HTTP Long Polling - Timeout best practice

Question

I play with Javascript AJAX and long-polling. Try to find best value for server response timeout.

I read many docs but couldn't find a detailed explanation for timeout.

Someone choose 20 secs, other 30 secs...

I use logic like on diagram

How can I choose better value for timeout? Can I use 5 minutes? Is it normal practice?

PS: Possible Ajax client internet connections: Ethernet RJ-45, WiFi, 3G, 4G, also, with NAT, Proxy.

I worry about connection can be dropped by third party in some cases by long timeout.

Answer 1

Maybe its your grasp of English which is the problem, but its the lifetime of the connection (time between connection opening and closing) you need to worry about more than the timeout (length of time with no activity after which the connection will be terminated).

Despite the existence of websockets, there is still a lot of deployed hardware which will drop connections regardless of activity (and some which will look for inactivity) where it thinks the traffic is HTTP or HTTPS - sometimes as a design fault, sometimes as a home-grown mitigation to sloloris attacks. That you have 3G and 4G clients means you can probably expect problems with a 5 minute lifespan.

Unfortunately there's no magic solution to knowing what will work universally. The key thing is to know how widely distributed your users are. If they're all on your LAN and connecting directly to the server, then you should be able to use a relatively large value, however setting the duration to unlimited will reveal any memory leaks in your app - sometimes its better to do refresh every now and again anyway.

Taking the case where there is infrastructure other than hubs and switches between your server and the clients, you need to provide a mechanism for detecting and re-establishing a dropped connection regardless of the length of time. When you have worked out how to do this, then:

1. dropped connections are only a minor performance glitch and do not have a significant effect on the functionality

2. it's trivial to then add the capability to log dropped connections and thereby determine the optimal connection time to eliminate the small problem described in (1)