Git Graphs, Hashes, and Compression, Oh My

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@matthewmccull matthew@github.com github.com/training

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GIT - the stupid content tracker "git" can mean anything, depending on your mood. * random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant. * stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang. * "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room. * "goddamn idiotic truckload of sh*t": when it breaks Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals. Git is an Open Source project covered by the GNU General Public License. It was originally written by Linus Torvalds with help of a group of hackers around the net. It is currently maintained by Junio C Hamano. “ ”

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2005 git

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Architecture Hashes

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centralized VCSs use sequential revision numbers

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Git uses a SHA-1 hash

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40 hex characters (20 bytes)

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9AB223D28B1AA46EF1780B22F304982E39872C34

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9AB223D28B1AA46EF1780B22F304982E39872C34

This is a test

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This is a test

9AB223D28B1AA46EF1780B22F304982E39872C34

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Hashed Content Git commits without Git

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# Green field project $ git init newproject $ cd newproject # ...start coding

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.git !"" COMMIT_EDITMSG !"" HEAD !"" MERGE_RR !"" config !"" description !"" hooks # !"" pre-commit.sample # $"" update.sample !"" index !"" info # $"" exclude !"" logs # !"" HEAD # $"" refs # $"" heads # $"" master !"" objects # !"" 54 # # $"" 3b9bebdc6bd5c4b22136034a95dd097a57d3dd # !"" info # $"" pack !"" refs !"" heads # $"" master $"" tags

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$ printf "blob 12\000Hello World\n" | shasum

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Architecture Hash Shortcuts

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use as little of it as is unique

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9AB223D28B1AA46EF 1780B22F304982E39 872C34 a certain commit

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9AB22F a certain commit

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git rev-parse 9AB22F expand the commit ref

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Architecture Storage

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Typical SCMs use delta storage

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CVS / Subversion / darcs / Mercurial

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B Δ Δ Δ Δ Δ Δ Δ

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Checkin Checkin Checkin Checkin Checkin Checkin Checkin Checkin Checkin Checkin Checkin Delta storage gets slower as the history of a ﬁle gets longer

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Directed Acyclic Graph

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Copy of the entire tree per checkin

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cp -r srcfolder srcfolder.prev

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Why?

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C

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hard link to existing identical blobs

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C ß

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zlib deﬂates each blob at commit

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C

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zlib deﬂates the entire repo

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C

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v1 v2 v3 v4 File A File B File C File A File B File B File C v5 File A File B File B File A File A File C File C File C

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2100 MB became 205 MB Act I

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Architecture Hash relationships

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‣Blob ‣Tree ‣Commit ‣Tag

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tree tree: 7e8b1 web blob: 9ab16 index.html a10b3 tree blob: 8d162 logo.jpg blob: 51d22 draw.js 7e8b1 commit tree: a10b3 parent: nil author: Fird committer: Matthew message: Major refactoring of the web content. c67db blob 9ab16 blob //Some more javascript var renderSize 51d22 blob 7D 8D B3 7F BD 12 9F E9 7B 78 9D 3F 5C A6 72 CB 8d162

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v1 v2 v3 commit tree: 9a87b parent: nil author: Fird committer: Matthew message: Major refactoring of the Javascript rendering engine. c67db commit tree: b22c1 parent: c67db author: Tim committer: Fird message: Minor update to HTML 9bd21 commit tree: b22c1 parent: 9bd21 author: Johnny committer: Joe message: New language transations 1c2d7

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RELEASE_1.0 HEAD bug979branch commit c67db commit 9bd21 commit 1c2d7 commit 8c2d1 commit 1bdcd commit 2daa1

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RELEASE_1.0 HEAD bug979branch commit c67db commit 9bd21 commit 1c2d7 commit 8c2d1 commit 1bdcd commit 2daa1

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RELEASE_1.0 HEAD bug979branch commit c67db commit 9bd21 commit 1c2d7 commit 8c2d1 commit 1bdcd commit 2daa1

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RELEASE_1.0 HEAD bug979branch commit c67db commit 9bd21 commit 1c2d7 commit 8c2d1 commit 1bdcd commit 2daa1

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RELEASE_1.0 HEAD bug979branch commit c67db commit 9bd21 commit 1c2d7 commit 8c2d1 commit 1bdcd commit 2daa1

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RELEASE_1.0 HEAD bug979branch commit c67db commit 9bd21 commit 1c2d7 commit 8c2d1 commit 1bdcd commit 2daa1

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Architecture Hash shortcuts

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commitish & treeish

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commitish = shorthand for commit hashes

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treeish = shorthand for tree hashes

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9AB22F a certain commit

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9AB22F^ one commit before a certain commit

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9AB22F^^ two commits before a certain commit

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9AB22F~5 ﬁve commits before a certain commit

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9AB223..56CD77 between these two commits

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HEAD the most recent commit on this branch

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HEAD^ one commit before the most recent commit

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HEAD~2 two commits before the most recent commit

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HEAD..HEAD^^^ between the given recent commits

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master the most recent commit on this branch

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master^^ two commits before the most recent commit on this branch

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master~5 ﬁve commits before the most recent commit on this branch

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remotes/origin/master the most recent commit on this remote tracking branch

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origin/master the most recent commit on this remote tracking branch

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The Graph Veriﬁcation

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git fsck

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git verify-pack -v .git/objects/pack/FILENAME

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The Graph More graph navigation

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master^{tree} ﬁnd this commit’s tree

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git describe HASH ﬁnd nearest tag

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:/some words commit message that starts with

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REF:FILE blob spec

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:0:FILE index

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:1:FILE common ancestor

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:2:FILE target

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:3:FILE merging in

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@matthewmccull matthew@github.com github.com/training