git internals

Buildings

Git is a content addressable system

  • It is a simple key-value data store
  • You can insert any kind of content into a Git Repo.
    • Git will hand you back a unique key you can use later to retrieve that content
  • git hash-object command
    • takes in some data and stores in .git/objects folder -- this object database
    • gives back an unique key which references this object

Let us play with git hash-object

~  >git init git-playground
hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change. To configure the initial branch name to use in all
hint: of your new repositories, which will suppress this warning, call:
hint: 
hint: 	git config --global init.defaultBranch <name>
hint: 
hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hint: 'development'. The just-created branch can be renamed via this command:
hint: 
hint: 	git branch -m <name>
Initialized empty Git repository in /Users/mchinnappan/git-playground/.git/

~  >cd git-playground 

~/git-playground [master] >tree
.

0 directories, 0 files
~/git-playground [master] >tree .git 
.git
├── HEAD
├── config
├── description
├── hooks
│   ├── applypatch-msg.sample
│   ├── commit-msg.sample
│   ├── fsmonitor-watchman.sample
│   ├── post-update.sample
│   ├── pre-applypatch.sample
│   ├── pre-commit.sample
│   ├── pre-merge-commit.sample
│   ├── pre-push.sample
│   ├── pre-rebase.sample
│   ├── pre-receive.sample
│   ├── prepare-commit-msg.sample
│   ├── push-to-checkout.sample
│   └── update.sample
├── info
│   └── exclude
├── objects
│   ├── info
│   └── pack
└── refs
    ├── heads
    └── tags

8 directories, 17 files

Let us add content to object database

~/git-playground [master] >echo 'test content' | git hash-object -w --stdin
d670460b4b4aece5915caf5c68d12f560a9fe3e4


~/git-playground [master] >tree
.

0 directories, 0 files
~/git-playground [master] >tree .git                                       
.git
├── HEAD
├── config
├── description
├── hooks
│   ├── applypatch-msg.sample
│   ├── commit-msg.sample
│   ├── fsmonitor-watchman.sample
│   ├── post-update.sample
│   ├── pre-applypatch.sample
│   ├── pre-commit.sample
│   ├── pre-merge-commit.sample
│   ├── pre-push.sample
│   ├── pre-rebase.sample
│   ├── pre-receive.sample
│   ├── prepare-commit-msg.sample
│   ├── push-to-checkout.sample
│   └── update.sample
├── info
│   └── exclude
├── objects
│   ├── d6 <----- first 2 characters of the SHA-1
│   │   └── 70460b4b4aece5915caf5c68d12f560a9fe3e4 <------- SHA-1 checksum of the content and its header.
│   ├── info
│   └── pack
└── refs
    ├── heads
    └── tags

9 directories, 18 files
~/git-playground [master] >

find .git/objects -type f
.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4

Let us examine the content of the content with git cat-file

git cat-file -p d670460b4b4aece5915caf5c68d12f560a9fe3e4 

test content

Let us add a file to the object database

echo 'Version: 1' > test.txt 
git hash-object -w test.txt
f8acaeb421037571491c643e96acc09faea03b24

# Let us the object created by this:
find .git/objects -type f
.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4
.git/objects/f8/acaeb421037571491c643e96acc09faea03b24 <------

Let us add version 2 of the test.txt

echo 'Version: 2' > test.txt 
git hash-object -w test.txt
939ba388fd33e17497577f5cfacd6aa84ef219b9

# Let us the object created by this:
find .git/objects -type f

.git/objects/93/9ba388fd33e17497577f5cfacd6aa84ef219b9 <-----
.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4
.git/objects/f8/acaeb421037571491c643e96acc09faea03b24
  • Now you can delete your local copy of that test.txt file, then use Git to retrieve, from the object database, either the first version or second version you saved
rm test.txt

ls -l

git cat-file -p 939ba388fd33e17497577f5cfacd6aa84ef219b9
Version: 2

# Version 1:
git cat-file -p f8acaeb421037571491c643e96acc09faea03b24

Type of this SHA-1 is blob

git cat-file -t f8acaeb421037571491c643e96acc09faea03b24 
blob
  • All the content in git is stored as tree and blob objects
    • with trees corresponding to UNIX directory entries
    • blobs corresponding more or less to i-nodes or file contents

Tree Objects

  • Tree object solves the problem of storing the filename and also allows you to store a group of files together
~/git-playground [master] >mkdir src
~/git-playground [master] >touch README.md
~/git-playground [master] >touch src/hello.js

tree
.
├── README.md --> blob
└── src --> tree
    └── hello.js --> blob


~/git-playground [master] >git add -A                 
~/git-playground [master] >git commit -m init
[master (root-commit) 27c4b42] init
 2 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 README.md
 create mode 100644 src/hello.js
~/git-playground [master] >tree .git         
.git
├── COMMIT_EDITMSG
├── HEAD
├── config
├── description
├── hooks
│   ├── applypatch-msg.sample
│   ├── commit-msg.sample
│   ├── fsmonitor-watchman.sample
│   ├── post-update.sample
│   ├── pre-applypatch.sample
│   ├── pre-commit.sample
│   ├── pre-merge-commit.sample
│   ├── pre-push.sample
│   ├── pre-rebase.sample
│   ├── pre-receive.sample
│   ├── prepare-commit-msg.sample
│   ├── push-to-checkout.sample
│   └── update.sample
├── index
├── info
│   └── exclude
├── logs
│   ├── HEAD
│   └── refs
│       └── heads
│           └── master
├── objects
│   ├── 27
│   │   └── c4b42327458637549fb9d00170bdc7b07ed77b
│   ├── 5a
│   │   └── 38b439d9a9e0a7d8d12f1e88cad5f881968a6b
│   ├── 93
│   │   └── 9ba388fd33e17497577f5cfacd6aa84ef219b9
│   ├── d6
│   │   └── 70460b4b4aece5915caf5c68d12f560a9fe3e4
│   ├── e1
│   │   └── 52c4c52532fda392dece4c503567d30a9ae059
│   ├── e6
│   │   └── 9de29bb2d1d6434b8b29ae775ad8c2e48c5391
│   ├── f8
│   │   └── acaeb421037571491c643e96acc09faea03b24
│   ├── info
│   └── pack
└── refs
    ├── heads
    │   └── master
    └── tags

18 directories, 29 files
  • master^{tree} syntax specifies the tree object that is pointed to by the last commit on your master branch.
~/git-playground [master] >git cat-file -p master^{tree}
100644 blob e69de29bb2d1d6434b8b29ae775ad8c2e48c5391	README.md
040000 tree e152c4c52532fda392dece4c503567d30a9ae059	src


~/git-playground [master] >vi src/hello.js 
~/git-playground [master] >cat src/hello.js 
console.log('hello');

git add -A
~/git-playground [master] >git commit -m 'hello added'



>tree
.
├── README.md
└── src
    └── hello.js

References

Videos