Some initial context to this question. Currently I have a application cluster deployed behind a ALB which maintains persistent keep alive connections with the application. This application is under continuous heavy load and must have very high uptime. The ALB has been sending back 502 Bad Gateway status codes from this service. Digging deeper and after taking a pcap capture and sysdig capture on the affected instances we see the following (ordered by sequence of events):

19:51:26.881806 HTTP    1068    POST /api HTTP/1.1  (application/json)
19:51:26.881838    TCP 66  80→52026 [ACK] Seq=7201 Ack=19033 Win=67072 Len=0 TSval=240987 TSecr=1566420
19:51:27.018305861 0 node (2989) > writev fd=120(<4t>> size=400
19:51:27.018326    HTTP    466 HTTP/1.1 200 OK  (application/json)
19:51:27.018341806 0 node (2989) < writev res=400 data=HTTP/1.1 200 OK..Content-Type: application/json; charset=
19:51:27.018601 TCP 66  52026→80 [ACK] Seq=19033 Ack=7601 Win=47360 Len=0 TSval=1566454 TSecr=241021
19:51:32.042525 HTTP    1066    POST /api HTTP/1.1  (application/json)
19:51:32.042538    TCP 66  80→52026 [ACK] Seq=7601 Ack=20033 Win=69120 Len=0 TSval=242277 TSecr=1567710
19:51:32.066469320 0 node (2989) > close fd=120(<4t>>
19:51:32.066470002 0 node (2989) < close res=0
19:51:32.066487    TCP 66  80→52026 [RST, ACK] Seq=7601 Ack=20033 Win=69120 Len=0 TSval=242283 TSecr=1567710

As stated above, it appears that our nodejs application reaches 5 seconds of inactivity on a keep-alive connection (default keep alive timeout period), then receives a network request, then closes the socket and finally responds to the queued network request with a RST.

So it appears that the 502's are due to a race condition where a new request is received from the load balancer between or during the TCP teardown.

The most apparent solution to this problem would to ensure that the load balancer is the source of truth when tearing down these connections, ensuring that the load balancers idle timeout is less than the timeout on the application server. This solution works with AWS classic load balancers but not ALB as according to their docs:

You can set an idle timeout value for both Application Load Balancers and Classic Load Balancers. The default value is 60 seconds. With an Application Load Balancer, the idle timeout value applies only to front-end connections.


Could someone speculate as to why AWS may have removed the backend idle timeout (I'm assuming it is infinity)? I could set the keep alive timeout on the node server to Infinity as well but should I worry about leaking sockets? Are their any other server technologies that handle this problem more gracefully that I could apply to fix this issue (without using classic load balancers)?

Also AWS support states they will not respect a Keep-Alive header sent back from the service.


We cannot know the reason why they don't have it. We cannot know AWS design / implementation decisions which cause the behaviour. Only people who work with Amazon on this feature know, and they are most likely under NDA.

Your only chance to get a valid answer for this is to ask AWS.

  • Modified the title, I suppose knowing exactly why AWS made this decision is more of an artifact of the question. – andrsnn Dec 19 '17 at 7:34
  • 1
    Any speculation as to why it might behave like this? Seems strange that a load balancer backend idle would have no timeout. – andrsnn Dec 19 '17 at 7:35

It doesn't seem reasonable to believe that ALB keeps back-end connections open indefinitely but rather that the configurable timeout only applies to front-end connections.

I'm a little concerned by how much time elapsed between the arrival and ACK of the request at :32.042 and the fd close at :32.066. You're timing out a connection that is arguably not actually idle -- it accepted a request 24ms earlier. (!?) To me, that's a surprisingly "long" time.

As noted on the same page in the docs,

Classic Load Balancers use pre-open connections but Application Load Balancers do not.

You should not need to worry about leaking descriptors since ALB won't open connections it doesn't actually need for serving requests... but you should not need an infinite timeout, either.

The question seems to be how long ALB holds idle back-end connections open -- which appears to be undocumented but I will review my logs and see if I can find evidence to suggest what the timer may be set to, assuming it's static. (Holding back-end connections open is of course intended as a performance optimization.)

Intuition suggests you might try a 75 second timer on your side. Those are the defaults I established based on classic balancer behavior and I have observed no issues with ALBs dropped into their place.

  • 1
    As this is a node server I'm guessing that 24ms is spent processing a cpu bound task so the event queue is blocked. That probably does play a role in this problem or at least exacerbates it. According to AWS they expect the backend to close the connection and also reiterated that the ALB has no backend idle (without specifying timeout). They recommend implementing a retry. Somewhat surprising seems like this edge case would always exist. – andrsnn Dec 20 '17 at 16:44

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.