Git Submodule Slides - From PHPUK 2012

sumodule-slides.md

% Git Submodules - The Good + The Bad % Paul Matthews % March 2012

This talk

• Submodules
• Essentials
• Using submodules
• Using distributed submodules
• Alternatives

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Submodules

• Include other git repos
  • Anywhere in the tree
• Submodules != svn:externals
• Only entire repos
• Use in conjunction with symlinks (linux)

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Submodules use cases

• Include other git projects

• Including OS projects, such as frameworks

• Create a custom structure for your codebase

• Deployment?

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Essentials

Git isn't like SVN

• Data structure like linked list
• Everything is a reference
  • Branches
  • Tags

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Git isn't like SVN

• Commit can be multiple branches / tags
• Branch HEAD is only branch

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Using Submodules

• Submodules refer to a commit
• Look like a repo inside a repo
  • Inside the submodule you can't tell you're in submodule

Definition

• Parent : Amazon
• Child : Magento

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Creating child

• Do Nothing from child

Create parent

• Just Add them

git submodule add <repo> <location>

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Cloning parent

• 2 stage process
• Need to initialise the submodules

git submodule init

• Then pull the submodules down

git submodule update

• Alternatively use "--recursive" clone

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Updating a submodule

• Submodules don't update themselves
  • Good / Bad
• Add the submodule like a file
• Work-flow:
  • Work on child
  • Commit
  • Goto parent
  • Add child reference
  • Commit

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Updating a submodule (demo)

• Work on child

cd ~/amazon/magento
vim index.php

• Commit

commit -am "Updating magento"

• Goto parent

cd ~/amazon

• Add child reference

add ~/amazon/magento

• Commit

commit -m "Getting latest version of magento"

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Advanced Submodules

• Submodules don't support multiple remotes
  • by default
• Can be used to checkout multiple repos
  • Into a specific state
• Useful for distributing many repos to teams
• User can check in an inaccessible commit
• Utility command

git submodule foreach [command]

• e.g:

git submodule foreach git branch

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Submodules controlling features

• Children contain feature branches
• So too can the parent
  • Ensures correct libraries per feature
  • Ensures system state

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Overcoming multiple remotes

• Create an init script
  • Adds all the remotes
• Beware:
  • HEAD commit of submodule must be accessible on default remote

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Alternatives to submodules

• Subtree Merging
  • More useful for frameworks and libraries
  • You commit the child code to your parent branch

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Roundup

• Submodules
• Essentials
• Using submodules
• Using distributed submodules
• Alternatives