadata. One example is
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listing and retrieving past versions of a file, which it can do
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without reading all the changesets. This metadata will also allow it
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to perform better merges as described above.
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