Understanding Git Internals

GitJeff Kunkle

Understanding

January 6, 2010

Git is a free & open source, distributed version control system designed to handle everything from small to very

large projects with speed and efficiency.

Every Git clone is a full-fledged repository with complete history and full revision tracking capabilities, not dependent

on network access or a central server. Branching and merging are fast and easy to do.

What’s Git?

./

Rakefile

init.rb

lib

grant.rb

Working Directory

./

Rakefile

init.rb

lib

grant.rb

Working Directory

>git init

.git

./

Rakefile

init.rb

lib

grant.rb

Working Directory

>git init

What’s in here?.git

Objects

Git Objects

header

content

object_type [content size]\0content of the filecan be textcan be binarycan be whatever

Zlib::Deflate

Git Objects

1 Blob 2 Tree 3 Commit 4 Tag

header

content

object_type [content size]\0content of the filecan be textcan be binarycan be whatever

Zlib::Deflate

1 Blobs

./

Rakefile

init.rb

lib

grant.rb

Working Directory Git Directory

./

Rakefile

init.rb

lib

grant.rb


>git add .>git commit -m “initial commit”

./

Rakefile

init.rb

lib

grant.rb


blob : 21a307


blob : 644eda

blob : a22a24

./

Rakefile

init.rb

lib

grant.rb


blob : 21a307


blob : 644eda

blob : a22a24

SHA-1 hashesof file content

Blob Content

Blob Content

>git cat-file -p 644eda

%w{ models }.each do |dir| path = File.join(File.dirname(__FILE__), 'lib', 'app', dir) $LOAD_PATH << path ActiveSupport::Dependencies.load_paths << path ActiveSupport::Dependencies.load_once_paths.delete(path)end

2 Trees

./

Rakefile

init.rb

lib

grant.rb


blob : 21a307

blob : 644eda

blob : a22a24

./

Rakefile

init.rb

lib

grant.rb


blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

Tree Content

Tree Content

>git cat-file -p f2eb1e

100644 blob 21a30738954b6bb164731d822efafa6c89c7bce7! Rakefile100644 blob 644eda506db859e011ccbca5a06421ee76782ac7! init.rb040000 tree 523fa41bd27fa29a00afb0bef6a10fde75aef501! lib

./

Rakefile

init.rb

lib

grant.rb


blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

3 Commits

./

Rakefile

init.rb

lib

grant.rb


blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

./

Rakefile

init.rb

lib

grant.rb


blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

commit : 3848fa

Commit Content

Commit Content

>git cat-file -p 3848fa

tree f2eb1e549e7e0d9cd5b7a580e63e9ce79d5a03aeauthor Jeff Kunkle <[email protected]> 1261341813 -0500committer Jeff Kunkle <[email protected]> 1261341813 -0500

initial commit

first commit

mailto:[email protected]




./

Rakefile

init.rb

lib

grant.rb


blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

commit : 3848fa

Commit Content

tree ac451e549e7e0d9cd5b7a580e63e9ce79d5b45a1parent 3848fa7e91490a99b77590ff1385c4b3eebb3de3author Jeff Kunkle <[email protected]> 1261351813 -0500committer Jeff Kunkle <[email protected]> 1261351813 -0500

second commit

next commit





4 Tags

./

Rakefile

init.rb

lib

grant.rb

blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

commit : 3848fa

./

Rakefile

init.rb

lib

grant.rb

blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

commit : 3848fa

tag : e9eff3>git tag -a v1.0

Tag Content

Tag Content

>git cat-file -p v1.0

object 3848fa7e91490a99b77590ff1385c4b3eebb3de3type committag v1.0tagger Jeff Kunkle <[email protected]> Tue Dec 29 21:02:04 2009 -0500

version 1.0



./

Rakefile

init.rb

lib

grant.rb

blob : 21a307

blob : 644eda

blob : a22a24

tree : f2eb1e

tree : 523fa4

commit : 3848fa

tag : e9eff3>git tag -a v1.0

References

blob

tree

commit

tag

Basic Data Model

blob

tree

commit

tag

Basic Data Model

ImmutableGit Objects

blob

tree

commit

tag

Basic Data Model


branch

blob

tree

commit

tag

Basic Data Model


branch

HEAD

blob

tree

commit

tag

Basic Data Model


branch

HEAD

MutableReferences

./

Rakefile

init.rb

lib

grant.rb

Working Directory

blobtree

commit

tagbranch

HEAD

blobtree

blob

blobtree

commit

tagbranch

HEAD

blobtree

blob

tree

commit

branch

HEAD

tree

blob

>git commit -a lib/grant.rb

>git commit -a Rakefile init.rb

blobtree

commit

tagbranch

HEAD

blobtree

blob

tree

commit

branch

HEAD

tree

blob

tree

commit

branch

HEAD

blob

blob

Branches

blob

tree

commit

branch

C1

branch

C

master

C

C

master

C

idea

>git checkout -b idea

C

master

C

C

idea

>git commit

C

master

C

C

idea

>git checkout master

C

master

C

C

idea

C

>git commit

C

master

C

C

idea

C

bug

>git checkout -b bug

C

master

C

C

idea

C

C

bug

>git commit

C

master

C

C

idea

C

C

bug

>git commit>git checkout master

C

master

C

C

idea

C

C

bug

>git commit>git merge bug

CC

C

idea

C

C

C

bug

master

>git commit>git merge bug>git commit

CC

C

idea

C

C

C

bug

master

>git commit>git merge bug>git commit>git branch -d bug

CC

C

idea

C

C

C

bug

master

>git commit>git merge bug>git commit>git branch -d bug>git checkout idea

CC

C C

C

idea

C

C

bug

master

>git commit>git merge bug>git commit>git branch -d bug>git checkout idea>git commit

CC

C C C

C

C

C

idea

bug

master

>git commit>git merge bug>git commit>git branch -d bug>git checkout idea>git commit>git commit

CC

C C C

C

C

C

idea

bug

master

>git commit>git merge bug>git commit>git branch -d bug>git checkout idea>git commit>git commit>git checkout master

CC

C C C

C

C

C

idea

bug

master

>git commit>git merge bug>git commit>git branch -d bug>git checkout idea>git commit>git commit>git checkout master>git merge idea

CC

C C C

CC

C

C

idea

bug

master

>git commit>git merge bug>git commit>git branch -d bug>git checkout idea>git commit>git commit>git checkout master>git merge idea>git commit

CC

C C C

CC

C

C

idea

bug

master

>git commit>git merge bug>git commit>git branch -d bug>git checkout idea>git commit>git commit>git checkout master>git merge idea>git commit>git branch -d idea

Recommendations

Starting new project?

Starting new project?

Use Git

Using SVN or CVS?

Using SVN or CVS?

Switch to Git

Using Team Studio?

Using Team Studio?

Maybe switch to Git

Using MKS?

Using MKS?

Switch to Gitor CVS, or Subversion, or anything else

References

Good Better Best*most diagrams in this presentation are based on Git internals

Understanding Git Internals

Technology

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