AWS Step Function Synchronous Task Token - amazon-web-services

Question

I have a use case which I want to use Step Functions to solve but I can't find a way to solve this problem. Your help would be greatly appreciated.
The problem goes like this: I have an Amazon API Gateway which has a /start endpoint. a POST to this endpoint should start a data processing session and return a URL to an app which the API client can use to capture some data. Once data capture is complete, some processing takes place before the final response is sent to the API client via a callback.
My thinking, as you can see below, is to generate a task token and send it to the Data Capture Service. Then, when the user data capture is complete, the service can send a request to the Step Function API to say that stage is complete. The problem with this is how can I return the URL to the client from within the Step Function? I don't want to use a callback to do this.
One option is to create the data capture session within the 'Step Function Initiator' Lambda but then how do I provide the Data Capture Service with a task token?
Really, what I need is some mechanism of synchronously returning something (either a URL from that call or the task token from the first stage) from within the Step Function to the Lambda which started the execution. Is this possible? How would you solve this?

Answer 1

In step function initiator lambda, you must be doing start-execution which returns an executionArn
Next, you can loop and call get-execution-history api and task token will be part of the 'capture data' task parameters. Since this is the first step, this really should be done with in couple of seconds, so, we can keep running this loop every second until desired step in step function is initiated and task token can be obtained.
Take this example, i am passing the task token to another step function call from current step function.
{
"StartAt":"ChildTask",
"States":{
"ChildTask":{
"End":true,
"Type":"Task",
"Resource":"arn:aws:states:::states:startExecution.waitForTaskToken",
"Parameters":{
"Input":{
"token.$":"$$.Task.Token",
"foo":"bar"
},
"StateMachineArn":"arn:aws:states:us-east-1:110011001100:stateMachine:ChildStateMachine",
"Name":"MyExecutionName"
}
}
}
}
Get Execution history:
aws stepfunctions get-execution-history --execution-arn arn:aws:states:us-east-1:110011001100:execution:ParentStateMachine:667102b3-b19c-b7ab-b119-9ec6cf23e505
Result:
one of the first few entries in execution history and task token is part of the parameters. we can exit the loop, grab that, send it back to Api Gateway.
{
"timestamp": "2021-03-12T13:56:58.097000-05:00",
"type": "TaskScheduled",
"id": 3,
"previousEventId": 2,
"taskScheduledEventDetails": {
"resourceType": "states",
"resource": "startExecution.waitForTaskToken",
"region": "us-east-1",
"parameters": "{\"Input\":{\"foo\":\"bar\",\"token\":\"o6QVQ9gls.......=\"},\"StateMachineArn\":\"arn:aws:states:us-east-1:110011001100:stateMachine:ChildStateMachine\",\"Name\":\"MyExecutionName\"}"
}
}