Git inside out

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

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.

GIT INSIDE OUT
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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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

GIT INSIDE OUT
GIT IS IMMUTABLE
PREVIOUS SNAPSHOTCURRENT SNAPSHOT

▸Branch
▸Commit
▸Fetch
▸Merge
▸Push
▸Rebase
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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

▸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
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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

▸Graft remote commits into graph
▸Updates refs in remote namespace
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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

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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

Git inside out

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