dissecting-gits-guts.md

DISSECTING GIT'S GUTS

@emilyxxie

• For most people, Git is complete sorcery
• Git happens 😆
• Why dissect Git? Gain comfort, better mental model. E.g. what is at the heart of a branch? What is a detached HEAD state?
• Git Porcelain: the high-level commands we use on a daily basis
• Plumbing: the internal, low-level commands

Agenda

• Git directory

• Git objects

• Git references

• Git packfiles

• Git directory

  • .git
  • git init puts the scaffolding in place needed to work with git on a project
  • A git clone just copies the .git folder
  • ls .git
    • objects
    • refs
    • …

• Objects

  • Where the content is!
  • Git’s database
    • Content-addressable filesystem
    • Git gives you a key to retrieve content from the database
    • git hash-object -w hello_world.txt plumbing command, gives use the SHA-1! 💥
    • For a given set of content, you reliably generate the same hash
      • A fingerprint
    • git cat-file -p allows you to read from the database, -p makes the output pretty, -t by the way, returns the object’s type (e.g. blob, tree)
    • Git’s database stores compressed objects, compressed with zlib

• Exercise

• Cheat sheet

• git hash-object -w bio.md

• git cat-file -p 9d6b6928c57e27ddd64bcad64b1a53623c9867b2

• Git doesn’t store diffs, for each version of a file, it installs an entire copy of the file (initially) but… [follow up later on this]

• Trees

  • A complete snapshot of your directory
  • To create a tree… Need an index first (git update-index --add, a low-level git add), index lives in .git/index
  • git ls-files —stage shows the index
  • git write-tree
  • Note, trees can reference other trees (sub-directories)

• Exercise: Make a tree!

git write-tree
0baa49142494d6bb979bcd36572662bdd62f2160
git cat-file -p 0baa49142494d6bb979bcd36572662bdd62f2160
100644 blob 9d6b6928c57e27ddd64bcad64b1a53623c9867b2	bio.md
040000 tree d35e12dfe16923b05e99fbf49c31e91c18eb1db5	subdir
git cat-file -p d35e12dfe16923b05e99fbf49c31e91c18eb1db5
100644 blob 8ab01e46b848c561cf37000562915c4ae4d116ba	barbaz.md
100644 blob 5ddc4a0635b7e92187567282868679136da1ec1f	foobar.md

• Commit objects

  • git commit-tree (echo “first commit” | git commit-tree $SHA -p $PARENT_SHA)
  • We’ve done a git commit! Up until now, we have not committed anything, nuttin’!

• Exercise: Commit an object!

echo "First commit" | git commit-tree 0baa49142494d6bb979bcd36572662bdd62f2160
cat >> foobar.md
git update-index --add foobar.md
git write-tree
echo "Second commit" | git commit-tree 4fa2797f288b553d0ed8cddb59b3719f817cdabe -p f6e6ec6a7806b894ccce06c57a6568798b11ab60
git cat-file -p ea99086abafac64f6598a3053d80c64a24fc3f04
tree 4fa2797f288b553d0ed8cddb59b3719f817cdabe
parent f6e6ec6a7806b894ccce06c57a6568798b11ab60
author Steve Winton <stevewinton@gmail.com> 1467069112 -0700
committer Steve Winton <stevewinton@gmail.com> 1467069112 -0700

Second commit

• 🎉 We’ve now performed all the low-level plumbing commands for git add, git commit

• Git is a both a DAG and a Merkle tree (Merkle DAG)

  • Merkle also used with Bitcoin

• Git references

• We have blobs, trees, commits as our low-level objects in our Git objects database

• How do we keep track of what commit we want to work with?

  • Branches, but what is a branch?
  • How does Git know what goes with a branch?
  • Branches are just pointers, that reside in .git/refs/heads

• git update-ref can be used to create a branch

  • git update-ref refs/heads/master $SHA
  • Just a file, containing the SHA the branch points to 😮

• What about .git/HEAD?

  • Just a pointer to current branch
  • Detached head is when HEAD points to a commit, not to a branch
  • Any commits made on detached HEAD will not belong to a branch, and will eventually get pruned by Git
  • To recover: Checkout a branch

• Exercise: References

 git update-ref refs/heads/master ea99086abafac64f6598a3053d80c64a24fc3f04
 cat .git/refs/heads/master
 git checkout -b my-feature-is-a-feature
 ls -a .git/refs/heads/
 cat .git/refs/heads/my-feature-is-a-feature
 cat bio.md
 cat >> bio.md
 git st
 git add bio.md
 git commit
 cat .git/refs/heads/my-feature-is-a-feature

• Packfiles
  • git gc garbage collects
  • git count-objects, returns total no. of lose objects, should significantly shrink after git gc
  • git verify-pack

Resources

• Pro Git
• Git from the Inside Out
• Git from the Bottom UP
• Advanced Git: Graphs, Hashes, and Compression, Oh My.
• Unpacking Git Pacifies
• Visualizing Git’s Merkle DAG with D3.js

Slides?

• This was recorded at Git Merge, recording available… 🔜?

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Mon Jun 27 15:05:30 PDT 2016