Mercurial for Perforce users
(Might also be usable as "Perforce for Mercurial users")
This document focuses on the similarities and differences of Mercurial with Perforce, a popular commercial CentralisedSCM. Some parts of the general discussion about Mercurial and other centralized SCM systems apply here too. Please refer to CvsConcepts for more details.
Architecture
Perforce is a quite good commercial CentralisedSCM. In Perforce the repository for all users and branches lives on a central server. The command-line utility p4 or a graphical client called p4v is used for communicating with the server and working with local/repository files. Perforce users create a "client" and check files out from the repository to a working directory in their local file system. Perforce requires network access to the central repository (the "depot") in order to perform any operation. It is very fundamental that multiple users can work on the same code at the same time. As a consequence of this files are write protected until they are opened for edit (and thus given an advisory lock), and only changes to the newest version can be submitted to the repository.
By contrast, Mercurial is a DistributedSCM. Each working directory is self-contained and contains the files being worked on as well as a complete Repository. The command hg is used for working with files/repositories. You do not need to be connected to any server in order to perform any operation. You can even share your changes with others without being directly connected - file transfer or email can be used. It is a basic assumption that users (or uses) have their own repository and pulls other changes to it. This one writer / multiple readers design allows files to be edited without coordination with server, and changes to any version can be submitted.
Revisions
Perforce changelist numbers are similar to Mercurial revision numbers. Perforce repositories are central and common for all users, and the changelist numbers are thus globally unique and used as such. A single Perforce server can be used to manage multiple "depots", and in this case changelist numbers are unique across all the depots managed by the same Perforce server!
Mercurial RevisionNumbers describe the history of a local repository and will thus be different even in repositories which share a part or even all of their change history. To uniquely identify ChangeSets Mercurial uses ChangeSetIDs instead of RevisionNumbers; changeset IDs are hashes of the content and are thus considered unique.
Modules, branching and tagging
Perforce basically knows nothing about branches - but does it quite well anyway. Any file can be branched anywhere in the depot, even though branch specs often are used to specify branching paths. Client specs are used to decide which part of the depot to put in the working directory.
Branches
In Mercurial, each WorkingDirectory is a potential "branch". Branches don't have to be anticipated in advance as in Perforce, they just happen naturally if two changesets are based on the same parent changeset (see also Heads). In Mercurial, a changeset can have two parents (in contrast, Perforce doesn't record this information, as all merge and branch operations are file based). It doesn't even matter whether this happens in the same or in different repositories. Mercurial can assign a branch name to a changeset (see NamedBranches), but conceptually, branches can and do exist in a Mercurial repository even if they are not named.
Mercurial can Merge two changesets (not files) by using the hg merge command, which does a p4 integrate and p4 resolve in one step for all files in both Manifests needing a classical file merge. See also the ImergeExtension, which provides a file by file (an incremental) resolve process similar to the one provided by p4 resolve of Perforce. In Mercurial, a revision of a file can have two parent revisions (see Design for how this is technically implemented), which happens if the file had to be merged as a consequence of the hg merge of two Mercurial changesets.
Tags
A Perforce tag is a symbolic name for a subset of files at certain revisions in a depot.
A Mercurial Tag is just a symbolic name for a changeset and thus the whole manifest.
Clients
In Perforce, a client is a collection of directories that you can check out under one name.
The equivalent Mercurial concept is the Repository. The ForestExtension can be used to something similar to Perforces mapping of arbitrary repository files into a working directory.
Converting a Perforce repository
Conversion from Perforce was added in Mercurial 1.2. See ConvertingRepositories.
The Perfarce extension allows you to pull changelists from Perforce, and to push Mercurial changesets back to it.
Collaborating with other people
With Perforce, the standard way to share changes with other people is that all contributors are licensed users on the same server and simply check them in.
In Mercurial, collaboration is much more flexible. You can share changes in any number of ways, including (but not limited to) the following:
- Export one or more changesets and
put the exported PatchFiles on a server for someone to download
- email the patchfile to a maintainer or mailing list
- copy the patchfile onto a memory stick and give them to someone else
Push (or rsync) the changesets into a repository from which other people can Pull them
- Clone a copy of the repository onto a CD-ROM and hand it to someone else
Perforce-to-Mercurial Cheat sheet
This table gives hints to some related Perforce and Mercurial commands or concepts. Some commands are the same. Some commands just use different words or syntax. Some commands have no direct counterpart, so there the relation is kind of "instead you could use this command which does something else".
