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Building Git From Scratch

Building Models

Demo: Tiny Git

  • Creating a repository

  • Commiting changes

  • Viewing changes in workking copy

  • Viewing change history

  • Creating branches

  • Switching branches

  • Merging changes from branches

Limitations

  • Tracks only a single file

  • Merge handling is not completely implemented

  • Push and pull from remote repository is not supported

V0: The Skeleton

docopt

  • All command line tools need to parse arguments.

  • Docopt considerably reduces the code for argument parsing.

  • From the command help message, Docopt figures out

    • sub-commands

    • options

    • positional arguments

  • Provides parsed result in a dictionary.

V1: Features

  • Create Repo

  • Commit

  • Checkout

  • Diff

Limitations

  • Checkout only latest revision

  • No logs

  • No branching

How to Store Files: Take 1

  • .tig/objects holds revisions of the file.

  • .tig/master contains the latest revision filename: file.txt.v3

proj/
  + .tig/
  |  + objects/
  |  | + file.txt.v1
  |  | + file.txt.v2
  |  | + file.txt.v3
  |  + master
  |
  + file.txt

  • Commit

    1. Get the current revision from .tig/master

    2. Increment that by 1: 3 → 4.

    3. Copy file.txt to .tig/objects/file.txt.v4

    4. Set .tig/master to file.txt.v4

How to Store Files: Take 1 (Contd.)

  • Checkout

    1. Get lastest revision from .tig/master

    2. Copy .tig/objects/file.txt.v4 to file.txt

DVCS and Seq. Versioning

dvcs

  • Distributed versions control systems allows each user to create commits, in their local repo.

  • Distributed version control system, sequential version no. does not work.

Use SHA1 to Identify the File

dvcs sha1

  • Idea: Use SHA1 sum of the content to identify the file.

  • SHA1 is similar to checksum algorithms: but 20-bytes long

  • Interesting property, no two files will have the same checksum. (very low probability)

  • Depicted as a 40 digit hex number: 4c7be7b2c3641a5e489c4ce667699eeee4e994c9

  • Only first 6 digits are shown: 4c7be7…​

How to store Files: Take 2

proj/
  + .tig/
  |  + objects/
  |  | + 4c7be7b2c3641a5e489c4ce667699eeee4e994c9
  |  | + c9360f5cd3d2c9f5656cd5ee10dedd3d235a4e0f
  |  | + da39a3ee5e6b4b0d3255bfef95601890afd80709
  |  + master
  |
  + file.txt

  • Commit

    1. Find SHA1 of file.txt: 58f3966…​

    2. Store file.txt to .tig/objects/58f3966…​

    3. Set .tig/master to 58f3966…​

  • Checkout

    1. Get lastest revision from .tig/master

    2. Copy .tig/objects/58f3966…​ to file.txt

Ordering Lost

  • The latest revision is in .tig/master

  • The previous revision is not known!

  • No meta information about the changes: who? why? when?

    no commit objects

V2: Features

  • Create Repo

  • Commit

  • Checkout

  • Diff

  • Log

Limitations

  • Checkout only latest revision

  • No branching

Commit Objects

  • Commit object: meta information stored in the object database.

{
        "log-msg": "Added goodbye world",
        "author": "[email protected]",
        "time": 1437790070,

        "content": eb12bc...,
        "parent": ef4d3e...,
}

  • Stores log message, author, and time information as well.

  • Stores SHA1 of file.txt that corresponds to this commit.

  • Stores SHA1 of parent commit.

Ordering Regained

commit objects

Ordering Regained (Contd.)

  • Commit

    1. Find SHA1 of file.txt: 58f3966…​

    2. Store file.txt to .tig/objects/58f3966…​

    3. Create and store commit object: 8fb439…​

    4. Set .tig/master to commit object

  • Checkout

    1. Get lastest commit object from .tig/master: 8fb439…​

    2. Get content SHA1 from commit object: 58f3966…​

    3. Copy it to file.txt

  • Log

    1. Get lastest commit object from .tig/master: 8fb439…​

    2. Print the commit info

    3. Get the parent commit

    4. Print the commit info

    5. …​ Repeat …​

V3: Features

  • Create Repo

  • Commit

  • Checkout (any version)

  • Diff

  • Log

Limitations

  • No branching

Checkout Any Version

  • .tig/master: the latest version of the file.

  • .tig/HEAD: the current checked out version.

  • Commits not allowed if not latest version

proj/
  + .tig/
  |  + objects/
  |  | + 4c7be7b2c3641a5e489c4ce667699eeee4e994c9
  |  | + c9360f5cd3d2c9f5656cd5ee10dedd3d235a4e0f
  |  | + da39a3ee5e6b4b0d3255bfef95601890afd80709
  |  + master
  |  + HEAD
  |
  + file.txt

Checkout Any Version (Contd.)

  • Checkout

    1. Determine the commit SHA1.

      1. If SHA1 specified, use it.

      2. If not specifed get lastest commit object from .tig/master

    2. Get content SHA1 from commit object.

    3. Copy it to file.txt

    4. Set HEAD to checked-out commit SHA1.

  • Commit

    1. If HEAD != master, cannot commit.

    2. Find SHA1 of file.txt: 58f3966…​

    3. Store file.txt to .tig/objects/58f3966…​

    4. Create and store commit object: 8fb439…​

    5. Set .tig/master to commit object

V4: Features

  • Support for branching

  • No merging

Branches

  • Only one branch so far: master

    o---o---o---o---o master

  • Each branch has it’s own series of commits.

    o---o---o---o---o  master
    \
     ---o---o  2.0-dev

Repository Format

  • .tig/refs/heads: contains one file per branch

  • Name of the file, corresponds to named of the branch

  • Just like master contains the SHA1 of the latest commit in the branch

  • .tig/HEAD:

    • Contains SHA1 if checkout done by SHA1

    • Contains the branch name if checkout done by branch name. Example: ref: refs/heads/master

proj/
  + .tig/
  |  + objects/
  |  | + 4c7be7b2c3641a5e489c4ce667699eeee4e994c9
  |  | + ...
  |  + refs/
  |    + heads/
  |      + master
  |      + 2.0-dev
  |  + HEAD
  |
  + file.txt

Branch Operations

  • Create Branch at commit

    1. Store commit SHA1 in ref: refs/heads/master

  • Checkout

    1. Determine the commit SHA1:

      1. If SHA1 specified, use it.

      2. If branch specified, get SHA1 from ref: refs/heads/your-branch

    2. Get content SHA1 from commit object.

    3. Copy it to file.txt

    4. Set HEAD:

      1. If checkout using SHA1, set to SHA1

      2. If checkout using branch name, set to ref: refs/heads/your-branch

  • Commit

    1. If HEAD does correspond to a branch, cannot commit.

    2. Find SHA1 of file.txt: 58f3966…​

    3. Store file.txt to .tig/objects/58f3966…​

    4. Create and store commit object: 8fb439…​

    5. Set ref: refs/heads/your-branch to commit object

V5: Features

  • Support for simple merge

  • Merge conflicts not supported