AWS AppSync Lambda authoriser always results in "Error: Request failed with status code 401"

Question

I am currently playing around a bit with AWS AppSync and I am trying to use the Lambda authoriser feature to do some custom auth for the GraphQL API.

I have the Lambda function set up with the correct resource-based policy to allow AppSync to invoke the function and I have AppSync's Default authorization mode set to invoke my Lambda.

This is my lambda code:

exports.handler = (event) => {
  console.log(JSON.stringify(event));
  const response = {
    isAuthorized: true,
  };
  console.log(JSON.stringify(response));
  return response;
};

Now I am facing the issue that the Lambda authoriser is always giving me the following error when I attempt to run a GraphQL quarry:

Error: Request failed with status code 401

After debugging this problem for two hours I can say the following things:

• The GraphQL endpoint is working fine, because if I set the Default authorization mode to API key or Amazon cognito user pool without changing anything else my Query executes successfully.
• The lambda function is definitely being invoked whenever I make a request to the API and the lambda also receives the correct event from AppSync.
• The Lambda returns {"isAuthorized":true} which means no Authorization Token would result in a 401.

So as far as I can tell everything is as it should but I am still getting the 401 no matter what I do and im getting pretty frustrated.

Answer 1

After some very frustrating debugging I finally figured out that the problem was the Lambda handler function. As it turns out a Node.js lambda handlers should be async.

So changing the lambda to the following code fixes the issue:

exports.handler = async (event) => {
    console.log(JSON.stringify(event));
    const response = {
        isAuthorized: true,
    };
    console.log(JSON.stringify(response));
    return response;
};

I didn't know this, since until no I only used Python for Lambdas, and the problem was hard to spot since the console.log's where still running correctly so I though the function was returning the correct data where as in fact it was returning null.