World loop/engine design with apache MINA

Tanner Gower · 2016-06-06T18:05:37

I'm currently writing a 2D MMO that will be pretty small scale, kind of like Zelda and Pokemon on GameBoy. I'm wondering how I should update the world and when I should send delta updates to connected clients. I'm using MINA, so each client is connected to a character in the world. When a client sends a command (like movement changes) it checks if the command is "legal" (velocity can't be more than X, etc) and then adds the movement to a queue for the server loop to process. Basically, I have asynchronous clients sending packets that all come together and become synchronized in the game loop. The command that gets received first, gets processed first. The only commands that are currently sent from the client are velocity changes (usually when you start moving or change direction) and then position updates when the character stops moving voluntarily(no collision). Collisions are detected clientside as well as server side but the server has the final say in what happens. The client does not send a collision update to the server because the server already knows where the character should be based on the last velocity update. For example: if a player A walks straight into a wall, A's client will stop A's movement and the server will also detect the collision and send an update to all other clients in the area telling them that A stopped at x,y location. Now, onto HOW the server knows. First, the server knows where each player is and knows the entire map, obviously. The client runs on a game loop set at 60fps, so each frame takes 16.6ms to process. Velocity is "added" to position and scaled using delta time between frames. I want to write a server engine loop that runs in the exact same way and corrects the client if need be. In this loop I want to also send delta updates to the client, but I don't want to send them every 16.6ms. I would rather keep the updates at a 50ms tick (20hz). Should I forget about updating the server at a 16.6ms tick since It wouldn't matter because the clients get updated at 50ms? Or use 16.6ms for more accurate collision detection and corrections while sending delta updates at 50ms but correcting a client if a collision is detected? Also, the combat will be turn based, so a fast tickrate isn't really necessary.

Networking and Multiplayer Programming

Started by triggdev June 02, 2016 02:45 AM

9 comments, last by hplus0603 7 years, 10 months ago

hplus0603

11,916

June 06, 2016 06:05 PM

Wont all updates received by the client be in the past?

Only if the client immediately applies commands before sending to server. This is a legitimate strategy, and the server (and other clients) will have to then back up and re-simulate their physics. (The "counter-strike" model does this, but only for shots/damage)

An alternative is to have the client time-stamp commands for the future, and then actually apply them in the future. There will be a small command lag, and your job is to hide this lag through UI. RTS games will do this by playing a "yes, sir!" animation before starting to move.

A third option is to just run commands in the order received on each node. Where there are disparities, just apply them as-received, and wait for the server to correct you. The server will then have to send periodic "baseline" updates for the state of each object, such that correction can happen. You'll probably want certain effects (death, falling off cliffs, etc) to only happen when the server says they have happened, rather than pre-simulating them on the client, for best consistent display.

For a gameboy-pokemon style RPG, the last option may be perfectly adequate. You still want a different update rate for network packets than for input and simulation, though. And, when commands are received, you put them in a queue, and run something like one command per player per physics tick. (Where "physics" really means "simulation.")

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World loop/engine design with apache MINA

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World loop/engine design with apache MINA

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