Understanding Git Internals

This is a quick introduction to the inner workings of Git, largely based on what I learned from reading Git Internals (https://peepcode.com/products/git-internals-pdf) and experimentation. It covers Git objects (blob, tree, commit, tag), references, and branches with incrementally building diagrams to help convey the concepts.

Transcript

1. Git Jeff Kunkle Understanding January 6, 2010
2. 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?
3. ./ Rakefile init.rb lib grant.rb Working Directory
4. ./ Rakefile init.rb lib grant.rb Working Directory >git init .git
5. ./ Rakefile init.rb lib grant.rb Working Directory >git init What’s in here? .git
6. Objects
7. Git Objects header content object_type [content size]\0 content of the file can be text can be binary can be whatever Zlib::Deflate
8. Git Objects 1 Blob 2 Tree 3 Commit 4 Tag header content object_type [content size]\0 content of the file can be text can be binary can be whatever Zlib::Deflate
9. 1 Blobs
10. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory
11. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory >git add . >git commit -m “initial commit”
12. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 >git add . >git commit -m “initial commit” blob : 644eda blob : a22a24
13. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 >git add . >git commit -m “initial commit” blob : 644eda blob : a22a24 SHA-1 hashes of file content
14. Blob Content
15. 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
16. 2 Trees
17. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24
18. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4
19. Tree Content
20. Tree Content >git cat-file -p f2eb1e 100644 blob 21a30738954b6bb164731d822efafa6c89c7bce7! Rakefile 100644 blob 644eda506db859e011ccbca5a06421ee76782ac7! init.rb 040000 tree 523fa41bd27fa29a00afb0bef6a10fde75aef501! lib
21. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4
22. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4
23. 3 Commits
24. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4
25. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4 commit : 3848fa
26. Commit Content
27. Commit Content >git cat-file -p 3848fa tree f2eb1e549e7e0d9cd5b7a580e63e9ce79d5a03ae author Jeff Kunkle <jkunkle@nearinfinity.com> 1261341813 -0500 committer Jeff Kunkle <jkunkle@nearinfinity.com> 1261341813 -0500 initial commit first commit
28. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4 commit : 3848fa
29. ./ Rakefile init.rb lib grant.rb Working Directory Git Directory blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4 commit : 3848fa
30. Commit Content tree ac451e549e7e0d9cd5b7a580e63e9ce79d5b45a1 parent 3848fa7e91490a99b77590ff1385c4b3eebb3de3 author Jeff Kunkle <jkunkle@nearinfinity.com> 1261351813 -0500 committer Jeff Kunkle <jkunkle@nearinfinity.com> 1261351813 -0500 second commit next commit
31. 4 Tags
32. ./ Rakefile init.rb lib grant.rb blob : 21a307 blob : 644eda blob : a22a24 tree : f2eb1e tree : 523fa4 commit : 3848fa
33. ./ 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
34. Tag Content
35. Tag Content >git cat-file -p v1.0 object 3848fa7e91490a99b77590ff1385c4b3eebb3de3 type commit tag v1.0 tagger Jeff Kunkle <jkunkle@ni.com> Tue Dec 29 21:02:04 2009 -0500 version 1.0
36. ./ 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
37. ./ 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
38. References
39. blob tree commit tag Basic Data Model
40. blob tree commit tag Basic Data Model Immutable Git Objects
41. blob tree commit tag Basic Data Model Immutable Git Objects branch
42. blob tree commit tag Basic Data Model Immutable Git Objects branch
43. blob tree commit tag Basic Data Model Immutable Git Objects branch HEAD
44. blob tree commit tag Basic Data Model Immutable Git Objects branch HEAD Mutable References
45. ./ Rakefile init.rb lib grant.rb Working Directory
46. blob tree commit tag branch HEAD blob tree blob
47. blob tree commit tag branch HEAD blob tree blob tree commit branch HEAD tree blob >git commit -a lib/grant.rb
48. >git commit -a Rakefile init.rb blob tree commit tag branch HEAD blob tree blob tree commit branch HEAD tree blob tree commit branch HEAD blob blob
49. Branches
50. blob tree commit branch C1 branch
51. C master C
52. C master C idea >git checkout -b idea
53. C master C C idea >git commit
54. C master C C idea >git checkout master
55. C master C C idea C >git commit
56. C master C C idea C bug >git checkout -b bug
57. C master C C idea C C bug >git commit
58. C master C C idea C C bug >git commit >git checkout master
59. C master C C idea C C bug >git commit >git merge bug
60. C C C idea C C C bug master >git commit >git merge bug >git commit
61. C C C idea C C C bug master >git commit >git merge bug >git commit >git branch -d bug
62. C C C idea C C C bug master >git commit >git merge bug >git commit >git branch -d bug >git checkout idea
63. C C C C C idea C C bug master >git commit >git merge bug >git commit >git branch -d bug >git checkout idea >git commit
64. C C 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
65. C C 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
66. C C 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
67. C C C 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 >git commit
68. C C C 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 >git commit >git branch -d idea
69. Recommendations
70. Starting new project?
71. Starting new project? Use Git
72. Using SVN or CVS?
73. Using SVN or CVS? Switch to Git
74. Using Team Studio?
75. Using Team Studio? Maybe switch to Git
76. Using MKS?
77. Using MKS? Switch to Git or CVS, or Subversion, or anything else
78. References Good Better Best *most diagrams in this presentation are based on Git internals