Mercurial

Mercurial
A Distributed Version Control System
DmitriyIassenev
GSC Game World
2011

Why Mercurial?
Pros
It has rich functionality
It is easy to customize
If you like Python
It scales good
Big files need to be handled carefully
It has good GUI client – TortoiseHG
It is easy to learn
Non-progammers could have problems with it

Why Mercurial?
Cons
Every user has the whole repository
User access rights problems – use subrepositories?
Every user should have enough HDD space

Mercurialvs Subversion
Pros
Simpler for non-programmers
Even better GUI client – TortoiseSVN (at the time we choose)
Handle big files well without any additional effort
Need less space on HDD
Mixed revisions working copy
Exclusive file lock

Mercurial vs Subversion
Cons
Centralized VCS
No local commits
No inter-repository commands
Slower on most operations
Complicated and restrictive merge and branch functionality
In some use cases need even more space on HDD
.svn folders
Impossible to setup ignore filter for future folders
Easier to loose your changes

Mercurial vsGit
Pros
Even more rich functionality
Faster on Linux

Mercurial vsGit
Cons
Worse performance on Windows (2009 year measurements)
You should “repack” your local repository to prevent performance degradation
No good free GUI client for Windows at the time we choose – TortoiseGit?
It is known to be more complicated for both programmers and non-programmers

How does Mercurial work?
Local Repository and Working Copy
commit
update
merge
rebase

Local repository and Working Copy
Merge
1
2 3

Local repository and Working Copy
Rebase
1
2

Inter-repository communication
clone
pull
After pull commands
Update
Fetch = update + merge + commit
Rebase
push
push after commit option
serve

Tags and Branches
Tags
Merge tags - .hgtags
Branches
Repository clones
Markers
Named branches
Anonymous branches (several heads in a branch)
Branch hierarchy doesn’t depend on folders structure (unlike Subversion)

Under the hood
Deltas even for binaries
Atomic transactions – only append to revlog
Fast retrieval – snapshots
File and folder renames
Avoid case sensitive renames on case insensitive FSs
Compression: deflate and bzip2
Concurrent access – lockless reading
Minimizing disk seeks
Hard links for cloning repositories locally

Recovering from errors
Revert
Rollback
Undo the last commit
Undo erroneous pull
Backout
Recovering from erroneous merge

Rollback
1
2

Backout
1
2

Recovering from erroneous merge
1 2
3 4

Hooks
changegroup – we use it to detect when someone pushed to repository
precommit – we use it to prevent case folding collisions

Extensions
Modified Bigfiles
Fetch
Rebase
Transplant
Mercurial Queues (for Strip)

How we use Mercurial
Multiplatform engine for video game and game itself (Win32, Win64, Xbox360, PS3)
C++ for engine and game, C# for tools (.NET + WPF)
Team: 11 programmers + 13 designers (up to 40 designers in a production stage)
~467k lines of code ~= 15Mb
~3.5M lines of code from various SDK and third party libraries which we build ~= 132Mb
Working Copy: ~33.6k files and folders ~= 16.8Gb
Local Repository (~0.4Gb) => 17.2Gb per user
Full repository ~= 135Gb

Continuous Integration Workflow
2 Central repositories + Build station
Programmers repository
Build station repository
Designers repository
We plan to use Central repository for Testers

Continuous Integration Workflow
on push to Programmers
pull Build station from Programmers
build binaries for all 16 projects configurations
run tests
commit to Build station repository on success or report failure otherwise
pull from Designers
push to Designers
on push to Designers
pull Programmers from Designers

Merge tool we use - Perforce p4merge

Questions?
Feel free to email me at
iassenev@gsc-game.com
Online resources:
“Mercurial: The Definitive Guide” by Bryan O'Sullivan - http://hgbook.red-bean.com/
Joel Spolsky Tutorial on Mercurial - http://hginit.com/

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Mercurial

by Kiev ALT.NET on Jul 10, 2011

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