How to retrieve the current state of a running Step Functions in AWS

Question

I'm giving the AWS' Step Functions a try and I'm interested in them for implementing long-running procedures. One functionality I would like to provide to my users is the possibility of showing execution's progress. Using describeExecution I can verify if some execution is still running or done. But progress is a logical measure and Step Functions itself has no way to tell me how much of the process is left.

For that, I need to provide the logic myself. I can measure the progress in the tasks of the state machine knowing the total number of steps needed to take and counting the number of steps already taken. I can store this information in the state of the machine which is passed among steps while the machine is running. But how can I extract this state using API? Of course, I can store this information is an external storage like DynamoDb but that's not very elegant!

Answer 1

The solution I have found my self (so far this is the only), is using getExecutionHistory API. This API returned a list of events that are generated for the Step Functions and it can include input or output (or neither) based on whether the event is for a starting a lambda function or is it for the time a lambda function has exited. You can call the API like this:

var params = {
  executionArn: 'STRING_VALUE', /* required */
  maxResults: 10,
  reverseOrder: true
};
stepfunctions.getExecutionHistory(params, function(err, data) {
  if (err) console.log(err, err.stack); // an error occurred
  else     console.log(data);           // successful response
});

By reversing the order of the list of events, we can get the latest ones first. Then we can look for the latest output in the list. The first one you'll find will be the latest version of the output which is the current state of the Step Functions.