How to pass parameters between Lambda Functions in AWS Step Functions

Question

I have tried to look throught the internet, but none of the solutions seem to work for me in python. Basicly I want to transfer some parameters from one lambda function to the next lambda function in a step function. I was a bit surprised how hard it is to find a simple example of how to do this?

So my first lambda function has an output like:

return {"user_name" : user_name, "region" : region, "instance_id" : instance_id}

And my second lambda function tries to import with:

user_name = event["user_name"]
region = event["region"]
instance_id = event["instance_id"]

My stepfunction looks like:

{
         "StartAt": "CreateEC2Instance",
         "States": {
           "CreateEC2Instance": {
             "Type": "Task",
             "Resource": "arn:aws:states:::lambda:invoke",
             "Parameters": {
               "FunctionName": "MY_ARN_1",
               "Payload": {
                  "region": "eu-central-1",
                  "user_name": "MY_USERNAME"
              }},
             "Next": "AttachEFStoEC2"
           },
            "AttachEFStoEC2": {
             "Type": "Task",
             "Resource":"MY_ARN_2",
             "End": true
         }

the error is:

{
  "error": "KeyError",
  "cause": {
    "errorMessage": "'user_name'",
    "errorType": "KeyError",
    "stackTrace": [
      "  File \"/var/task/lambda_function.py\", line 7, in lambda_handler\n    user_name = event[\"user_name\"]\n"
    ]
  }
}

samtoddler · Accepted Answer · 2021-01-31 13:03:29Z

4

{
         "StartAt": "CreateEC2Instance",
         "States": {
           "CreateEC2Instance": {
             "Type": "Task",
             "Resource": "arn:aws:states:::lambda:invoke",
             "ResultPath": "$.randomstring",
             "Parameters": {
               "FunctionName": "MY_ARN_1",
               "Payload": {
                  "region": "eu-central-1",
                  "user_name": "MY_USERNAME"
              }},
             "Next": "AttachEFStoEC2"
           },
            "AttachEFStoEC2": {
             "Type": "Task",
             "Resource":"MY_ARN_2",
                   "Parameters": {
                       "outputfromearlierfunction.$": "$.randomstring.Payload"
                   },
             "End": true
         }

and then try to access within the function like

receivedinput = event.get('outputfromearlierfunction')
user_name = receivedinput["user_name"]
region = receivedinput["region"]
instance_id = receivedinput["instance_id"]

edited Jan 31, 2021 at 13:03

answered Jan 31, 2021 at 12:33

samtoddler

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1 Comment

smallbirds Over a year ago

Thanks. Your explanation really helped me understand how parameters are stored :)

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