This document summarizes a presentation on understanding the internals of Git. It discusses key Git concepts like Git being distributed, content-addressable, and operating as both a database and a graph. It outlines paradigm shifts in thinking of commits as separate from sharing. The document then dissects common Git operations like branching, committing, fetching, merging, rebasing, and pushing to demonstrate how they manipulate the underlying graph. It also provides advice on commands to recover from errors like reset, reflog, and interactive rebasing.

1. INSIDE
GIT

2. GIT INSIDE OUT
MICHAEL NADEL
▸Developer @ Pine River Capital Management
▸New(ish) to .NET
▸3-year Git practitioner
▸Please reach out!
▸michael.nadel@gmail.com
▸@mnadel

3. GIT INSIDE OUT
GIT IS HARD
▸Linus Torvalds, creator of Git (and Linux)
▸Initial revision of “git”, the information manager from hell
▸I didn’t really expect anyone to use it because it’s so hard
to use.
▸Andrew Morton, lead Linux kernel developer
▸Git is expressly designed to make you feel less intelligent
than you thought you were.

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THE CHALLENGE WITH GIT
▸Plenty of rope
▸Paradigm shifts
▸Distributed
▸Content-addressable filesystem

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WHY INSIDE OUT?

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AGENDA
▸Paradigm shifts
▸Conceptual models
▸Overview of internals
▸Dissect common operations

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DISTRIBUTED
▸No central authority (except by convention)
YOUR REPO/CLONE OTHERS’ REPOS
CLONE
FETCH
PUSH

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COMMITTING != SHARING
▸Separate concerns
▸Crafting your history
▸Publishing your history
▸Richer workflows
▸Commit, commit, commit, squash, push
▸Reorder, push subset
▸Enforced code reviews

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CONTENT ADDRESSABLE
▸Version control is an abstraction on top of a primitive
key/value store
▸hash-object
▸cat-file
▸Prove
▸cat-file performs no magic

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CONTENT ADDRESSABLE
▸Given arbitrary content

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CONTENT ADDRESSABLE
▸Git primitive: hash-object

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CONTENT ADDRESSABLE
▸Git returns its key (SHA1 hash of contents)

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CONTENT ADDRESSABLE
▸Git primitive: cat-file

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CONTENT ADDRESSABLE
▸Git returns the original content

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CONTENT ADDRESSABLE
▸Final proof!

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CONTENT ADDRESSABLE FILESYSTEM
▸Instead of text, how about your filesystem?

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CONCEPTUAL MODELS
▸Git as a Database
▸Store, retrieve, search your source code & its history
▸Git as a Graph
▸CRUD operations are performed against a graph of
commits

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GIT AS A DATABASE
▸CRUD, search operations
▸Data types
▸Commit
▸Tree
▸Blob
Structured text
byte[]

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DB TYPE - COMMIT

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DB TYPE - COMMIT

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DB TYPE - COMMIT

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DB TYPE - TREE

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DB TYPE - TREE

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DB TYPE - TREE

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DB TYPE - TREE

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DB TYPE - BLOB

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CONTENT ADDRESSABLE FILESYSTEM
93b6
ae3e
7bbc63b7
4e3c 3ca5

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GIT AS A GRAPH
▸What operations must I perform to get the graph to look the
way I want?

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GIT COMMANDMENTS
▸Git is immutable
▸No updates, only appends
▸Git is a directed acyclic graph (DAG)
▸Directed: can only traverse in a single direction
▸Acyclic: no cycles — traversals only visit a node once
▸Every command is an operation on the graph

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GIT IS IMMUTABLE
PREVIOUS SNAPSHOTCURRENT SNAPSHOT

31. ▸Branch
▸Commit
▸Fetch
▸Merge
▸Push
▸Rebase
GIT INSIDE OUT
DISSECT COMMON OPERATIONS

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REFS, HEADS, BRANCHES
▸Ref is a pointer to a commit
▸Branch is a ref
▸HEAD is a pointer to your current branch
▸Branches have “namespaces”

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BRANCH (BEFORE)
▸A branch is a ref
▸A ref is a pointer to a commit

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BRANCH (AFTER)
▸A branch is a ref
▸A ref is a pointer to a commit

35. ▸Heads contain your branches
▸Remotes contain remote
branches (eg origin)
▸“Namespaces” are directories
▸Branches are 40-byte files
containing a SHA1 hash of a
commit object
GIT INSIDE OUT
BRANCH IMPLEMENTATION

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COMMIT (BEFORE)
▸A commit references its parent
▸HEAD, branch point at commit

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COMMIT (AFTER)
▸Add a new node
▸Advance branch

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FETCH (BEFORE)
▸Fetch brings remote objects into repo
▸Refs, commits
▸But Git is immutable

39. ▸Graft remote commits into graph
▸Updates refs in remote namespace
GIT INSIDE OUT
FETCH (AFTER)

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MERGE (BEFORE)
▸Merge is just a commit object
▸But which refers to two parents

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MERGE (AFTER)
▸Creates new commit with two parents
▸Current branch, other branch
▸Advance branch

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FAST-FORWARD MERGE (BEFORE)
▸Type of merge
▸Is there a traversal from your branch to the other’s?

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FAST-FORWARD MERGE (AFTER)
▸Simplest merge
▸Pointer manipulation only

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REBASE (BEFORE)
▸Type of merge
▸Without merge commit objects

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REBASE (AFTER)
▸Replay your commits onto the other branch

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PUSH
▸Two-step process
▸“Reverse” fetch — push commits, refs
▸Attempt a fast-forward merge

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PUSH
▸What will happen if you push?
▸How do you recover?

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SAVING YOURSELF - RESET

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SAVING YOURSELF - REFLOG

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SAVING YOURSELF - INTERACTIVE
REBASE

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RECAP - CONCEPTUAL MODELS
▸Duality of Git
▸As a database
▸As an immutable DAG
▸Reasoning through problems
▸Launch SmartGit & observe the result of commands
against the DAG

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RECAP - “OH SHIT!” COMMANDS
▸git reflog
▸git reset
▸—soft won’t affect your workspace
▸—hard will make your workspace reflect where your
HEAD moved to (you can lose work)
▸git rebase -i

53. GIT INSIDE OUT
RESOURCES
▸https://git-scm.com/book/en/v1/Git-Internals
▸https://github.com/pluralsight/git-internals-pdf/releases
▸https://pinboard.in/u:mnadel/t:git/
▸http://www.syntevo.com/smartgit/
▸Free for open source projects
▸Also: https://www.sourcetreeapp.com/
▸michael.nadel@gmail.com