Node.js game logics

Question

I'm in process of making realtime multiplayer racing game. Now I need help writing game logics in Node.js TCP (net) server. I don't know if it's possible, I don't know if i'm doing that right, but I'm trying my best. I know it's hard to understand my broken english, so i made this "painting" :)
Thank you for your time

Answer 1

To elaborate on driushkin's answer, you should use remote procedure calls (RPC) and an event queue. This works like in the image you've posted, where each packet represents a 'command' or RPC with some arguments (i.e. movement direction). You'll also need an event queue to make sure RPCs are executed in order and on time. This will require a timestamp or framecount for each command to be executed on (at some point in the future, in a simple scheme), and synchronized watches (World War II style).

You might notice one critical weakness in this scheme: RPC messages can be late (arrive after the time they should be applied) due to network latency, malicious users, etc. In a simple scheme, late RPCs are dropped. This is fine since all clients (even the originator!) wait for the server to send an RPC before acting (if the originating client didn't wait for the server message, his game state would be out of sync with the server, and your game would be broken).

Consider the impact of lag on such a scheme. Let's say the lag for Client A to the server was 100ms, and the return trip was also 100ms. This means that client input goes like:

• Client A presses key, and sends RPC to server, but doesn't add it locally (0ms)
• Server receives and rebroadcasts RPC (100ms)
• Client A receives his own event, and now finally adds it to his event queue for processing (200ms)

As you can see, the client reacts to his own event 1/5 of a second after he presses the key. This is with fairly nice 100ms lag. Transoceanic lag can easily be over 200ms each way, and dialup connections (rare, but still existent today) can have lag spikes > 500ms. None of this matters if you're playing on a LAN or something similar, but on the internet this unresponsiveness could be unbearable.

This is where the notion of client side prediction (CSP) comes in. CSP is made out to be big and scary, but implemented correctly and thoughtfully it's actually very simple. The interesting feature of CSP is that clients can process their input immediately (the client predicts what will happen). Of course, the client can (and often will) be wrong. This means that the client will need a way of applying corrections from the Server. Which means you'll need a way for the server to validate, reject, or amend RPC requests from clients, as well as a way to serialize the gamestate (so it can be restored as a base point to resimulate from).

There are lots of good resources about doing this. I like http://www.gabrielgambetta.com/?p=22 in particular, but you should really look for a good multiplayer game programming book.

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I also have to suggest socket.io, even after reading your comments regarding Flex and AS3. The ease of use (and simple integration with node) make it one of the best (the best?) option(s) for network gaming over HTTP that I've ever used. I'd make whatever adjustments necessary to be able to use it. I believe that AIR/AS3 has at least one WebSockets library, even if socket.io itself isn't available.

Answer 2

This sounds like something socket.io would be great for. It's a library that gives you real time possibilities on the browser and on your server.

Answer 3

You can model this in commands in events: client sends command move to the server, then server validates this command and if everything is ok, he publishes event is moving.

In your case, there is probably no need for different responses to P1 (ok, you can move) and the rest (P1 is moving), the latter suffices in both cases. The is moving event should contain all necessary info (like current position, velocity, etc).

In this simplest form, the one issuing command would experience some lag until the event from server arrives, and to avoid that you could start moving immediately, and then apply some compensating actions if necessary when event arrives. But this can get complicated.