Introductory Notes on the Git Source Control Management

1
Introductory Notes on the Git Source Control
Management

• Ric Holt, 8 Oct 2009

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Git is a SCM (Source Control Management System)

• Management of changes to documents, programs, and
  other information stored as computer files.
• Its first usage have been for Linux and for Git
  itself.
• It is open source and free

3
Torvalds and Git

• Written by Linus Torvalds -- Mr. Linux
• What does the word git mean?
• Nothing?
• A person who is deemed to be despicable or
  contemptible ?
• From http//www.google.ca/search?hlenclientfire
  fox-arlsorg.mozillaen-USofficialhsLl1defle
  nqdefinegiteiwi7NSofxA4rflAevs-TgBQsaXoig
  lossary_definitioncttitle

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Git A Fast Version Control System

• Git
• Is distributed
• Has no master copy
• Has fast merges
• Is controversial
• Scales up
• Convenient tools still being built
• Safeguards against corruption

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Bibliography

• Overview of Git (1 hour video). by Randal
  Schartz, 2007, Google talk, no diagrams
  http//www.youtube.com/watch?v8dhZ9BXQgc4
• Tv's cobweb Git for Computer Scientists (nice
  diagrams)
• http//eagain.net/articles/git-for-computer-scient
  ists/
• A tutorial introduction to Git (key examples uses
  of commands)
• http//www.kernel.org/pub/software/scm/git/docs/v1
  .2.6/tutorial.html

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UW CS746 Seminar Course on Software Architecture

• Studying Git architecture (Fall 2009)
• Suggested/clarified various ideas given here

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Fundamental Concepts of Source Control Management

• Source control management SCM. Records
  information about an evolving software project).
  Key example CVS.
• Project (set of files and directories, typically
  comprising the source files of a software system
  under development, but could be any other
  content)
• Version (one instance of a project) (called a
  commit in Git)

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More Fundamental Concepts of Source Control
Management

• Branch. A sequence of versions, each one evolved
  from the previous one
• To branch. Split development into two parallel
  branches.
• To merge. Combine two branches into single
  branch
• Repository. In this case, a specialized database
  to store an evolving project with its branches

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Gits Distributed Architecture P2P
Ovals are servers
R2
R1
R3
Boxes are individual repositories
R4
R5
System of distributed repositories R1, R2, ...
Mostly Git repositories, but can be CVS etc.
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Architecture of Git Individual Repository

• Object storage. Some times called simply a
  repository. Stores representation of the
  versions of project (in a directory called .git
  )
• Gits Index. Caches objects from work space that
  have changed (added to index), and that will be
  stored in the repository with the next commit
• Work space. Users sandbox (ordinary files and
  directories) for active version    

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Architecture of Git Individual Repository
One Repository
add
commit
Working tree (sand box)
Index (cache)
Object store
pull, push
More Repositories
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Data Structure of Repository ERD Defines Allowed
DAGs
Each commit (version) is based on zero or more
previous versions
commit
version
tree
Each folder contains other folders and files
(blobs)
tree of folders
blob
file
Generally, committed structure is immutable
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Naming Nodes by Their Hashes

• The name (or key) of a node is the hash (SHA1) of
  its contents.
• A hash can be used as a pointer to locate its
  content
• Identical files have the same hash and are
  represented by a single blob

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Layered Structure of Implementation of Repository
GUI
Porcelain (High Level Operations)
Plumbing (Low Level Operations)
Operations are separate executables (not API)
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Key Git Operations

• 1)      Init. Create an empty Git repository
• 2)      Clone. Copy a repository into a new
  directory. After cloning, edit, create and
  remove files for new version
• 3)      Add. Add file contents from work space
  to the index. (The files are edited locally)
• 4)      Remove rm. Remove files from work
  space and from the index
• 5)      Commit. Store the changes (that are
  added) to the repository, using the index.
  Completes this version.
• 6)      Branch. Create (or delete) a branch

• 7)      Merge. Join two or more branches
• 8)      Rebase. Combine/restructure a set of
  commits to simplify them
• 9)      Checkout. Checkout files etc from a
  commit, and switch work space to that new branch
• 10)      Fetch. Download objects and refs from
  another repository
• 11)      Pull. Fetch from and merge with another
  repository or a local branch
• 12)      Push. Update remote refs (in another
  repo) along with associated objects