Conditional step execution

You can use the when keyword, along with the equals and notEquals functions to control whether or not Relay executes a step.

If when is present, Relay executes the step once the conditional data defined under when is true. The when key is an array of expressions that must output either boolean true or boolean false. If all booleans in the array are true, the condition is considered successful and Relay executes the step. If any of the values are false, the condition fails and the step does not continue.

If the when key outputs a non-boolean data type, the step returns an error. In practice, the when keyword is only effective when combined with the equals and notEquals functions. Here is an example of a step that contains a when block that uses the equals function:

- name: example
  image: alpine:latest
  when:
  - !Fn.equals [!Parameter env, development]

In this example, Relay only executes the example step if the env parameter is development.

Using conditional functions

The equals and notEquals functions provide a way to compare data sources:

• !Fn.equals returns true if the underlying values and types of the arguments are equal.
• !Fn.notEquals returns true if the underlying values and types of the arguments are not equal.

Note: The string "true" does not equal the boolean true.

Conditional functions must receive two arguments. If the number of arguments is more or less than 2, the step returns an ArityError and the step fails.

Here are all the supported expressions and types you can pass into equals and notEquals:

• a parameter: !Parameter example-parameter
• an output: !Output stepName outputName
• a string: "Just a normal string."
• a boolean: true or false
• an integer: 10, 100, 35
• a float: 10.5, 1.0, 54.99999
• an array: ["apple", "banana", "orange", "peach"]
• a map: {"vegetable": "carrot", "fruit": "apple"}

Note: You can't pass !Secret into a conditional function.

Example of a workflow with conditional execution

Consider a scenario where you have two environments, development and production. You want to deploy a monitoring system only against production. In the following workflow, the deploy-monitoring step uses when along with the !Fn.notEquals condition to make sure that Relay only deploys the monitoring system when env is not development.

Similarly, in this scenario, you want to send a notification to Slack when you deploy to production. The slack-notify step uses when together with !Fn.equals to ensure that Relay only notifies Slack when env is production.

description: Deploy a thing to environments then conditionally notify slack

parameters:
  channel:
    description: Slack channel (include preceding hashtag)
  env:
    description: the environment to deploy to
    default: development

steps:
- name: deploy-monitoring
  image: alpine:latest
  input:
  - echo "Deploying monitoring to system"
  when:
  - !Fn.notEquals [!Parameter env, development]

- name: slack-notify
  image: projectnebula/slack-notification
  spec:
    apitoken: !Secret slack-token
    channel: !Parameter channel
    message: "production was deployed"
  dependsOn:
  - deploy-monitoring
  when:
  - !Fn.equals [!Parameter env, production]