Taming Complex GitHub Actions Workflows with Reusable Workflows

July 15, 2023 - 10 min read (1838 words)

GitHub Actions workflows are implemented in YAML which is a format absent of functions making it hard to utilize the DRY principle when creating complex workflow configurations. Even in small enterprise examples, it is easy for a workflow definition to get out of hand with repeating similar blocks of code, a lack of modularity and lengthy workflow definition files. While YAML itself does not provide an easy solution to this problem, GitHub Actions includes a feature that allows complex workflows to be decomposed into reusable and parameterized modules: reusable workflows.

This article covers the refactoring of a complex workflow implemented in GitHub Actions from a single YAML file with many repeated similar blocks of code into a modular design using multiple reusable workflows and a single top-level caller.

The original workflow definition grew organically over time, developed a number of problems and was quickly becoming difficult to maintain. The scope of what the workflow needed to accomplish was not even particularly large for a real-world piece of enterprise software. It included:

  • 493 lines in a single workflow file
  • 12 environment configuration variables
  • 4 repository secrets
  • 8 jobs
  • 47 total steps
GitHub Actions Workflow Screenshot

The objective of this workflow is to deploy an Azure Static Web App containing an Angular application and linked Azure Functions App implementing its backend API. It works with two separate environments: production and preview. When a push is made to the default branch, the production environment should be built and deployed. When a pull request into the default branch is created or updated, the preview environment should be built and deployed. Several other jobs including unit testing for the frontend application, CodeQL analysis for the code base and a series of end-to-end deployment sanity tests are also included in the workflow.

For reference, the original workflow file can be found in its state prior to refactoring at this location. The resulting series of files that were the output of the refactoring can be found in this commit.

Table of Contents

Use the GitHub Actions VS Code Extension

The GitHub Actions Extension for Visual Studio Code is an incredible time saver when working on workflows. It offers a number of features that come in handy:

  • Syntax highlighting
  • Integrated documentation
  • Validation and code completion
  • Parsing parameters, inputs, and outputs for referenced actions and reusable workflows

Workflow Configuration Patterns

Even simple workflow files require configuration. Common practice is to include a series of variables containing these configuration values at the top of the workflow as environment variables. These environment variables are then referenced throughout the rest of the workflow as needed.

However, when decomposing a large workflow into small units using the reusable workflow feature, another similar pattern is needed. Reusable workflows have a series of limitations. Among those limitations is the note that environment variables are not passed from the calling workflow to the reusable workflow. Additionally, the env context is not available in the with block of the caller.

Any environment variables set in an env context defined at the workflow level in the caller workflow are not propagated to the called workflow.

As a result, another pattern is required: the use of output variables from a previously executed job.

Configuration through Environment Variables

Environment variables may be set at the workflow, job and step levels using the env context. In a single file workflow pattern, it is common to set environment variables used throughout the workflow for configuration at the top of the workflow file. These variables are then accessed when needed using the ${{ env.APP_WORKING_DIR }} syntax.

The original workflow definition used in this project followed this pattern prior to the refactoring.

  APP_WORKING_DIR: "ng-resume-app"
  APP_LOCATION: "/ng-resume-app/dist/ng-resume/browser"
  AZURE_FUNCTIONAPP_NAME: "personal-site-api"
  PREVIEW_AZURE_FUNCTIONAPP_NAME: "personal-site-api-preview"
  RESOURCE_GROUP: "personal-site"
  SLOT_NAME: "staging"
  BASE_URL: "https://www.jpatrickfulton.com"
  PREVIEW_BASE_URL: "https://preview.jpatrickfulton.com"

Configuration through Job Output Variables

Due to the limitations discussed above, the standard pattern of workflow-level environmental configuration variables does not work in a workflow that is decomposed into multiple reusable workflows. However, environment variables may still prove useful within a workflow file or within a self contained job or step.

An alternate pattern is required in a reusable workflow pattern. The env context is not available. However, several other contexts are available including the jobs and needs contexts.

The needs context contains outputs from all jobs that are defined as a direct dependency of the current job.

The solution is to define a job in the calling workflow that declares a series of outputs that contain the configuration data. Subsequent jobs that require these variables may declare a dependency on that job using a needs clause. For example, a subsequent job in the workflow may declare needs: [export_vars] and then access the values of the configuration variables using the following syntax: ${{ needs.export_vars.outputs.app-working-dir }}.

The updated calling workflow uses this pattern to pass configuration data to jobs and steps in the reusable workflows that were split from the original workflow.

    name: Export Variables to Output
    runs-on: ubuntu-latest
      app-working-dir: "ng-resume-app"
      app-location: "/ng-resume-app/dist/ng-resume/browser"
      api-location: ""
      output-location: ""
      azure-functionapp-name: "personal-site-api"
      preview-azure-functionapp-name: "personal-site-api-preview"
      azure-functionapp-package-path: "ng-resume-api"
      dotnet-version: "7.0.x"
      resource-group: "personal-site"
      slot-name: "staging"
      base-url: "https://www.jpatrickfulton.com"
      preview-base-url: "https://preview.jpatrickfulton.com"
      - run: echo "Exporting variables to outputs."

