Solving problems with dependency management in Go

In a previous blog post we talked about how we use the programming language Go. But it only briefly touched on the issue of dependency management, where Go is reliant on backwards compatible or vendored software. This post is about what we’ve done about that.

As developers we regularly use other libraries or packages to help us solve a problem rather than writing everything ourselves. Most programming languages offer a dependency system for distributing libraries. At GDS we use languages like Ruby, Python and Java and each language has its preferred tool to manage these dependencies: bundler, pip and gradle, for example.

Libraries installed through a package system can be installed system-wide (known as site packages) or copied somewhere into the directory containing the project (known as vendoring). However this tripped us up when we started to use Go. This is because Go doesn’t currently provide a way of defining specific versions of dependencies to use. The language maintainers have publicly endorsed vendoring of dependencies, but this isn’t always productive when you want an easy way to keep various dependencies up to date.

Dependencies in Go
Dependencies in Go are downloaded using the go get tool and placed into a directory determined by the $GOPATH environment variable. Dependencies aren't explicitly pinned to a version and the recommended model of development is to always keep the master branch backwards compatible and up-to-date, or to vendor dependencies as required.

If two developers were to work on a single project at two points in time, with some difference in-between. And the dependencies that were used had changed during this time. Then the software they'd be working on would be different and could cause issues.

This was a problem for us because we wanted to make sure that the various services we write in Go can always be compiled using specific versions of each library we were using.

Pinning dependencies to specific versions
The most common way to define specific versions of dependencies is to create a dependency file that references specific tags or commits in the different packaging systems the code is located in.

The dependency management tool is used to get, build and install the library. One advantage is that your project repository continues to only use the specific library with the state you expect.

Pinning dependencies to specific versions in Go
go get does not have an explicit concept of versions. As a result, different members of a team could be importing different versions of any libraries being used. This is less than ideal.

Different dependencies may introduce incompatibilities, security vulnerabilities, or be removed entirely. For these reasons, it's better to have more control over the upgrade process when building a real-world application.

What have we chosen?
As well as being able to pin versions of libraries we use, another requirement for us was being able to compile and run Go code without requiring a global $GOPATH to be in place.

Our current solution is either Godep or gom, depending on the project. We’ve chosen two options because different projects have different requirements. For some projects we want to pin a specific version but don’t mind downloading dependencies and don’t require a fixed $GOPATH. Whereas for others we want to vendor the dependencies because of how we view the code and the respective binary we generate.

Godep vs. gom
Godep's preferred specification format is JSON. It will create a dependency file and create copies of all of your libraries into a Godeps directory at the root of your package. The expectation being that the libraries will be checked in with the rest of your code once you start using Godep.

In comparison, gom is very similar to Ruby's bundler. It expects a Gomfile to list the dependencies of the project, in a Ruby-like syntax which will be familiar to Bundler users. The dependencies are downloaded into a _vendor directory and there's no need to explicitly check this into source control if you don't need to. One added bonus for gom is that if the gom tool is available on your $PATH there's no need to compile your project within a $GOPATH as this will be automatically set to use the specific dependencies you reference in your Gomfile.

What next?
For now, both Godep and gom works for us. Should a consensus emerge from the Go community over what to use, we'll look at switching again.

If work like this appeals to you, take a look at Working for GDS – we’re usually in search of talented people to come and join the team.

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