AWS API Gateway returning different status via Postman to Method Test

Question

I have an AWS Lambda integrated into API Gateway. The lambda is current just returning an error:

exports.handler = (event, context, callback) => {
  context.fail(JSON.stringify({
    status: 500,
    errors: 'This is definitely the same endpoint.'
  }));
};

I have added an HTTP status 500 response in method response of API Gateway, and I have mapped an integration response using regex *"status":500.* to the 500 response.

When I test this using the Method Test functionality in AWS, I get the 500 response I expect:

But when send command to the endpoint with Postman, I get a 200 status:

How can this be? The Method Test seems to suggest my Integration Response and Method Response setups are correct, and I have my regex set up correctly, but what am I missing between the API Gateway and the rest of the world that would produce this different result?

Umair Ahmed · Accepted Answer · 2019-06-11 09:09:22Z

3

Have you tried Deploying the API? Maybe a redeployment might fix the issues.

answered Jun 11, 2019 at 9:09

Umair Ahmed

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2 Comments

Paul T Davies Over a year ago

Yes, that was it.

justHelloWorld Over a year ago

This helped me as well after creating an API through CDK

Niclas Meier · Accepted Answer · 2019-06-11 08:22:34Z

did you stumble across the Handle Custom Lambda Errors in API Gateway guide already?

The format suggested there is:

{
   "isBase64Encoded" : "boolean",
   "statusCode": "number",
   "headers": { ... },
   "body": "JSON string"
}

So you basically need to return this JSON structure. I check this with AWS serverless express library (line 100-105) - which we use - and it's returning this structure.

The major challenge is, that sending the HTTP response to the client is a task for the API Gateway and to do so, you must return a valid JSON response from the lambda in a format that the API gateway unterstands. The execution status of the lambda will be also 200 (because it worked A-Okay) but the API Gateway will transform your response into a 500. This is also why you nedd this double/triple JSON encoding (JSON-as-string in JSON-attributes).

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