Applying Reusable Workflows

Reusable workflows in GitHub Actions allow the developer to create modular and parameterized units of work to decompose a complex workflow. This overcomes the absence of functions in the YAML specification. A monolithic single workflow file can be subdivided into multiple reusable workflows each of which can be passed inputs to control their flow and configuration.

Creating a Reusable Workflow

Reusable workflows must be created in the .github/workflows/ folder. Notably, the use of subdirectories under that folder is not supported. A reusable workflow is structured in the same fashion as a traditional workflow including an on block and a jobs block including at least one job.

What differentiates a reusable workflow is the trigger included in the on block. To create a reusable workflow, the on block must include a workflow_call trigger. This section, optionally composed of inputs and secrets that can be passed to the workflow by its callers, marks the workflow in GitHub as callable by other workflows.

Input variables are declared in the inputs block. Each input variable may be marked as required and declares an associated type. Only boolean, number and string types may be used. Sequence types are not currently supported. At the start of workflow execution, the parser examines the required inputs and should one be missed by a caller or of an incorrect type, the workflow will generate errors before hitting the runner. Input variables may be referenced later in the reusable workflow file using the following syntax and the inputs context: ${{ inputs.dotnet-version }}.

Secrets may also either be passed or inherited.

If the secrets are inherited by using secrets: inherit in the calling workflow, you can reference them even if they are not explicitly defined in the on key.

The advantage to explicit passing of secrets, as shown in this example, is that caller may elect to choose which secret value to pass to the reusable workflow. In the complete example used here, there are two Azure Service Principal secrets within the caller: one for the production environment and one for the preview environment. The caller then selects the correct secret to explicitly pass to the reusable workflow thereby abstracting logic about which secret to use into the calling workflow.

In either case, once secrets have been passed to the underlying reusable workflow, they may be accessed for use with the following syntax: ${{ secrets.azure-sp-credentials }}.

name: Reusable API CD Workflow
  workflow_call:    inputs:      dotnet-version:
        required: true
        type: string
    secrets:      azure-sp-credentials:
        required: true

jobs:  api_cd_job:
    name: API CD Job
      contents: read
      pull-requests: write
    runs-on: windows-latest
      - name: "Checkout GitHub Action"
        uses: actions/checkout@v3

The complete version of this reusable workflow is available here.

Creating a Caller Workflow

In a caller workflow that invokes a separate reusable workflow, the caller defines a job and adds a uses key to it. Beneath the uses key, the caller passes input variables in a with block followed by any necessary secrets in a secrets block.

In this example, the reusable workflow exists in the same repository as the caller and the desired outcome is to use a version of that reusable workflow that exists on the same branch and commit as the caller. This is accomplished with the following syntax: uses: ./.github/workflows/api-cd-job.yml.

However, reusable workflows can also be accessed from other sources from a uses clause. Per the documentation, reusable workflows from other repositories may be used with the following syntax: uses: {owner}/{repo}/.github/workflows/{filename}@{ref}. The {ref} may be a commit SHA, a release tag, or a branch name.

Using the commit SHA is the safest for stability and security.

The documentation encourages consumers of reusable workflows in separate repositories or from third parties to refer to the security hardening document as the risks associated with a compromised workflow are significant.

In this example, you can see the pattern referenced in earlier sections of this article to utilize output variables from a previously executed job to configure the inputs to the reusable workflow.

  name: (Preview)
  needs: [export_vars, analyze, frontend_ci_job]  if: (github.event_name == 'pull_request' && github.event.action != 'closed')
    contents: read
    pull-requests: write
  uses: ./.github/workflows/api-cd-job.yml  with:    dotnet-version: ${{ needs.export_vars.outputs.dotnet-version }}
    function-app-name: ${{ needs.export_vars.outputs.preview-azure-functionapp-name }}
    function-app-package-path: ${{ needs.export_vars.outputs.azure-functionapp-package-path }}
    resource-group: ${{ needs.export_vars.outputs.resource-group }}
    slot-name: ${{ needs.export_vars.outputs.slot-name }}
  secrets:    azure-sp-credentials: ${{ secrets.PREVIEW_AZURE_SP_CREDENTIALS }}

The complete version of this caller workflow is available here.

A Complete View of the Refactoring Results

By splitting the original workflow file into a caller and then a series of reusable workflows, the main file was made significantly more maintainable, readable and modular. The environment variables were converted to job output variables using the pattern discussed above and configuration for the workflow remains in-file as a result. The output of the final refactoring can be found in this folder. Following the refactoring, it now includes:

  • 1 top-level caller workflow (174 lines)
  • 5 reusable workflows leveraged by the caller
  • 12 configuration variables converted to job output variables
  • 4 repository secrets
  • 9 jobs (including the new variable output job)
  • 48 total steps
Folder Structure Screenshot
analyze-job.ymlA reusable workflow containing a CodeQL analysis job.
api-cd-job.ymlA reusable workflow containing a CD job for function APIs.
ci-and-cd-workflow.ymlThe top-level caller workflow for the complete CI/CD process for the project.
e2e-job.ymlA reusable workflow containing an E2E testing job.
frontend-cd-job.ymlA reusable workflow containing a CD job for the frontend application.
frontend-ci-job.ymlA reusable workflow containing a CI job for the frontend application.

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Written by J. Patrick Fulton.

github actions reusable workflows yaml devops