AWS ALB returns 502 Bad Gateway from lambda

Question

I have a lambda function which return base64 string, when I invoke lambda from code it works, but when I call lambda behind ALB and base64 string is large size, ALB gives me error 502 Bad Gateway. Note:for small size string ALB also works.

// Lambda function handler

'use strict';


module.exports.handler = async (event, context) => {
  // ALB expects following response format
  // see: https://docs.aws.amazon.com/lambda/latest/dg/services-alb.html
  const response = {
    headers: {
      'Access-Control-Allow-Origin': '*',
      'Content-Type': 'application/json',
    },
    isBase64Encoded: true,
    statusCode: 200,
    statusDescription: '200 OK',
  };
// ALB gives error 502 when buffer size is large
  const answer = 'This is my audio buffer'.toString('base64');
  response.body = answer
  return response;
  };

Helmut

Answer 1

Check that you are not exceeding the limits. As per the AWS docs, when using Lambda as an ALB target the maximum response size is 1MB; if the response is more than 1MB you will get an error.

You can see this link for more information on using Lambda as a target for your ALB: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html

Parsaye

Answer 2

This could be due to couple of reasons as mentioned in the AWS ALB docs -

• response body exceeds 1 MB
• Lambda function that did not respond before its configured timeout was reached

There is nothing much that can be done if the payload size limit is reached. 1 MB is the limit if lambda is configured as a target for ALB.

If the reason is due to lambda timeout, then this can be extended in the lambda configuration. The default is 3 seconds and it can be extended upto 15 minutes. There is nothing much one can do if the 15 minutes limit is reached. docs

Amortize

Any idea when it's none of these things? I can see that my lambda runs successfully, but I always get a 502 Bad Gateway response? – Luz

@Luz Did you find out why this might have happened? Facing the same problem. – Whippoorwill

@bob Yes I fixed my issue by adding a 3rd "callback" param to the exports.handler, then instead of just returning response object, I do callback(null, response); – Luz

@bob I've added an answer which might help. – Luz

Answer 3

Just in case it's helpful to someone. I was able to resolve a similar issue by changing the code to be like this...

// Lambda function handler

'use strict';

//note the 3rd 'callback' parameter
module.exports.handler = (event, context, callback) => {

  // Construct the response object like normal

  callback(null, response);
};

Luz