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A real world usage for git subtrees.
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example-subtree-usage.md

Let's say you have an iOS project, and you want to use some external library, like AFNetworking. How do you integrate it?

With submodules

Add the project to your repo:

git submodule add git@github.com:AFNetworking/AFNetworking.git Vendor/AFNetworking

or something to that effect.

Well, what happens if you find a bug in AFNetworking that you want to fix? With submodules, I'd usually

  1. Fork AFNetworking
  2. Go through the pain of changing my project's submodules to point to my new fork
  3. Make my change and commit it to my fork
  4. Submit a pull request to AFNetworking repo
  5. Wait to see if the pull request is accepted, but keep my fork up-to-date in the meantime
  6. If it is accepted, do the whole dance to switch my submodule back to the official AFNetworking repo
  7. Continue as usual

Okay, that sucks. If you've ever done it, you know how painful it is and how finicky submodules can be.

With subtrees

Add the project to your repo:

git subtree add --prefix=Vendor/AFNetworking --squash git@github.com:AFNetworking/AFNetworking.git master

This is pretty similar so far except the other members of your team won't have to remember to run git submodule update because subtrees actually store the source in your repo. Nice.

Now, let's say we have the same bug. What do I do differently now that I'm using subtrees?

I make my change and commit it to my project's repository. Technically, I could stop now if I wanted to since the bug fixed code is in my repository. But, I want to be a good open source citizen, so what do I do?

Be a good open source citizen

  1. I'll fork AFNetworking into my account on Github.
  2. Back in my local repo:
git subtree split --prefix=Vendor/AFNetworking/ --branch AFNetworking

to set up being able to push changes to my fork. 3. I'll push my change to my fork, but on a branch to make the pull request more awesome.

git push git@github.com:kvnsmth/AFNetworking.git AFNetworking:critical-bug-fix
  1. I would issue a pull request and hope it gets accepted, but a big difference is that the acceptance of my change doesn't keep me from being able to easily stay in sync with the official AFNetworking repo.

I can still do:

git subtree pull --prefix=Vendor/AFNetworking --squash git@github.com:AFNetworking/AFNetworking.git master

to stay up-to-date with the latest in the official repository.

Now, I think that is much better than using submodules and a lot less invasive to my repo.

@YueLinHo
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Could someone comment about this from enterprise perspective

the entire Windows codebase is moving to a single Git repo... and From the Design History:

Submodules
...
In the end, we dropped that approach, because it created nearly as many problems as it fixed. 

For one, we found that we were complicating people’s workflows
...

Second, it’s not really possible to do atomic commits or pushes across multiple repos
...

And third, most developers are not interested in becoming version control experts, 
...

Can't find anything about subtree there, perhaps it's not even an option. :P

Another ref.:

We started down at least 2 failed paths to scale Git.
Probably the most extensive one was to use Git submodules to stitch together lots of repos into a single “super” repo.
I won’t go into details but after 6 months of working on that we realized it wasn’t going to work
– too many edge cases, too much complexity and fragility.
We needed a bulletproof solution that would be well supported by almost all Git tooling.

(From https://blogs.msdn.microsoft.com/bharry/2017/02/03/scaling-git-and-some-back-story/)

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