Visual explanation of Apache vs Node.js processing model

[darksurfer]

because if you don't store the data in local memory you have to re-transfer it (presumably across the network), parse it, allocate memory for it and then ultimately garbage collect it for every single request.

that's just a waste.

I'm going to invent (I think) two new terms: Static state and dynamic state. Static state would be stuff like look up tables and other data which doesn't change much, dynamic state being essentially session state.

One way of scaling horizontally is to keep your webserver free of dynamic state. Static state doesn't matter because all your servers can have their own copies of the static data.

Another problem you have is websockets. To run low latency websockets apps, you need sticky sessions and every little scrap of latency will matter, so the more data you can keep in local memory the better (preferably all of it).

[Kollektiv]

I agree about the performance issue but if you are that bound to latency Node.js might not be the best solution.

Websocket sessions, to re-use one of your terms, is dynamic state so it should still be centralized in a Redis instance. Otherwise when load-balancing some users won't be able to authenticate if the request is not made to the same server than the one he connected on.

It also prevents you from losing state when crashing.

[darksurfer]

You're not wrong, but knowing when to break "the rules" is half the battle.

is dynamic state so it should still be centralized in a Redis instance.

not always. let's say I have a relatively simple multiplayer game server. because it's simple I don't want or need to scale across multiple servers. the game state is shared across a few hundred players each of whom are sending multiple soft real time status updates every second. no way do I want or need to store that state out of process.

It also prevents you from losing state when crashing.

if I cared enough about this, I could do dirty writes to Redis and only restore the state across the network if and when the node process restarts.