Managing dependencies

To ensure that components are deployed in the correct order, Mach Composer will create a dependency graph based on the configuration file. This graph will be used to determine the order in which components should be deployed. It will also batch any changes to unrelated components so they can be run in parallel. See the configuration documentation for more information on what options there are to configure dependencies.

Determining dependencies

Mach Composer will determine the dependencies between components based on what variables a component requires from another component. For example, if a component requires an url provided in the output of another url, it is considered dependent on that component.

For example the following configuration will tell Mach Composer that my-component depends on my-parent-component based on the input variable url:

sites:
  - identifier: my-site
    components:
      - name: my-component
        variables:
          parent_url: ${component.my-parent-component.url}
      - name: my-parent-component

Component outputs

For this to work correctly, the component that is depended on must actually have url as an output. See terraform outputs. Make sure your component configuration code actually includes the output, as Mach Composer will only fail at runtime if the output is missing.

Alternatively, these dependencies can be specified explicitly using the depends_on configuration for site components:

sites:
  - identifier: my-site
    components:
      - name: my-component
        depends_on:
          - my-parent-component
      - name: my-parent-component

By default, if no dependencies are specified, Mach Composer will consider the component only dependent on the site it is part of.

sites:
  - identifier: my-site
    components:
      # Both components are allowed to be deployed at the same time
      - name: my-component
      - name: my-parent-component

If a dependency is specified that would lead to a circular dependency, Mach Composer will throw an error when reading the configuration file.

sites:
  - identifier: my-site
    components:
      # This configuration is impossible and will throw an error
      - name: my-component
        depends_on:
          - my-parent-component
      - name: my-parent-component
        depends_on:
          - my-component

Visualizing dependencies

To help you understand the dependencies between your components, Mach Composer can plot a graph of the dependencies between components. This can be done using the mach-composer graph command:

$ mach-composer graph -f my-site.yml

This will create a graph in the DOT language

For example take the following configuration:

# my-site.yml
mach_composer:
  version: 1
  plugins: { }

global:
  cloud: ""
  environment: test
  terraform_config:
    remote_state:
      plugin: local
      path: ./state
sites:
  - identifier: my-site
    components:
      - name: my-component
        variables:
          parent_url: ${component.my-parent-component.url}
          other_parent_url: ${component.my-other-parent-component.url}
      - name: my-parent-component
        variables:
          grandparent_url: ${component.my-grandparent-component.url}
      - name: my-other-parent-component
        variables:
          grandparent_url: ${component.my-grandparent-component.url}
      - name: my-grandparent-component

components:
  - name: my-component
    source: ./my-component/terraform
    version: "v1.0.0"
    branch: main
  - name: my-parent-component
    source: ./modules/application
    version: "v1.2.1"
    branch: main
  - name: my-other-parent-component
    source: ./modules/application
    version: "v1.2.1"
    branch: main
  - name: my-grandparent-component
    source: ./modules/application
    version: "v2.0.0"
    branch: main

This will create the following graph:

strict digraph {


    "deployments/main" [weight = 0];

    "deployments/main" -> "deployments/main/my-site" [weight = 0];

    "deployments/main/my-site" [weight = 0];

    "deployments/main/my-site" -> "deployments/main/my-site/my-grandparent-component" [weight = 0];

    "deployments/main/my-site/my-component" [weight = 0];

    "deployments/main/my-site/my-parent-component" [weight = 0];

    "deployments/main/my-site/my-parent-component" -> "deployments/main/my-site/my-component" [weight = 0];

    "deployments/main/my-site/my-other-parent-component" [weight = 0];

    "deployments/main/my-site/my-other-parent-component" -> "deployments/main/my-site/my-component" [weight = 0];

    "deployments/main/my-site/my-grandparent-component" [weight = 0];

    "deployments/main/my-site/my-grandparent-component" -> "deployments/main/my-site/my-other-parent-component" [weight = 0];

    "deployments/main/my-site/my-grandparent-component" -> "deployments/main/my-site/my-parent-component" [weight = 0];
}

This is the DOT language representation of the graph at the top of the page.