Designing Multiplayer Game Engines?

mikera asks: "I'm a pretty experienced programmer but I've just embarked on my first (massively?) multiplayer strategy game. To make things even more interesting, I've decided to write it in C# and pick up a new language at the same time. I've chosen a client-server architecture where the server holds the one 'true' state of the world. The only communication from clients would be commands to units, which is simple enough, but the server will have to propagate game state changes (events) to all clients. I would like to ask Slashdot readers if they have solved a problem like this before or can offer some opinions on the best way to implement a solution."

"Lag is not really critical, but I still want things to be responsive and it must scale up well with the number of clients. The size of the map data, the complexity of the objects and bandwidth constraints rule out sending the complete game state, so only incremental updates will work. The situation is further complicated by the need to limit updates to just the areas of the map that are visible to a given player/team - this is clearly necessary to prevent client-side hacks such as gaining full map knowledge.

I understand the theory well enough, but I'm interested in practical advice on how to implement a solid architecture. What should the object model look like? How do I propagate events that are only partly within a client's field of view? Are there any novel features in C# that might make my life easier? How can I make the networking code as transparent as possible so I don't have to write SendUpdate() after every assignment?"

Your comments, insights, hints and flames are eagerly awaited."

Comments

Some Advice

[Shelrem]

I'll avoid most of my comments about your choice of language because most of it is of a political nature, rather than practical one; however, I really wouldn't suggest trying to make a massively multiplayer game with a language you're unfamiliar with. It's quite an undertaking even with a language you know. I know; i'm working on one.

As for the networking code transparency, this one seems fairly obvious to me.. Just keep a datastructure containing all the changed or "tainted" objects as you go. Make mutator functions of your classes set objects as tainted. Then, just do the networking updates once or twice every time through the main loop (assuming it's in the same thread. Otherwise, you can implement something that might end up being a little more efficient).

As for updating only what the player needs to be updated on, this seems like a question of algorithm efficiency. I don't know the specifics of your game, but with most massively multiplater games, transmitting the entire world state, or even the entire list of changes to every client, every cycle would be insane. So, you have to only update the section of the world that the player can see. How to do this well depends on the internal structure of the world, and what sort of stuff the player can "see". If the game is room-based, then this is easy. If the player can always just see a specific size circle or rectangle around him, this is also easy (each event can check distance to all players in its regeon). If it works like most RTS with arbitrary viewing areas, then you might have to be a little more clever. Whether this is even much of a concern is really a question of the number of people supported, and the expected hardware this'll be running on.

Hope that helps,
ben.c

Try AOP (Aspect Oriented Programming)

[hieronymous72]

AOP might be a great way to handle the problem of dealing with the update issue. You can program as though they are on the same box and code your aspects to deal with what to send and when. It's not an easy thing to learn (think when you went from procedural to OOP) but it can pay huge dividends.

I use AspectJ (www.aspectj.org) but that's for Java...I don't know of any AOP toolkits available for C# since it's such a new language.

Good luck.

Thoughts

[Joe Rumsey]

If you've never written a client-server game at all, you might check out Netrek for some basic ideas. It isn't massive, but it is client-server, and it's where I learned most of the important things I know about network programming in general (and I do, in fact, get paid to write network code for games now.) Quake or Quake 2 source are probably also good things to look at, though I haven't seen their code personally.

None of those solve the major problems you're really asking about though: how to decide who gets to know about what. Worldforge is the only open source project I can think of to point you to in that area. Perhaps some MUDs might be useful as well, but they tend to be based on rooms, not areas, which doesn't translate well at all to most other games.

Probably the biggest problem is avoiding N^2 operations as much as possible. At some level, there's no way around it: N players in an area generate events that have to be propagated back out to N (or at least N-1) players. This obiously makes scaling to arbitrarily large populations difficult. On the other hand, if you can guarantee a set maximum number of players in an area/server/whatever, you can target that maximum and not worry about it a whole lot.

A slashdot post isn't really the right medium to answer this question. There aren't any quick and easy answers, you need to figure out what makes sense for your game on your own. I've spent a large portion of the last year thinking about these problems myself, and I wish you the best of luck on it. I'm having a blast myself, but it's also the most challenging work I've ever done.

I am interested in hearing your reasons for using C#. That's actually somewhat shocking, but maybe it's just ignorance on my part. I can't imagine why you'd want to bet the farm on such an immature system, regardless of the strengths you've perceived in it. What does it do for you that C++ doesn't, and is it really worth it?

Don't forget Client-side Prediction

[Rayonic]

It sounds like you're taking a lot of the stuff off the client's hands to add security. While this is a good idea, I can't stress enough the importance of the client predicting the positions of players and objects. Objects especially are easy to predict (an arrow arcing through the air, something rolling, etc.) Movement prediction can make or break a game; you'd be surprised how little tolerance players have for people/things jumping around with no movement in between.

Heck, players can be pretty predictable objects themselves, if you want to get fancy. Most games have pretty simple algorithms predicting the future state of a player character (if player X is moving in a straight line, he'll probably continue to do so.) But I think you can get fancier than that. What about curves? (circle strafing) Or if the player is repeatedly hitting a button every half-second. (chopping wood?)

It'd take a lot of coding, but the master server could conceivably take note of each player's input characteristics and compile some sort of "personality" profile for each one, which it could transmit to clients every time they start up, to help with their local prediction.

Please note that I have no idea how much CPU time this would take up,

Two strikes ...

[Ldir]

If this is a commercial project, please tell us the name of the game so we can avoid it.

