Multiplayer game with JavaScript backend and frontend. What are the best practices?

Question

I'm thinking of creating a web multiplayer game in Node.js. This means I'll be using the same language in the backend and in the frontend. It would be in realtime and about 20 people max in each 'room', so I have a few thoughts:

1. How do I compensate for the delay among all users so that everyone sees the same thing the same time? I'm thinking of tracking the average ping time of each player, find the slowest one, and inform the other clients of the time (in milliseconds) they have to be delayed each one so that everyone is as synchronised as possible.

2. I'm thinking of running the game code in the backend as well as in the frontend (since it's JavaScript on both ends) and just have an error-correction mechanism to synchronize with the 'real game' in the backend. That way the game should perform smoothly on the frontend and with only few glitches when the synchronisation happens. Also that would minimize frontend JavaScript hacking since cheaters would be synchronised down to the backend game.

3. Should I receive player actions through the socket (keypresses), inform all other clients of the other players' actions and in the mean time 'playing' the game in the backend and send synchronisation information to everyone of the entire game state every once in a while to synchronise them?

What do you think? Are there more stuff I should consider or pay attention to?

Please post any thoughts or links to documentation or articles regarding multiplayer gaming.

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EDIT: These are useful:

• Gaffer on Games - Very good articles by Glenn Fiedler on multiplayer game networking and physics
• The Quake3 Networking Model

Answer 1

1 - is impossible. You don't know exactly how long a message will take to arrive on a client and no measurement you take will necessarily be applicable to the next message you send. The best you can do is an approximation, but you always need to assume that people will see EITHER slightly different things OR the same things at slightly different times. I'd recommend just sending the current state to everybody and using interpolation/extrapolation to smooth out the gameplay, so that everybody sees the game a few milliseconds in the past, with the delay varying both between players and over time. In general this is rarely a big problem. If you really want to buffer up some past states on the server you can interpolate between them and send different old data to different people in an attempt to sync what they see, but combined with the client side simulation and the jitter in transmission times you'll still see some differences across machines.

2 - the typical way is to run the simulation on the server, and send regular (small) state updates to clients. Clients typically run their own simulations and have a way to blend between their own predicted/interpolated state and the authoritative state that the server is sending them. All decisions other than user input should be made server-side. Ultimately the way you blend these is just a tradeoff between a smooth appearance and an accurate state so it's a cosmetic decision you'll have to make.

3 - your client should typically translate a keypress to a logical action. Your server doesn't care about keys. Send that logical action to the server and it can broadcast it to other clients if they need it. Generally though you wouldn't need to do anything here - any relevant change caused by the action will typically just change the game state, and thus will get sent out in the normal broadcast of that state.