Choice States

Cumulus supports AWS Step Function Choice states. A Choice state enables branching logic in Cumulus workflows.

Choice state definitions include a list of Choice Rules. Each Choice Rule defines a logical operation which compares an input value against a value using a comparison operator. For available comparison operators, review the AWS docs.

If the comparison evaluates to true, the Next state is followed.

Example

In examples/cumulus-tf/parse_pdr_workflow.tf the ParsePdr workflow uses a Choice state, CheckAgainChoice, to terminate the workflow once meta.isPdrFinished: true is returned by the CheckStatus state.

The CheckAgainChoice state definition requires an input object of the following structure:

{
  "meta": {
    "isPdrFinished": false
  }
}

Given the above input to the CheckAgainChoice state, the workflow would transition to the PdrStatusReport state.

"CheckAgainChoice": {
  "Type": "Choice",
  "Choices": [
    {
      "Variable": "$.meta.isPdrFinished",
      "BooleanEquals": false,
      "Next": "PdrStatusReport"
    },
    {
      "Variable": "$.meta.isPdrFinished",
      "BooleanEquals": true,
      "Next": "WorkflowSucceeded"
    }
  ],
  "Default": "WorkflowSucceeded"
}

Advanced: Loops in Cumulus Workflows

Understanding the complete ParsePdr workflow is not necessary to understanding how Choice states work, but ParsePdr provides an example of how Choice states can be used to create a loop in a Cumulus workflow.

In the complete ParsePdr workflow definition, the state QueueGranules is followed by CheckStatus. From CheckStatus a loop starts: Given CheckStatus returns meta.isPdrFinished: false, CheckStatus is followed by CheckAgainChoice is followed by PdrStatusReport is followed by WaitForSomeTime, which returns to CheckStatus. Once CheckStatus returns meta.isPdrFinished: true, CheckAgainChoice proceeds to WorkflowSucceeded.

Further documentation

For complete details on Choice state configuration options, see the Choice state documentation.