community/contributors/devel/godep.md

Using godep to manage dependencies

This document is intended to show a way for managing vendor/ tree dependencies in Kubernetes. If you do not need to manage vendored dependencies, you probably do not need to read this.

Background

As a tool, godep leaves much to be desired. It builds on go get, and adds the ability to pin dependencies to exact git version. The go get tool itself doesn't have any concept of versions, and tends to blow up if it finds a git repo synced to anything but master, but that is exactly the state that godep leaves repos. This is a recipe for frustration when people try to use the tools.

This doc will focus on predictability and reproducibility.

Theory of operation

The go toolchain assumes a global workspace that hosts all of your Go code.

The godep tool operates by first "restoring" dependencies into your $GOPATH. This reads the Godeps.json file, downloads all of the dependencies from the internet, and syncs them to the specified revisions. You can then make changes - sync to different revisions or edit Kubernetes code to use new dependencies (and satisfy them with go get). When ready, you tell godep to "save" everything, which it does by walking the Kubernetes code, finding all required dependencies, copying them from $GOPATH into the vendor/ directory, and rewriting Godeps.json.

This does not work well, when combined with a global Go workspace. Instead, we will set up a private workspace for this process.

The Kubernetes build process uses this same technique, and offers a tool called run-in-gopath.sh which sets up and switches to a local, private workspace, including setting up $GOPATH and $PATH. If you wrap commands with this tool, they will use the private workspace, which will not conflict with other projects and is easily cleaned up and recreated.

To see this in action, you can run an interactive shell in this environment:

# Run a shell, but don't run your own shell initializations.
hack/run-in-gopath.sh bash --norc --noprofile

Restoring deps

To extract and download dependencies into $GOPATH we provide a script: hack/godep-restore.sh. If you run this tool, it will restore into your own $GOPATH. If you wrap it in run-in-gopath.sh it will restore into your _output/ directory.

hack/run-in-gopath.sh hack/godep-restore.sh

This script will try to optimize what it needs to download, and if it seems the dependencies are all present already, it will return very quickly.

If there's every any doubt about the correctness of your dependencies, you can simply make clean or rm -rf _output, and run it again.

Now you should have a clean copy of all of the Kubernetes dependencies.

Making changes

The most common things people need to do with deps are add and update them. These are similar but different.

Adding a dep

For the sake of examples, consider that we have discovered a wonderful Go library at example.com/go/frob. The first thing you need to do is get that code into your workspace:

hack/run-in-gopath.sh go get -d example.com/go/frob

This will fetch, but not compile (omit the -d if you want to compile it now), the library into your private $GOPATH. It will pull whatever the default revision of that library is, typically the master branch for git repositories. If this is not the revision you need, you can change it, for example to v1.0.0:

hack/run-in-gopath.sh bash -c 'git -C $GOPATH/src/example.com/go/frob checkout v1.0.0'

Now that the code is present, you can start to use it in Kubernetes code. Because it is in your private workspace's $GOPATH, it might not be part of your own $GOPATH, so tools like goimports might not find it. This is an unfortunate side-effect of this process. You can either add the whole private workspace to your own $GOPATH or you can go get the library into your own $GOPATH until it is properly vendored into Kubernetes.

Another possible complication is a dep that uses gopdep itself. In that case, you need to restore its dependencies, too:

hack/run-in-gopath.sh bash -c 'cd $GOPATH/src/example.com/go/frob && godep restore'

If the transitive deps collide with Kubernetes deps, you may have to manually resolve things. This is where the ability to run a shell in this environment comes in handy:

hack/run-in-gopath.sh bash --norc --noprofile

Updating a dep

Sometimes we already have a dep, but the version of it is wrong. Because of the way that godep and go get interact (badly) it's generally easiest to hit it with a big hammer:

hack/run-in-gopath.sh bash -c 'rm -rf $GOPATH/src/example.com/go/frob'
hack/run-in-gopath.sh go get -d example.com/go/frob
hack/run-in-gopath.sh bash -c 'git -C $GOPATH/src/example.com/go/frob checkout v2.0.0'

This will remove the code, re-fetch it, and sync to your desired version.

Removing a dep

This happens almost for free. If you edit Kubernetes code and remove the last use of a given dependency, you only need to restore and save the deps, and the godep tool will figure out that you don't need that dep any more:

Saving deps

Now that you have made your changes - adding, updating, or removing the use of a dep - you need to rebuild the dependency database and make changes to the vendor/ directory.

hack/run-in-gopath.sh hack/godep-save.sh

This will run through all of the primary targets for the Kubernetes project, calculate which deps are needed, and rebuild the database. It will also regenerate other metadata files which the project needs, such as BUILD files and the LICENSE database.

Saving deps in staging repos

Kubernetes stores some code in a directory called staging which is handled specially, and is not covered by the above. If you modified any code under staging, or if you changed a dependency of code under staging (even transitively), you'll also need to update deps there:

./hack/update-staging-godeps.sh

Sanity checking

After all of this is done, git status should show you what files have been modified and added/removed. Make sure to sanity-check them with git diff, and to git add and git rm them, as needed. It is commonly advised to make one git commit which includes just the dependencies and Godeps files, and another git commit that includes changes to Kubernetes code to use (or stop using) the new/updated/removed dependency. These commits can go into a single pull request.

Before sending your PR, it's a good idea to sanity check that your Godeps.json file and the contents of vendor/ are ok:

hack/run-in-gopath.sh hack/verify-godeps.sh

All this script will do is a restore, followed by a save, and then look for changes. If you followed the above instructions, it should be clean. If it is not, you get to figure out why.

Manual updates

It is sometimes expedient to manually fix the Godeps.json file to minimize the changes. However, without great care this can lead to failures with the verifier scripts. The kubernetes codebase does "interesting things" with symlinks between vendor/ and staging/ to allow multiple Go import paths to coexist in the same git repo.

The verifiers, including hack/verify-godeps.sh must pass for every pull request.