Mercurial's decentralized development model can be confusing to new users. This page attempts to illustrate some of the basic concepts. See the ["Tutorial"] for step-by-step instructions.
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What's in a Repository
Mercurial [:Repository:repositories] contain a [:WorkingDirectory:working directory] coupled with a store:
The store contains the complete history of the project. Unlike traditional [:SCM:SCMs], where there's only one central copy of this history, every working directory is paired with a private copy of the history. This allows development to go on in parallel.
The working directory contains a copy of the project's files at a given point in time (eg rev 2), ready for editing. Because [:Tag:tags] and [:.hgignore:ignored files] are revision-controlled, they are also included.
Committing Changes
When you [:Commit:commit], the state of the working directory relative to its [:Parent:parents] is recorded as a new [:ChangeSet:changeset] (also called a new "[:Revision:revision]"):
Note here that revision 4 is a [:Branch:branch] of revision 2, which was the revision in the working directory. Now revision 4 is the working directory's parent.
Revisions, Changesets, Heads, and Tip
Mercurial groups related changes to multiple files into single atomic [:ChangeSet:changesets], which are revisions of the whole project. These each get a sequential [:RevisionNumber:revision number]. Because Mercurial allows distributed parallel development, these revision numbers may disagree between users. So Mercurial also assigns each revision a global [:ChangeSetID:changeset ID]. Changeset IDs are 40-digit hexadecimal numbers, but they can be abbreviated to any unambiguous prefix, like "e38487".
Branches and [:Merge:merges] in the revision history can occur at any point. Each unmerged branch creates a new [:Head:head] of the revision history. Here, revisions 5 and 6 are heads. Mercurial considers revision 6 to be the [:Tip:tip] of the repository, the head with the highest revision number. Revision 4 is a [:MergeChangeset:merge changeset], as it has two parent changesets (revisions 2 and 3).
Cloning, Making Changes, Merging, Pulling and Updating
Let's start with a user Alice, who has a repository that looks like:
Bob [:Clone:clones] this repo, and ends up with a complete, independent, local copy of Alice's store and a clean checkout of the tipmost revision d in his working directory:
Bob can now work independently of Alice. He then [:Commit:commits] two changes e and f:
Alice then makes her own change g in parallel, which causes her repository store to diverge from Bob's, thus creating a [:Branch:branch]:
Bob then [:Pull:pulls] Alice's repo to synchronize. This copies all of Alice's changes into Bob's repository store (here, it's just a single change g). Note that Bob's working directory is not changed by the pull:
Because Alice's g is the newest head in Bob's repository, it's now the tip.
Bob then does a [:Merge:merge], which combines the last change he was working on (f) with the tip in his working directory. Now, his working directory has two parent revisions (f and g):
After examining the result of the merge in his working directory and making sure the merge is perfect, Bob commits the result and ends up with a new [:MergeChangeset:merge changeset] h in his store:
Now if Alice pulls from Bob, she will get Bob's changes e, f, and h into her store:
Note that Alice's working directory was not changed by the pull. She has to do an [:Update:update] to synchronize her working directory to the merge changset h. This changes the parent changeset of her working directory to changeset h and updates the files in her working directory to revision h.
Now Alice and Bob are fully synchronized again.
A Decentralized System
Mercurial is a completely decentralized system, and thus has no internal notion of a central repository. Thus users are free to define their own topologies for sharing changes (see CommunicatingChanges):
What Mercurial can't do
Many SVN/CVS users expect to host related projects together in one repository. This is really not what hg was made for, so you should try a different way of working. This especially means, that you cannot check out only one directory of a repository. If you absolutely need to host multiple projects in a kind of meta-repository though, you could try the ForestExtension.
For a hands-on introduction to using Mercurial, see the ["Tutorial"].