Question

Whilst running an application on an EC2 instance behind an Elastic Load Balancer, an administrator receives a 504 error on their browser. What does this mean?

A. The ELB instance has stopped running
B. The application running on the EC2 instance is serving the 504 error page because it has exceeded its response timeout
C. The URL for the application has expired
D. The application is unresponsive so the ELB instance serves the 504 error page

Answer 1

A 504 error indicates that the application on the EC2 instance is unresponsive and the ELB serves the 504 error due to a timeout. It suggests performance issues or resource overutilization, differentiating it from a 403 error that signifies a request being blocked.

Step-by-step explanation:

When an administrator receives a 504 error while running an application on an EC2 instance behind an Elastic Load Balancer, it implies that the application is unresponsive, and as a result, the Elastic Load Balancer (ELB) serves the 504 error page. This error typically means that the ELB successfully connected to the target instance, but the target did not respond within the expected time frame. Therefore, the correct answer is: The application is unresponsive so the ELB instance serves the 504 error page.

A 504 error generally indicates that there is a gateway timeout. The potential causes for this can be related to application performance issues or resources being overutilized on the EC2 instance, leading to slower response times or no response at all. It is different from a 403 error, which means the request was valid but the server is refusing action. With a 504 error, the gateway did not receive a timely response from the upstream server.