Cloning Git in Go - Speaker Deck

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Cloning Git in Go Aditya Mukerjee Risk & Systems Engineer at Stripe

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Why write Gitgo? •Git operations from Go applications •Portability •Simplicity •…why not? @chimeracoder

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The dirty secret about Git •Git wasn’t always meant to be a version control system •Git is a decentralized, userland filesystem for preserving historical state and synchronizing data @chimeracoder

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What We’ve Enabled •Backup •Content management •Distributed build systems •Establishing consensus @chimeracoder

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Have you ever seen a “friend’s” computer that looked like this? @chimeracoder

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@chimeracoder 107724969b2af751a1ffd74e3f38d0d76cb8aa55

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@chimeracoder commits trees blobs objects

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Discovery #1: Layered interfaces are more flexible than union types @chimeracoder commits trees blobs objects

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Bootstrapping Git in Go “We can just use the real .git directory for our tests, right?” @chimeracoder …nope.

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hash(“I love Go”) != hash(“I love Go!”) @chimeracoder Packfiles

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Reference: Unpacking Git Packfiles binarydiff(1, 2, 3) 1. “I love Go” 2. “I love Go!” 3. “You love Go, and so do I!” “I love Go” + “!”

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Discovery #2: Pay attention to interface contracts @chimeracoder zlib data zlib data

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@chimeracoder type Reader interface { Read(p []byte) (n int, err error) } Object 1 Object 2 Object 3 Object 4 …

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• Discovery #3: Use concurrency to define state machines, and use channels for the input/output @chimeracoder f1 f3 f2 g g1 if f g2

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• Discovery #4: Treat errors as values, and ask how they behave @chimeracoder httpdir.Open() ParsePackfile() GitFetch()

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• Discovery #5: Well-architected interfaces perform very well @chimeracoder

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#1: Layered interfaces are more flexible than union types #2: Pay attention to interface contracts #3: Use concurrency to define state machines, and use channels for the input/output #4: Treat errors as values, and ask how they behave #5: Well-architected interfaces perform very well @chimeracoder

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Further References • “Unpacking Git Packfiles”: https://codewords.recurse.com/issues/three/unpacking-git-packfiles/ • Gitgo: https://github.com/ChimeraCoder/gitgo/ • “Git from the Inside Out”: https://codewords.recurse.com/issues/two/git-from-the-inside-out

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Aditya Mukerjee @chimeracoder https://github.com/ChimeraCoder