An explanation about the organization of a Git repo, the type of objects it contains inside and the relations between them.

1. GIT Internals
Pedro Melo <{mailto,xmpp}:melo@simplicidade.org>

2. A short GIT History
• 2002 Apr 2005: The BitKeeper Wars
• Apr 2005: Episode IV - A New Hope
• July 2005: Hamano is the new maintainer
• Late 2008: GitHub hits the spotlight
“I’m an egotistical bastard, and I name
all my projects after myself.
First Linux, now git.” – Linus

3. on
A short GIT History
Person al take
• 2002 Apr 2005: The BitKeeper Wars
• Apr 2005: Episode IV - A New Hope
• July 2005: Hamano is the new maintainer
• Late 2008: GitHub hits the spotlight
“I’m an egotistical bastard, and I name
all my projects after myself.
First Linux, now git.” – Linus

4. GIT rules (without !!)
• Track content, not changes
• Simple repository
• Complex software
• Its easier to update the software, complex
to update all the repos so far
Git Mantra: http://bit.ly/git-phylosophy

5. In other words, I'm right. I'm
always right, but sometimes
I'm more right than other
times. And dammit, when I say
"files don't matter", I'm really
really Right(tm).
Linus

6. Strong Points
• Non-Linear development
• Distributed Development
• Centralized development is a subcase
• Efficiency
• Toolkit Design

7. Objects
• Git repositories store objects
• Stored in the Object Database
• Inside the Git directory
• .git at the root of your project
• Four major object types
• Objects are compressed for storage (zlib)
• SHA1 of header+content ID

8. The Blob
• Files are stored as blobs
• Only content, no metadata

9. Meet the blob
blob [content_size]0
Your content goes here after the header
I like pizza with apples

10. The tree
• Trees store directories
• Mode, type, pointer and name
• Recursive, trees can contain trees
• Stored as a simple text file

11. Meet the tree
tree [content_size]0
100644 blob b5f21a README
100644 blob afe433 Makefile.PL
040000 tree a42cd0 lib

12. The commit
• The object that makes history
• Pointer to a tree and the parent(s)
commits if any
• Author, committer and commit
message

13. Meet the commit...
commit [content_size]0
tree 23edfc
author Pedro Melo <melo@mini.me> 1243036800
committer Pedro Melo <melo@mini.me> 1243036800
commit without a parent
usually called first commit

14. ...and its child the other
commit
commit [content_size]0
tree fde45c
parent 3454df
author Pedro Melo <melo@mini.me> 1243036932
committer Pedro Melo <melo@mini.me> 1243036932
and we fixed that nasty bug
after all, they do tend to crop up

15. The tag
• A name for a particular commit
• Can contain a message
• Optionally GPG signed
• Allows for cryptographically secure
releases

16. Meet the tag
tag [content_size]0
object 123fec
type commit
tag v1
tagger Pedro Melo <melo@mini.me> 1243037423
made it to 1.0!

17. Git Data Model Recap
• Immutable objects
• A file per object
• Repacked into object packs for efficiency
• Organized as a directed acyclic graph

19. proj/
Makefile.PL
lib/
Cool.pm

20. proj/
Makefile.PL
lib/
Cool.pm

21. proj/
Makefile.PL
lib/
Cool.pm
Cool.pm

22. proj/
Makefile.PL
lib/
Cool.pm
Cool.pm

23. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL
lib/
Cool.pm

24. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL
lib/
Cool.pm

25. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL
lib/
Cool.pm

26. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL
lib/
Cool.pm

27. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL
lib/ lib/
Cool.pm

28. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL
lib/ lib/
Cool.pm

29. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL
lib/ lib/
Cool.pm

30. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL
lib/ lib/
Cool.pm Cool.pm

31. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL Makefile.PL
lib/ lib/ lib/
Cool.pm Cool.pm

32. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL Makefile.PL
lib/ lib/ lib/
Cool.pm Cool.pm

33. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL Makefile.PL
lib/ lib/ lib/
Cool.pm Cool.pm

34. References
• “Names” for commits
• Mutable, they point to a specific commit
and move to a new one after each
commit
Name
• A branch is a reference, a name to a
commit
• Special HEAD reference: points to a
reference

35. proj/
Makefile.PL
lib/
Cool.pm
Makefile.PL Makefile.PL Makefile.PL
lib/ lib/ lib/
Cool.pm Cool.pm

36. master HEAD

37. master HEAD

38. master HEAD
test

39. master
test HEAD

40. master
test HEAD

41. master HEAD
test

42. test master
HEAD

43. test master
Merge

44. test
Merge
master

45. test master
Rebase

46. master
Rebase
test

47. master
Rebase
test

48. Rebase + Merge
master
test

49. Rebase + Merge
master
test

50. Non-SCM uses for Git
• Leverage strengths
• immutable
• over network pulls only missing objects
• fast checkout (compare to copy, less to
read)
• easy rollback

51. Beware of weak points
• Always stores full copy of files
• not good for backups of DB dumps
• Full history more disk space
• this might chance as “shallow clones” gain
funcionality...

52. Content distribution
• Updates done in a master, central
repository
• Hierarchy of slave repositories
• Fast sync between repositories, fast
checkout
• Can be automated with hooks
• Useful if you have lots of static files, faster
than rsync

53. Read-only filesystem
• Design web server that fetch objects
directly from the object database
• Compact storage, efficient retrieval
• Packs of objects also very VM friendly,
mmap ready
• Some solutions already available OSS

54. Wiki/Ticketing backend
• Use git repository as storage for wiki or
ticketing systems
• Good match for distributed developement
• Several solutions already available OSS
• ... but similar to SCM usages

55. That’s all folks!
• I’ll be around #codebits, feel free to ask me
stuff
• If you want a git as a SCM demo, lets get
organized and I’ll do a impromptu
presentation, or even private
lapdan^H^H^H^H^Hdemos
• After #codebits
<{mailto,xmpp}:melo@simplicidade.org

56. About Git
http://git-scm.com/
Git Internals: http://peepcode.com/products/git-internals-pdf
Git book: http://progit.org/
About Me
http://simplicidade.org/notes/
@pedromelo
{mailto,xmpp}:melo@simplicidade.org
skype:melopt
http://github.com/melo
http://www.slideshare.net/melopt