DRY: reusing code in GitHub Actions

In this post I want to make a quick overview of the approaches of reusing steps of your workflow to avoid duplication of the same steps across different workflows or jobs.

🔸 Reusing workflows

The obvious option is using the "Reusable workflows" feature that allows you to call extract some steps into a separate "reusable" workflow and call this workflow as a job in other workflows.

🥡 Takeaways:

• Reusable workflows can't call other reusable workflows.
• The strategy property is not supported in any job that calls a reusable workflow.
• Env variables and secrets are not inherited.
• It's not convenient if you need to extract and reuse several steps inside one job.
• Since it runs as a separate job, you have to use build artifacts to share files between reusable workflow and your main workflow.
• You can call reusable workflow in synchronous or asynchronous manner (managing it by jobs ordering using needs keys).
• A reusable workflow can define outputs that extract outputs/outcomes from performed steps. They can easily used to pass data to the "main" workflow.

🔸 Dispatched workflows

Another possibility that GitHub gives us is workflow_dispatch event that can trigger a workflow run. Simply put, you can trigger a workflow manually or through GitHub API and provide its inputs.

There are actions available on the Marketplace which allow you to trigger a "dispatched" workflow as a step of "main" workflow.

Some of them also allow doing it in a synchronous manner (wait until dispatched workflow is finished). It is worth to say that this feature is implemented by polling statuses of repo workflows which is not very reliable, especially in a concurrent environment. Also, it is bounded by GitHub API usage limits and therefore has a delay in finding out a status of dispatched workflow.

🥡 Takeaways

• You can have multiple nested calls, triggering a workflow from another triggered workflow. If done careless, can lead to an infinite loop.
• You need a special token with "workflows" permission; your usual secrets.GITHUB_TOKEN doesn't allow you to dispatch a workflow.
• You can call run multiple dispatched workflows inside one job.
• There is no easy way to get some data back from dispatched workflows to the main one.
• Works better in "fire and forget" scenario. Waiting for a finish of dispatched workflow has some limitations.
• You can observe dispatched workflows runs and cancel them manually.

🔸 Composite Actions

In this approach we extract steps to a distinct composite action, that can be located in the same or separate repository.

From your "main" workflow it looks as a usual action (a single step), but internally it comprises of multiple steps each of which can call own actions.

🥡 Takeaways:

• Supports nesting: each step of a composite action can use another composite action.
• Bad visualisation of internal steps run: in the "main" workflow it's displayed as a usual step run. In raw logs you can find details of internal steps execution, but it doesn't look very friendly.
• Shares environment variables with a parent job, but doesn't share secrets, which should be passed explicitly via inputs.
• Supports inputs and outputs. Outputs are prepared from outputs/outcomes of internal steps and can be easily used to pass data from composite action to the "main" workflow.
• A composite action runs inside the job of the "main" workflow. Since they share a common file system, there is no need to use build artifacts to transfer files from the composite action to the "main" workflow.
• You can't use continue-on-error option inside a composite action.

👏 Thank you for reading

Any comments, critics and sharing your own experience would be appreciated!

If you are interested in developing own Actions, I also recommend you reading "GitHub Actions Testing" series.