Using Submodules.

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In this post we will learn about frequently used method to include Submodules in github repository.Before learning submodules you should know the dependency management.

What is dependency management ?

While making API if it depends upon several other modules it is necessary to include them in one in order to ensure proper functioning of API.This is especially done when bigger project are involved and all dependency should be kept on one level. Dependency tool is available for dependency management.

What are submodules ?

A submodule in a git repository is like a sub-directory which is really a separate git repository in its own right. This is a useful feature when you have a project in git which depends on a particular versions of other projects.

Here are some languages and thier dependency management ;

And many more examples can be found.

You should have proper version of git.It is important to have proper git version. When you add a submodule in Git, you don’t add the code of the submodule to the main repository, you only add information about the submodule that is added to the main repository. This information describes which commit the submodule is pointing at. This way, the submodule’s code won’t automatically be updated if the submodule’s repository is updated. This is good, because your code might not work with the latest commit of the submodule, it prevents unexpected behaviour.

How do we add submodule ?
   git submodule add <github-repo-which-you-want-to-include>
How to get submodule ?
  git submodule init
  git submodule update

  OR
  git submodule update --init

submodule add command adds new .gitmodule file and the repository which is included as submodule.Then to commit changes in repository

  git commit -m "Adding submodule"
Submodules or Subtree ,Confuse ?

Many of us may have confusion regarding submodule and subtree. we have seen submodule above now we will see how to make subtree.

git subtree add --prefix=submodule-name <link-to-submodule> master --squash

you can consider subtree as copy or integrating another repository into your own repository. The subtree command adds the the submodule-name to your own repository. Usually this approach is not used as it takes more space in your repository .Consequently ,using submodule creates link to the repository which user may or may not use it depends on user whether to use submodule or just use the source code in repository(If the submodule is optional).

How submodules are represented ?

This below information I found useful on Mark’s blog.So I thought it might be useful for others.More information can be found on his blog

$ git ls-tree HEAD^{tree}
100644 blob 1e38e022c1c7d27f6dd9b765793087b59d147ef8    .cvsignore
100755 blob 44c881fe25b8dc1413d9195677f492121a3789f0    INSTALL.txt
100644 blob 37312d9a1bcc80ac334547f047a2cece38dd24dc    README
100644 blob 3bb0e8592a41ae3185ee32266c860714980dbed7    Rakefile
040000 tree e326ffb3d697e7ac83fa19d93a8a3305120c719e    app
160000 commit fd91ab69279f1e0cfed53353e64811d5aa9c4b5f    commonlib
040000 tree ae93b14ec7ab01ee33053c32eca340a31ce6449f    config
100644 blob bfc265e33e47ffa9796fe7bb7ae7d1fe7e633593    todo.txt
040000 tree 2999c0a790c0033ad93e312c0bc62ecdc9a18f81    vendor

Various file object such as blob -> Indicates File. tree -> Subdirectory. commit -> Indicates Submodule 1e38e022c1c7d27f6dd9b765793087b59d147ef8 -> SHA1 hash

How to see status of submodules ?

To view status of submodule we use just git submodule without any arguments.

git submodule status --recursive

If –recursive is specified, this command will recurse into nested submodules, and show their status as well.If your repository has multiple embeded submodules.

You can see each SHA-1 hash begins with a space, a ‘+’ or a ‘-‘ which indicate the following things:

+ The version checked out in the submodule is different from that specified in the super-project. The object name shown is that of the commit that the submodule is currently at.

– The submodule hasn’t been initialized or you may not have committed the changes in github repository.

U This option indicates if the given module has merge conflicts.

[space] The submodule’s HEAD is at the correct version .

90287f0250542be256f67ade4e29a618bf6e688f TrakEM2 (0.7m-227-g90287f0)
+f25db2a43b95480c780d865323fce659a1135c2d VIB (tracer-1.4.0-candidate-849-gf25db2a)
e4d3eb47a8f9d4e62d1f356636652c3ecc739d92 batik (remotes/origin/svn/git-svn@216063-588-ge4d3eb4)9fa7f4d993f57e27e3134b016c7d36fbfd33e34c ij-plugins (9fa7f4d)
7ffa48359cdbf7a47735b719a605ea322c58d694 java/linux (heads/master)
-cc218f05fdc0bb55f40f904d5d1f804e8751d0d2 java/linux-amd64
-4f3964234f4e6fd78247e5e7fad9c8becad53e8f java/macosx-java3d

More can be learnt regarding submodules.

Downside of using submodule

One of the advantage that dependency manager such as Apache ivy has transitive dependency resolution.It downloads already compiled packages. straight from Apache ivy pages With Apache Ivy, it’s different: simply write a dependency file once for the component, and benefit from the work already done anytime this component is reuse.

Resources

Git

Heroku Dev center

Atlassian blog