Git.From thorns to the stars

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Git.From thorns to the stars

  1. 1. Git. From the thorns to the stars. Сергей Моренец 25 апреля 2013 г.
  2. 2. Agenda • Versioning and revision systems overview • Git under the microscope • Examples • Q & A
  3. 3. Glossary • VCS • SCM • RCS
  4. 4. Requirements • Storing content • Tracking changes to the content • Distributing the content and history with collaborators
  5. 5. Lost in selection
  6. 6. Magic pill
  7. 7. SCCS • First VCS available on any Unix system • Developed in SNOBOL at Bell Labs in 1972 • Prepared for IBM Systems/370 computers running OS/360 • Its file format is used in BitKeeper and other VCS • Introduced repositories and locking mechanism
  8. 8. CVS • Ancestor of the revision control systems • First released in 1986 by Dick Grune • Simple technology with small learning curve • Useful for sharing and backing up the files • Tortoise CVS is a de facto client for CVS on Windows • Introduces merging • Lifecycle ended in 2008
  9. 9. Apache Subversion • Created in 2000 • Used to host Apache software products, also Mono, SourceForge, Google Code • Most adopted SCM • Atomic commits • Maintains versioning for directories, renames, and file metadata • Better support for branches and tagging
  10. 10. Centralized VCS
  11. 11. Distributed VCS
  12. 12. Distributed workflow
  13. 13. Git • Distributed revision control and source code management system • Designed and developed by Linus Torvalds for Linux kernel development • Based on BitKeeper system • The development began on April 2005 • Current version 1.8.2
  14. 14. Linus Torvalds • Swedish-speaking Finnish American • Chief architect and the project's coordinator of the Linux kernel • Names after Linus Pauling and Linus Van Pelt • Second lieutenant of the Finnish Army • Winner of Millennium Technology Prize in 2012 • Calls himself egotistical bastard
  15. 15. Git The information manager from hell
  16. 16. Git Global information TRACKER
  17. 17. Junio Hamano • Graduated from Tokyo university • Git coordinator since 2005 • Participated in the Linux development • Currently Google developer
  18. 18. Design Principles • Take CVS as an example of what not to do • Support distributed workflow • Scaling to thousand developers • Strong consistency and integrity support • Free
  19. 19. Features • Rapid branches and merging • Distributed development • Compatibility and emulation • Performance breakthrough • Revisions hashing • Garbage collector • Packed data storage
  20. 20. Git Repository • Database containing revisions and history of the project • Retains complete copy of entire project • Maintains object store and index • Object store contains data files, log files and audit information
  21. 21. Git Repository
  22. 22. Git Object Types • Blobs • Trees • Commits • Tags
  23. 23. Blobs • Each version of a file is represented as a blob. • Blob internal structure is ignored by Git. • A blob holds a file’s data but does not contain any metadata about the file or even its name. • git show command examines contents of the blob
  24. 24. Trees • A tree object represents one level of directory information. • It records blob identifiers and path names for all the files in one directory. • It can also recursively reference other sub-trees objects • Can be examined by git show or git ls-tree commands
  25. 25. Commits • A commit object holds metadata for each change including the author, commit date, and log message. • Each commit points to a tree object that captures, the state of the repository at the time the commit was performed. • git tag stable-1 1b2e1d63ff
  26. 26. Tags • A tag object assigns an arbitrary yet presumably human readable name to a specific object, usually a commit. • Contains tag type, tag message, author and object name. • Can be examined by git cat-file command.
  27. 27. Git Repository
  28. 28. Git Object Model • Object store is organized and implemented as a content-addressable storage system. • Each object has a unique name produced by applying SHA1 to the contents of the object. • SHA1 hash is a sufficient index or name for that object in the object database. • SHA1 values are 160-bit values that are represented as a 40-digit hexadecimal number • 9da581d910c9c4ac93557ca4859e767f5caf5169
  29. 29. Advantages • Git can determine equality of the objects by comparing names. • The same content stored in two repositories will always be stored under the same name. • Corruptions errors can be detected by checking that the object's name is still the SHA1 hash of its contents.
