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| 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. ''(Translations: [:BrazilianPortugueseUnderstandingMercurial:Brazilian Portuguese], [:ChineseUnderstandingMercurial:Chinese], [:FrenchUnderstandingMercurial:French], [:GermanUnderstandingMercurial:German], [:ItalianUnderstandingMercurial:Italian], [:JapaneseUnderstandingMercurial:Japanese], [:KoreanUnderstandingMercurial:Korean], [:SpanishUnderstandingMercurial:Spanish] )'' [[TableOfContents]] == What's in a Repository == Mercurial [:Repository:repositories] contain a [:WorkingDirectory:working directory] coupled with a store: {{{#!dot digraph G { compound=true; rankdir = LR background="#999999"; subgraph cluster_1 { style=filled; color="#eeeeee"; node [shape=box,style=filled,color=lightgray]; "rev 0" -> "rev 1" -> "rev 2" -> "rev 3"; label = "store"; } subgraph cluster_0 { style=filled; color=lightgrey; node [style=filled,color=white]; edge [style=invis]; "main.c"-> "main.h" -> ".hgignore" -> ".hgtags" label="working directory"; } "rev 2" -> ".hgtags" [lhead = cluster_0 constraint=false] } }}} 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 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 [:Revision:revision]: {{{#!dot digraph G { compound=true; rankdir = LR background="#999999"; subgraph cluster_1 { style=filled; color="#eeeeee"; node [shape=box,style=filled,color=lightgray]; "rev 0" -> "rev 1" -> "rev 2" -> "rev 3"; "rev 2" -> "rev 4"; label = "store"; } subgraph cluster_0 { style=filled; color=lightgrey; node [style=filled,color=white]; edge [style=invis]; "main.c"-> "main.h" -> ".hgignore" -> ".hgtags" label="working directory"; } "rev 2" -> ".hgtags" [style = dotted lhead = cluster_0 constraint=false] "rev 4" -> ".hgtags" [lhead = cluster_0 constraint=false] ".hgtags" -> "rev 4" [color = red label = "commit" ltail = cluster_0 constraint=false] } }}} Note here that revision 4 is a '''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". {{{#!dot digraph { rankdir = LR node [shape=box] "rev 0:838e" -> "rev 1:34ef" -> "rev 2:4563" "rev 1:34ef" -> "rev 3:fe56" "rev 2:4563" -> "rev 4:ac98" "rev 3:fe56" -> "rev 4:ac98" "rev 4:ac98" -> "rev 5:0345" "rev 4:ac98" -> "rev 6:19e3 (tip)" label="example history" } }}} 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. == Cloning, Making Changes, Merging, and Pulling == Let's start with a user Alice, who has a store that looks like: {{{#!dot digraph { label="Alice's Repo" rankdir = LR node [shape=box] a->b->c->d } }}} Bob [:Clone:clones] this repo, and ends up with a complete copy of Alice's store (though his working directory is independent!): {{{#!dot digraph { label="Bob's Repo" rankdir = LR node [shape=box] a->b->c->d } }}} Bob then [:Commit:commits] a couple changes: {{{#!dot digraph { label="Bob's Repo" rankdir = LR node [shape=box] a->b->c->d->e->f e [color=blue] f [color=blue] } }}} Alice then makes her own change in parallel: {{{#!dot digraph { label="Alice's Repo" rankdir = LR node [shape=box] a->b->c->d->g g [color=red] } }}} Bob then [:Pull:pulls] Alice's repo to synchronize. This copies all of Alice's changes into Bob's repo: {{{#!dot digraph { label="Bob's Repo" rankdir = LR node [shape=box] a->b->c->d->e->f e [color=blue] f [color=blue] d->g g [color=red;label="g (tip)"] } }}} 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, commits the result, and ends up with: {{{#!dot digraph { label="Bob's Repo" rankdir = LR node [shape=box] a->b->c->d->e->f e [color=blue] f [color=blue] d->g g [color=red] f->h g->h h [color=green;label="h (tip)"] } }}} Now if Alice '''pulls''' from Bob, she will get Bob's changes e, f, and h, and they will be fully synchronized: {{{#!dot digraph { label="Alice's Repo" rankdir = LR node [shape=box] a->b->c->d->g d->e->f e [color=blue] f [color=blue] g [color=red] f->h g->h h [color=green;label="h (tip)"] } }}} == 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): {{{#!dot digraph { Alice -> Central Central -> Alice Bob -> Central Alice -> Bob Alice -> Carl Carl -> Central Bob -> Carl Carl -> Bob "Carl's Laptop" -> Carl Carl -> "Carl's Laptop" "Carl's Laptop" -> Central Central [style=fill;color=blue;label="Main Public Repo"] label="A Mercurial Network" } }}} == 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 thou, you could try the ForestExtension. For a hands-on introduction to using Mercurial, see the ["Tutorial"]. |
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