Long-polling (user notification) with a RESTful architecture

Question

I\'m working on a simple RESTful api (on NodeJs). I understand that being restful implies that horizontal scaling will be much easier. Unfortunately, I need some way for cli

Answer 1

First off, I don't know why you wouldn't just use something like socket.io on both client and server to handle your notification channel. That will use web sockets if available (more efficient than long polling) and then fallback to long polling if no web sockets.

There are a number of ways to handle the issue of notifying a user that may not be connected to the server that originates the notification:

Connected Database

When a user connects, they are load balanced to a random server, but which server they end up connecting to is stored in a central database (e.g. perhaps a redis store) from which any of your servers can find out which server any user is currently connected to. This gives you the ability to lookup which server any user is currently connected to. Each server that handles user connections just adds each user to the database with a reference to their server id when they connect and removes them from the database when the user disconnects. Note: since this information doesn't need to be stored persistently to disk and the size of the data is small, you can select a database that excels at keeping most or all of the info in memory.

Hashed Connection

Based on some well-known characteristic of the user such as their userID, a hash value is computed and a repeatable algorithm is created to map hash values across the server pool. This is a predictable assignment. Thus, when a server wants to notify Bob, they can call the same function that will figure out which server Bob has connected to. The hash algorithm can be adaptive such that if today you have three servers, the hashed users are spread evenly among the three servers and if tomorrow you have four servers, then the hashed users are spread evenly among the four servers.

Multi-Connection

Since low-activity websockets are pretty scalable these days, all users could connect to all servers and thus each server would have a socket connection to the user. This is simpler infrastructure, but not ultimately as scalable.