  30. 30. Name Vs Content • Git stores each version of file not differences • Path name is separated from file contents • Object store is based on hashed computation on file contents, not name System Index mechanism Data store Database Indexed Sequential Access Method Data records Unix FS Directories(/path) Blocks of data Git .git/objects/hash Blob/tree objects
  31. 31. Git Directory • Stores all Git's history, configuration and meta information for your project • There is only one git directory per project • By default it’s '.git' in the root of your project
  32. 32. Git Directory • Configuration: - config - description - info/exclude • Helps configuring local repository
  33. 33. Git Directory • Hooks: -hooks • Scripts that are run on certain lifecycle events of the repository
  34. 34. Git Directory • Object Database: -objects • Default Git object database • Contains all content or pointers to local content. • All objects are immutable
  35. 35. Git Directory • References: -refs • Stores reference pointers for branches, tags and heads. • A reference is a pointer to an object, usually of type tag or commit. • References changes as the repository evolves
  36. 36. Working Directory • Holds the current checkout of the files • Files can be removed or replaced by Git as branches are switching • Working directory is temporary checkout place
  37. 37. Index • The index is a temporary and dynamic binary file that captures a version of the project’s overall structure • The project’s state could be represented by a commit and a tree from any point in the project’s history • The index allows a separation between incremental development steps and the committal of those changes.
  38. 38. Index • Staging area between your working directory and your repository • With commit data files from index are committed, not from working directory • Can be viewed by git status command.
  39. 39. Data flow
  40. 40. Git Usage • Command-line tool(Git Bash) • Git GUI • IDE Plugin(JGit-based)
  41. 41. Git Bash • Command-line tool • UNIX-style utility • Last straw
  42. 42. Git GUI • MinGW – based • Former WinGit • No support
  43. 43. JGit • Lightweight, pure Java library implementing the Git • EGit - Eclipse team provider for Git • NBGit - Git Support for NetBeans
  44. 44. Git Commands • init • checkout • fetch • pull • reset • merge • log
  45. 45. Git Commands • add • commit • push • branch • tag
  46. 46. First steps • Clone repository • Initialize repository
  47. 47. Clone Repository • git clone git://git.kernel.org/pub/scm/git/git.git • git clone http://www.kernel.org/pub/scm/git/git.git
  48. 48. Branching • Branch is graph of commits • Master branch is created by default • HEAD is pointer to the current branch • “git branch test” creates branch test. • “git checkout master” switches to branch master. • “git merge test” merges changes from test to master. • Merges are done automatically.
  49. 49. Conflicts • If conflict cannot be resolved index and working tree are left in the special state • “git status” shows unmerged files with conflict markers • git add file.txt • git commit
  50. 50. Roll Back • Reset • Checkout • Revert
  51. 51. Reset • git reset --hard HEAD • git reset --hard ORIG_HEAD
  52. 52. Checkout • git checkout HEAD MyClass.java
  53. 53. Revert • Rollbacks the last commit(s) in the repository • git revert HEAD • git revert HEAD~1 –m 2
  54. 54. Git References • All references are named with a slash-separated path name starting with "refs“. • -The branch "test" is short for "refs/heads/test". • The tag "v1.0" is short for "refs/tags/v1.0". • "origin/master" is short for “refs/remotes/origin/master"
  55. 55. Git References • The HEAD file is a symbolic reference to the branch we are currently using • git symbolic-ref HEAD • ref: refs/heads/master
  56. 56. Advanced Git
  57. 57. Branching strategy • master • develop
  58. 58. Branching strategy • origin/master contains production-ready code • origin/develop contains development changes
  59. 59. Branching strategy • Feature branches • Release branches • Hotfix branches
  60. 60. Feature Branches • Feature branches (or topic branches) are used to store new features • Can be added to develop or disregarded • git checkout –b newfeature develop
  61. 61. Release Branches • Release branches support preparation of a new production release
  62. 62. Hotfix branches • Hotfix branches are related to new production release. • Created in response to critical bugs in a production environment. • Separates developing of the current version and hotfix.
