Keep Elastic Load Balancer connection alive during long AJAX request

Question

I am running into this problem :

• I am sending a request to the server using AJAX, which takes some parameters in and on the server side will generate a PDF.
• The generation of the pdf can take a lot of time depending on the data used
• The Elastic Load Balancer of AWS, after 60s of "idle" connection decides to drop the socket, and therefore my request fails in that case.

I know it's possible to increase the timeout in ELB settings, but not only my sysadmin is against it, it's also a false solution, and bad practice.

I understand the best way to solve the problem would be to send data through the socket to sort of "tell ELB" that I am still active. Sending a dummy request to the server every 30s doesn't work because of our architecture and the fact that the session is locked (ie. we cannot have concurrent AJAX requests from the same session, otherwise one is pending until the other one finishes)

I tried just doing a get request to files on the server but it doesn't make a difference, I assume the "socket" is the one used by the original AJAX call.

The function on the server is pretty linear and almost impossible to divide in multiple calls, and the idea of letting it run in the background and checking every 5sec until it's finished is making me uncomfortable in terms of resource control.

TL;DR : is there any elegant and efficient solution to maintain a socket active while an AJAX request is pending?

Many thanks if anyone can help with this, I have found a couple of similar questions on SO but both are answered by "call amazon team to ask them to increase the timeout in your settings" which sounds very bad to me.

Answer 1

Another approach is to divided the whole operations into two services:

1. The first service accepts a HTTP request for generating a PDF document. This service finishes immediately after request is accepted. And it will return a UUID or URL for checking result
2. The second service accepts the UUID and return the PDF document if it's ready. If PDF document is not ready, this service can return an error code, such as HTTP 404.

Since you are using AJAX to call the server side, it will be easy for you to change your javascript and call the 2nd servcie when the 1st service finished successfully. Will this work for your scenario?

Answer 2

Have you tried to following the trouble shooting guide of ELB ? Quoted the relevant part below:

HTTP 504: Gateway Timeout

Description: Indicates that the load balancer closed a connection because a request did not complete within the idle timeout period.

Cause 1: The application takes longer to respond than the configured idle timeout.

Solution 1: Monitor the HTTPCode_ELB_5XX and Latency metrics. If there is an increase in these metrics, it could be due to the application not responding within the idle timeout period. For details about the requests that are timing out, enable access logs on the load balancer and review the 504 response codes in the logs that are generated by Elastic Load Balancing. If necessary, you can increase your capacity or increase the configured idle timeout so that lengthy operations (such as uploading a large file) can complete.

Cause 2: Registered instances closing the connection to Elastic Load Balancing.

Solution 2: Enable keep-alive settings on your EC2 instances and set the keep-alive timeout to greater than or equal to the idle timeout settings of your load balancer.