In all seriousness, in my opinion (unless you're doing this solely as a personal learning experience), you are starting with two critical strikes:

1. You're trying to do a major project in a language you don't know (and an immature one at that).

2. You're trying to do a major project in a genre with which you have no experience.

Either one could cripple the project. Put them both together and you're doomed before you start. You may eventually make it work - sort of - but it will never work well, and it will be riddled with bugs.

I encourage you to start by developing a small multi-player game in a language in which you are already proficient. This will let you focus on the design and structure without fighting the language. Keep it simple, manage the scale, but incorporate the kinds of capabilities you want in the final version.

When you've got that working, throw it awy and develop it again in C#. Since you're starting with a working design , you're now free to focus on the mechanics of the language. You need time to learn its limitations and idiosyncracies, and to become proficient. (I will let others debate the wisdom of C# - I'm skeptical of all proprietary languages, especially until they're field-proven.)

Once you have succesfully finished a small project in C#, you can begin planning your real game. Based on your experiences, you may decide to scrap C# entirely. If you choose to stick with C#, then throw away ALL of your original code and start over. No matter how good you think your first code was, by the time you finish the big project you will know that it's crap. Might as well get it out of the way up front to reduce re-work and improve the overall quality.

Of course, if this is a project you've been assigned as a commercial effort, you won't be given the luxury of doing it well. You probably already have a deadline pulled out of thin air, and you're probably already behind schedule. Speaking as a pointy-haired boss who actually has significant coding experience (a long time ago, in a galaxy ... etc.), most PHB's have no clue when it comes to software development. They work with the suits to draw up pretty little Gantt charts, and haven't the foggiest notion as to why they are complete fantasies. You can see some of the results in the bargain bin of your local Best Buy, or in the "still delayed" list of your favorite gaming magazine.

In any case, good luck.

Re:Not doing two things at once

[JasonAsbahr]

Hey, thanks for the plug. Here's an open letter to developers about my new project, Netsu. It uses Python, the Nebula Device, *and* Twisted...

Greetings,

I'm a professional game developer who has committed a large portion of his adult life working on virtual worlds of one sort of another. Recently, I worked on Ultima Online 2, which some of you may have been looking forward to playing. I know I was! :-)

I'd like to talk to you a bit about my current project, 'Netsu', which is the culmination of many years of background work. "Netsu", Japanese for "heat" and "fever", is an anime-inspired world which incorporates multiple perspectives of play and a reputation network for players to self-organize. It's going to be fun.

Currently, I'm working on combining a number of existing open source technologies to build the new virtual world system. These include the 'Nebula Device' 3D engine from Radon Labs on the client side, the Twisted framework for server-side and client-server communication, and (as the project moves forward) the OpenCyc knowledge base as server-side archetype repository and artificial intelligence server.

As I develop use cases and other design documentation for Netsu, I intend to keep them as portable as possible between gaming systems, so that they can benefit the most people. I anticipate this work feeding back into Twisted, expanding it to be a more general simulation framework, and into Nebula, adding network support and in-game world editing tools.

So where did this project come from?

The genesis of the project began with my work on virtual reality systems in the early 90's. I developed virtual building walkthroughs for architects, VR games, and created the first hardware accelerated PC-based virtual actor system for Compaq.

This work was all done with C and C++, and it was during this period that I ran up against the need for a dynamic and interactive way to "reach inside" the simulation and manipulate the code and data directly. And that's how I first became acquainted with the language Python.

The next major phase was to build a 3D client engine with Python embedded as a control language. The fruits of that effort are described in the paper I presented at Python 7, "Beyond: A Portable Virtual World Simulation Framework". (This was also the first mention of 'Netsu'.)

In 1998, the path of development took a turn when I was recruited into Origin Systems to work on developing the Python foundation for the Ultima Online 2 project. The focus during the period was on innovation on the server side and on client-server communication. The results of that labor are described in another paper presented at Python 9, "Python for Massively Multiplayer Virtual Worlds".

Both papers are here.

However, to this point, all the software developed for these projects was closed source. The desire for an open source virtual world system was lurking in my head, but it wasn't until the O'Reilly Open Source conference in 2000 that my plan for the open source virtual world crystallized.

At the 2001 O'Reilly conference, while presenting on the UO2 design work, I took the opportunity to discuss my plans for the future open source virtual world system. The response was extremely positive.

So what are next steps to realizing this goal?

Glyph (the originator of the Twisted project and a fellow ex-Origin colleague) and I have been meeting periodically over the last few months to discuss these issues, which intersect nicely with his long term plans for gaming in Twisted. I've also been meeting with the Cycorp folks (who happen to office just down the road from me) to discuss integrating Cyc and driving Python class generation from Cyc.

I recently integrated Python into the Nebula Device, which allowed me to easily integrate the PB remote object communication protocol and made Nebula the first 3D clients for Twisted. Over the next six months, several major components will come online, including character creation, character motion, the GUI system, basic terrain management, character inventory, and combat.

As I wrap up my current consulting project (porting Python to the Playstation 2 and developing a GUI for designers to build Python logic), I'm devoting 100% of my time to Netsu and open source development.

It is my hope that this collaborative approach for virtual world development grows, spreading outward and providing a foundation to help other developers build their own virtual worlds and enable the sharing of code and art resources between worlds..

If you are interesting in finding out more about the project, drop me a line via email and tell me a bit about your interests and background.

Cheers,

Jason Asbahr
jason@asbahr.com