gitinternals

8/6/2019 gitinternals

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GIT InternalsPedro Melo
mailto:[email protected]:[email protected]


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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

Im an egotistical bastard, and I nameall my projects after myself.First Linux, now git. Linus


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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

Im an egotistical bastard, and I nameall my projects after myself.First Linux, now git. Linus

Person

altakeo

n


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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
http://bit.ly/git-phylosophyhttp://bit.ly/git-phylosophyhttp://bit.ly/git-phylosophy


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In other words, I'm right. I'm

always right, but sometimesI'm more right than other

times. And dammit, when I say"files don't matter", I'm really

really Right(tm).Linus


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Strong Points

Non-Linear development

Distributed Development Centralized development is a subcase

Efficiency

Toolkit Design


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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


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The Blob

Files are stored as blobs Only content, no metadata


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blob [content_size]\0

Your content goes here after the header

I like pizza with apples

Meet the blob


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The tree

Trees store directories

Mode, type, pointer and name Recursive, trees can contain trees

Stored as a simple text file


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tree [content_size]\0100644 blob b5f21a README100644 blob afe433 Makefile.PL040000 tree a42cd0 lib

Meet the tree


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The commit

The object that makes history Pointer to a tree and the parent(s)

commits if any

Author, committer and commitmessage


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commit [content_size]\0tree 23edfc

author Pedro Melo 1243036800committer Pedro Melo 1243036800

commit without a parent

usually called first commit

Meet the commit...
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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commit [content_size]\0tree fde45c

parent 3454dfauthor Pedro Melo 1243036932committer Pedro Melo 1243036932

and we fixed that nasty bug

after all, they do tend to crop up

...and its child the other

commit
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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The tag

A name for a particular commit

Can contain a message Optionally GPG signed

Allows for cryptographically securereleases


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tag [content_size]\0

object 123fectype committag v1tagger Pedro Melo 1243037423

made it to 1.0!

Meet the tag
mailto:[email protected]:[email protected]


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Git Data Model Recap

Immutable objects

A file per object Repacked into object packs for efficiency

Organized as a directed acyclic graph


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proj/Makefile.PLlib/

Cool.pm


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Cool.pm


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Cool.pm


Cool.pm


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Cool.pm


Cool.pm


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Cool.pm


Cool.pm

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/

Cool.pm


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/

Cool.pm

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/

Cool.pm

Makefile.PLlib/


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/

Cool.pm

Makefile.PLlib/


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References

Names for commits

Mutable, they point to a specific commit

and move to a new one after eachcommit

A branch is a reference, a name to acommit

Special HEAD reference: points to areference

Name


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Cool.pm


Cool.pm

Makefile.PLlib/

Makefile.PLlib/

Cool.pm

Makefile.PLlib/


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mas

ter

HE

AD


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mas

ter

HE

AD


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master

HEAD

te

st


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master

HEAD

te

st


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mas

ter

HE

AD

te

st


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mas

ter

HE

AD

te

st


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master

HEAD

t

est


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masterMerge


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master

Merge


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t

est

Rebase


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t

est

Rebase


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masterRebase


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master

Rebase + Merge


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master

Rebase + Merge


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Non-SCM uses for Git

Leverage strengths

immutable

over network pulls only missing objects fast checkout (compare to copy, less to

read)

easy rollback


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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 gainfuncionality...


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Content distribution

Updates done in a master, centralrepository

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


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Read-only filesystem

Design web server that fetch objectsdirectly from the object database

Compact storage, efficient retrieval Packs of objects also very VM friendly,

mmap ready Some solutions already available OSS


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Wiki/Ticketing backend

Use git repository as storage for wiki orticketing systems

Good match for distributed developement Several solutions already available OSS ... but similar to SCM usages


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About Git

About Mehttp://simplicidade.org/notes/

@pedromelo{mailto,xmpp}:[email protected]

skype:melopthttp://github.com/melo

http://git-scm.com/Git Internals: http://peepcode.com/products/git-internals-pdf

Git book: http://progit.org/
http://www.slideshare.net/melopthttp://github.com/melomailto:[email protected]://www.slideshare.net/melopthttp://github.com/melohttp://peepcode.com/products/git-internals-pdfhttp://git-scm.com/http://git-scm.com/http://www.slideshare.net/melopthttp://github.com/melohttp://github.com/melomailto:[email protected]:[email protected]://simplicidade.org/notes/http://simplicidade.org/notes/http://peepcode.com/products/git-internals-pdf

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