CODEWITHSUNDEEP
amazon-web-servicesaws-lambdaamazon-sqsaws-step-functions
Jalaj Varshney
Jalaj Varshney

Reputation: 133

Invoking a step function synchronously from a lambda function

I have a lambda function which is triggered by a FIFO SQS. I only want one instance of the function running. The function invokes a state machine and the state machine takes longer to finish than the lambda function. I want the lambda function to finish only after the step function has completed it's execution.

Upvotes: 2

Views: 7579

Answers (3)

slim
slim

Reputation: 41203

Edit - note that the solution below is usually unnecessary now, since AWS has introduced ways to invoke express step functions in a synchronous manner

If you expect your state machine to complete in a reasonable time, then you can get your Lambda (or anything else that starts a state machine execution) to await a response.

This is an accepted pattern, but has quite a few moving parts:

  • Set up an SNS topic; state machine results will be published onto it
  • During your Lambda's initialisation:
    • generate a random "gateway ID"
    • create a SQS queue
    • subscribe to the SQS queue, filtering by gateway ID
  • When your Lambda invokes the state machine:
    • generate a random "correlation ID"
    • include both the gateway and correlation IDs in the inputs to the state machine
  • As the final step of the state machine, publish the result to the SNS topic. Include the gateway and correlation IDs in the response.
  • In the lambda, after submitting the state machine execution:
    • await a message on the SQS queue.
    • give some thought to timeouts on this. You should set a timeout for the response. You can therefore set a time-to-live on the state machine -- if its outcome is going to be useless once this Lambda dies.
    • When it arrives, ensure that its correlation ID matches yours, then use the data in that message as your response.

Upvotes: 2

Michael Ortiz
Michael Ortiz

Reputation: 777

AWS Express Step Functions now supports synchronous invocations.

Keep in mind that Express Step Functions are meant for workloads that require higher event rates and have shorter durations. Also, keep in mind that you will be billed based on the number of requests and the duration of the workflow just like lambda.

Upvotes: 5

A.Khan
A.Khan

Reputation: 3992

AWS Step Functions are only invoked asynchronously. A state machine can run for up to 1 year so synchronous invocation is not possible. Depending on your workflow you might find Activities useful.

Activities are an AWS Step Functions feature that enables you to have a task in your state machine where the work is performed by a worker that can be hosted on Amazon Elastic Compute Cloud (Amazon EC2), Amazon Elastic Container Service (Amazon ECS), mobile devices—basically anywhere.

Upvotes: 3

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