You implement a REST API using a Lambda Function. The API is exposed through the AWS API gateway. The Lambda Function calls a third-party service to retrieve the data. But this third-party service may not respond in time. You already increased the timeout of the Lambda Function to be 15 minutes. However, sometimes users still get an HTTP 504 error after about 30 seconds. Which of the following options is the most possible reason?

Answer options:

A.The error code indicates that there is an Internal Server Error. Check the CloudWatch Logs of the Lambda Function.
B.The third-party service has returned an HTTP 504 error after 30 seconds to the Lambda Function. Then the Lambda Function forwards the error to the API Gateway.
C.The maximum of integration timeout for AWS API Gateway is 29 seconds. Although the timer for Lambda Function does not expire yet, the API Gateway already hits its limitation.
D.When Lambda Function integrates with API Gateway, the effective Lambda Function execution time limit is 30 seconds although the limit is set to 15 minutes.

Correct Answer – C
Please check https://docs.aws.amazon.com/apigateway/latest/developerguide/limits.html for the Amazon API Gateway limits. The integration timeout is 50 milliseconds - 29 seconds for all integration types. And the limit cannot be increased.
Option A is incorrect: Because HTTP 504 is Gateway Timeout instead of Internal Server Error.
Option B is incorrect: When the third-party service does not respond in time, the API Gateway reaches its limit after 29 seconds. It is not the third-party service that returns the HTTP 504 error code in this scenario.
Option C is CORRECT: Because the API call through API Gateway cannot exceed 29 seconds due to the limits. This is the most possible reason to cause the issue.
Option D is incorrect: Because the limit of Lambda Function can be 15 minutes according to https://aws.amazon.com/about-aws/whats-new/2018/10/aws-lambda-supports-functions-that-can-run-up-to-15-minutes/. The limit of API Gateway has caused the issue.