Extension Review: Funky Async Extension

Wakeem's World
3 min readFeb 5, 2023


Intro to our comic characters that will help us describe our lambda extension

Do you ever find your self needing to build an API endpoint on lambda that executes some code after your lambda handler has finished running?

Maybe you have done this where your API lambda function just immediately invokes another lambda function. Maybe you hook up a step function directly to an http endpoint. Maybe your pockets are deep enough to use API Gateway v1 and utilize the X-Amz-Invocation-Type header.

What if I told you it is possible to use an API Gateway v2 API and still perform an async task with lambda?

Bare with me on this “review” because this is an extension I built myself and I am currently using in a production. I probably haven’t made the code super pretty or robust enough for public consumption but I still thought it would be fun to share.

This extension leverages AWS Lambda’s extensions API to execute an ASYNC_HANDLER after your lambda sends its final response.

Allow me to demonstrate with a cartoon 👇

Our client making a request to our main lambda handler, our main lambda handler asking the async handler to finish the job

As you can see

  1. Our client makes a request
  2. The lambda tells the async handler to start
  3. The lambda returns a response to the client
  4. The async handler finishes the work

How could I try this for myself?

Take a look at my repo for this extension

If you take a look at the serverless.yml file there are three examples.

  • longRunning - This function demonstrates an async handler that runs after a typical HTTP API would be required to return a response. It is also using the proxy handler included with the async extension layer that will take care of passing the event and context object to the async handler.
  • customResponse - This function demonstrates an async handler that uses the built in layer proxy handler and customizes the response message.
  • customHandler - This function demonstrates a custom handler paired with an ASYNC_HANDLER. This also demonstrates how to use the sendArgs method exposed by our layer.


If you include this extension and set your main handler to /opt/proxy.handler then set an environment variable for the ASYNC_HANDLER that points to your normal lambda handler function.


This extension is written using nodejs and is referencing the async handler using require. So as it stands right now this extension would only really work with a nodejs lambda.

Also this extension may or may not work out of the box with some build tools. It will technically work but you may need to set the ASYNC_HANDLER path to the output file path in the build directory.


Lol maybe I am being a little cheeky calling this a “review”. I don’t think I can actually review someething I built myself so maybe I should ask that you review it for me and leave some feedback in the comments or on GitHub. 😉

Feel free to try out this extension for yourself and let me know what you think. I plan to make this a public extension at some point so you don’t have to build/distribute the layer on your own but in the meantime feel free to use the layer code as you please. 😎



Wakeem's World

Attempting to weekly blog about whatever I am thinking about. Skateboarding, tech, film, life, etc.