Perforce |
Description |
Mercurial |
Description |
Concepts |
|
|
|
client/workspace |
read-only files, all meta info is in server depot |
WorkingDirectory (WD) |
plain files and .hg store with dirstate and history |
depot |
central repository |
.hg store |
the whole Repository is in each WD |
changelist |
changes to a set of files, depot-global |
changes to a set of files, local to .hg store |
|
base version |
the version a file is based on |
parent version |
the changeset a WD is based on |
head |
the latest change in a branch, new changes must be based on this |
the changeset that happens to have appeared last in .hg store |
|
branch |
location in depot dedicated to one project |
WD or Head |
usually a branch is a WD, but a repo can also contain branches |
Working on files |
changes are pending in WD until commit |
|
changes are pending in WD until commit |
p4 edit |
open a file for edit |
- |
hg detects automatically |
p4 add |
add files |
hg add |
add files |
p4 delete |
remove files |
hg remove |
remove files |
p4 diff |
show diff for files opened for edit |
hg diff |
show diff for all changed files |
$P4DIFF |
external (graphical) diff program |
external (graphical) diff program |
|
p4 opened |
show scheduled changes |
hg status |
show scheduled changes |
- |
|
hg status |
list unknown files - they should be added or listed in .hgignore |
schedule add added and remove removed files |
hg addremove |
automatic add+remove if not .hgignore'd |
|
p4 integ; p4 delete |
move file keeping history |
hg rename |
move file keeping history |
p4 integ |
copy file keeping history |
hg copy |
copy file keeping history |
p4 revert |
drop all changes, revert to base version |
hg revert |
restore files content to any version from repo, no meta data changed |
p4 revert |
drop all changes, revert to base version |
hg update -C |
drop all meta changes, revert WD to known state |
p4 sync |
update p4 client to other repo version |
hg update |
update hg WD to other repo version |
Looking in repository |
changes nothing |
|
changes nothing |
p4 print |
show repository file |
hg cat |
show repository file |
p4 annotate |
show modification info for each line in a file |
hg annotate |
show modification info for each line in a file |
p4 changes |
show history of (branch of) repo |
hg log |
show history of repo |
p4 filelog |
show history of file |
hg log |
show history of file |
p4 tag |
add a tag for a changeset |
hg tag |
add a Tag for a changeset |
p4 have |
show info for client files |
hg parent |
show parent of working directory |
p4 have |
show info for client files |
hg identify |
show info for working directory |
p4 have |
show info for client files |
hg manifest |
list files in repo version |
p4 files |
show info for repo files |
hg manifest |
list files in repo version |
p4 branches |
list branch specifications - that is officially related branches |
hg heads |
show heads, that is changes not merged |
p4 describe |
show changelist with its changes |
hg log -p |
show patch-like changelist |
Working with repository |
all changes are pending until submit |
|
changes local repo immediately - and perhaps WD |
p4 client |
create a new branch+client from scratch |
hg init |
create hg WD from scratch |
p4 client; p4 sync |
create a new workspace for existing repo |
hg clone |
create copy of a repo and create corresponding WD |
p4 client; p4 integ |
create a new branch+client based on other |
hg clone |
create new hg WD/branch based on other |
p4 integ; p4 resolve |
get changes from other p4 branch |
hg pull; hg merge |
get changes from other repo |
p4 integ |
integrate changes from other branch |
hg pull; hg merge |
get other version from a hg repo and merge with own repo |
p4 integ |
integrate changes to other branch |
hg push |
push own repo to other repo - but it is recommended to pull instead |
(diff; merge) |
can't move changes to other repos - must be done manually |
hg bundle |
create distributable file with all repo info |
p4 sync; p4 resolve |
rebase scheduled changes to other repo version |
- |
don't mess with uncommitted changes - commit and merge instead |
p4 sync |
get new changes from central repository |
hg pull |
get changes from (possibly more central) repository |
p4 sync; p4 resolve |
get changes from central repo and merge them with local changes |
hg pull; hg merge; hg commit |
get changes from another repo, merge them, and commit them to local repo |
p4 submit |
add pending changes to the chain of changesets in branch |
hg commit |
save changes in own repo, possibly creating new head/"branch" |
p4 submit |
submit a change to the central repository |
hg push |
push a set of changes to another (possibly more central) repository |
p4 triggers |
run commands (send email) when certain events happen |
run commands when certain actions are performed, send email notifications for changes |
|
p4 protect |
control user's access |
control access to repository |