  63. 63. Branching strategy
  64. 64. Rebasing • git checkout -b mywork origin • git commit • git commit
  65. 65. Rebasing
  66. 66. Rebasing • git merge origin
  67. 67. Rebasing • git checkout mywork • git rebase origin
  68. 68. Rebasing
  69. 69. Stashing • git stash save “Stashing reason“ • … • git stash apply
  70. 70. Treeishes • 980e3ccdaac54a0d4de358f3fe5d718027d96aae • 980e3ccdaac54a0d4 • 980e3cc
  71. 71. Treeishes • 980e3ccdaac54a0d4de358f3fe5d718027d96aae • origin/master • refs/remotes/origin/master • master • refs/heads/master • v1.0 • refs/tags/v1.0
  72. 72. Issues search • git bisect start • git bisect good v1.0 • git bisect bad master • git bisect bad • git show • git bisect reset
  73. 73. Blamestorming • git blame sha1_file.c • 0fcfd160 (Linus Torvalds 2005-04-18 8) */ • 0fcfd160 (Linus Torvalds 2005-04-18 9) #include "cache.h" • 1f688557 (Junio C Hamano 2005-06-27 10) #include "delta.h" • a733cb60 (Linus Torvalds 2005-06-28 11) #include "pack.h"
  74. 74. Git Hooks • Scripts placed in $GIT_DIR/hooks directory to trigger action at certain points • pre-commit • commit-msg • post-commit • post-checkout • post-merge
  75. 75. Object Store • All objects are stored as compressed contents by their SHA-1 values. • They contain the object type, size and contents in a gzipped format. • Loose objects and packed objects.
  76. 76. Loose Objects • Compressed data stored in a single file on disk • Every object written to a separate file • SHA1 ab04d884140f7b0cf8bbf86d6883869f16a46f65 • GIT_DIR/objects/ab/04d884140f7b0cf8bbf86d68838 69f16a46f65
  77. 77. Packed Objects • Packfile is a format which stores the part that has changed in the second file • Uses heuristic algorithm to define files to pack • git gc packs the data • git unpack-objects converts data into loose format
  78. 78. Ignoring files • # Ignore any file named sample.txt. • sample.txt • # Ignore Eclipse files • *.project • # except my.project with manual setting. • !my.project • # Ignore objects and archives. • *.[oa]
  79. 79. Scripting • Ruby • PHP • Python • Perl
  80. 80. Migration • Script support • CVS • SVN • Perforce • Mercurial • fast-support tool
  81. 81. Migration • git-svn clone http://my- project.googlecode.com/svn/trunk new-project • ~/git.git/contrib/fast-import/git-p4 clone //depot/project/main@all myproject
  82. 82. GitHub
  83. 83. GitHub • Web-based hosting service • Was launched in April 2008 • Git repository, paid for private projects and free for open-source projects • Run by Ruby on Rails & Erlang • Provides feeds and followers
  84. 84. Growth Period State 2009 100000 users and 50000 repositories 2011 1 million users 2012 2 million users and 4 million repositories 2013 3 million users and 5 million repositories
  85. 85. Octocat • Introduced by Tom Preston-Werner, cofounder of GitHub • Composed of octopus and cat words
  86. 86. Octocat
  87. 87. Resources • Version Control with Git, 2nd Edition, 2012 • Pro Git, 2009
  88. 88. Pros • Painless branching • Separation between local repository and upstream • Simplifies work in the distributed teams • Dramatic increase in performance • Integration with major VCS
  89. 89. Cons • Repository security risks • Latest revision question • Pessimistic locks • Big learning curve • Commit identifiers • Not optimal for single developers
  90. 90. Q&A • Сергей Моренец, morenets@mail.ru

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