Getting the response results from an asynchronous call to AWS lambda in python

Question

I have an AWS lambda function which I can call synchronously and get results back alright with below code

response = lambda_client.invoke(
        FunctionName=FUNCTION_NAME,
        InvocationType='RequestResponse',
        LogType='Tail',
        Payload=payload,
        Qualifier=$LATEST
    )

The response Payload is of type <botocore.response.StreamingBody object at 0x115fb3160> So I use below code to extract the payload which works fine.

response_body = response['Payload']
response_str = response_body.read().decode('utf-8')
response_dict = eval(response_str)

Now, I need to call my lambda asynchronously, so I change the invocation type with InvocationType='Event'

It gives me a response with payload of the same type as before, botocore.response.StreamingBody object but I am getting error with this line - response_dict = eval(response_str)

The error message says

    response_dict = eval(response_str)
  File "<string>", line 0

    ^
SyntaxError: unexpected EOF while parsing

What am I missing? If the response payload is same type as synchronous call, why is this parsing error? Any suggestion?

EDIT

For clarity, I understand that if the InvocationType='Event', then we only get the status of the invoke call, not the lambda function result. In my case though, I need both - launch the lambda async and get the result back when done. How do I do that? Is writing the result back to s3 and periodically checking that the only option?

Answer 1

InvocationType='Event' means you aren't getting a response. An asynchronous Lambda invocation means you just want to invoke the function, not wait for the response. The response payload from the function is discarded by the service.

When you invoke a function asynchronously, Lambda sends the event to a queue. A separate process reads events from the queue and runs your function. When the event is added to the queue, Lambda returns a success response without additional information. (emphasis added)

https://docs.aws.amazon.com/lambda/latest/dg/invocation-async.html

Note that the queue mentioned here is a queue inside the Lambda service, not to be confused with Amazon Simple Queue Service (SQS).