Replicating the Travis Build Matrix with Docker

It’s beneficial to test your open-source software library works correctly with multiple versions of its dependencies, and multiple versions of the programming language. You want to have confidence the library works as intended for your users, and prevent regressions.

In the Ruby world there are great tools - like Appraisal - which ease the process of testing against various versions of dependencies.

Continuous integration services can support testing multiple language versions:

• Workflows on CircleCI 2.0 can use a job-per-version
• Travis has a build matrix feature to combine different language and environment configurations

But what if you want to test multiple configurations, dependencies and language versions locally? Or debug a failure in a specific combination without having to push and wait for CI?

With these goals in mind I’ve created dmatrix. It allows you to define a matrix of Docker build-arg and env variables:

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matrix:
  build_arg:
    FROM_IMAGE:
      - ruby:2.4-alpine
      - ruby:2.5-alpine
      - ruby:2.6-alpine
    BUNDLE_GEMFILE:
      - gemfiles/factory_bot_4_8.gemfile
      - gemfiles/factory_bot_5.gemfile

Each possible combination will then be built and run in parallel:

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bundle exec dmatrix -- <your command>

Dmatrix is written in Ruby, but you can use it to test projects written in any language if you have Ruby & Docker available locally.