Merge branch 'realtime-methods-docs' into 'master'

Add docs for realtime polling for the frontend.

See merge request !9998

doc/development/frontend.md

@@ -32,6 +32,21 @@ You can find the Frontend Architecture experts on the [team page][team-page].
 
 You can find documentation about the desired architecture for a new feature built with Vue.js in [here][vue-section].
 
+### Realtime
+
+When writing code for realtime features we have to keep a couple of things in mind:
+1. Do not overload the server with requests.
+1. It should feel realtime. 
+
+Thus, we must strike a balance between sending requests and the feeling of realtime. Use the following rules when creating realtime solutions.
+
+1. The server will tell you how much to poll by sending `X-Poll-Interval` in the header. Use that as your polling interval. This way it is easy for system administrators to change the polling rate. A `X-Poll-Interval: -1` means you should disable polling, and this must be implemented. 
+1. A response of `HTTP 429 Too Many Requests`, should disable polling as well. This must also be implemented.
+1. Use a common library for polling.
+1. Poll on active tabs only. Use a common library to find out which tab currently has eyes on it. Please use [Focus](https://gitlab.com/andrewn/focus). Specifically [Eyeballs Detector](https://gitlab.com/andrewn/focus/blob/master/lib/eyeballs-detector.js).
+1. Use regular polling intervals, do not use backoff polling, or jitter, as the interval will be controlled by the server.
+1. The backend code will most likely be using etags. You do not and should not check for status `304 Not Modified`. The browser will transform it for you.
+
 ### Vue
 
 For more complex frontend features, we recommend using Vue.js. It shares