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MMOs! Y u no EZ?

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ApochPiQ

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A common aspiration among nascent game developers is to make an MMO. It's actually a pretty common aspiration in many perfectly functional, well-established studios, as well; but at least on GDNet, we mainly see newbies who want to make the next World of Warcraft, and so it's easy to be misled into believing that the MMO fascination is somehow peculiar to those who "don't know any better."

The reality, though, is that plenty of non-newbies have tried and failed to make successful MMOs. Many projects begin and never see the light of day; still more launch and are ultimately failures as business ventures. There's a common cause here, I think: making an MMO is hard, and for reasons which are not altogether obvious. The newbie wants to make an MMO because he doesn't understand how hard it is; the veteran industry studio fails to make an MMO because they don't understand how hard it is, either.

To do something hard, and do it well, requires preparation and a lot of awareness. Even understanding why it is hard can be tough in and of itself. It doesn't help that basically every successful MMO developer ever is highly protective of their secrets - and rightly so.

As a successful MMO developer, I'm going to uphold the tradition, and not talk about the secret sauce that makes it possible to solve the hard problems of MMOs. But I will take some time to shed some light on what those problems really are, and why they are hard.


Content Volume
This is the one most people get hung up on, so I'll dispense with it right away. Producing the sheer amount of content that goes into a genuine MMO is hard, yes, but only because it's expensive. If you have a boatload of money and a lot of talented content creators, this problem isn't really that big of a deal. After all, huge single-player games get made all the time.

If you're not driving home in a dump truck full of hundred dollar bills, then this problem is obviously going to be pretty hard.


The Speed of Light
This is a genuinely hard problem because we can't change the speed of light. However, the speed of light (or more accurately, the speed of signal propagation in physical network equipment) represents a very real limit to how responsive a multiplayer game can be. The farther away your computer is from the "other" computer, the longer it takes to send a signal to that computer. For the sake of simplicity in this article, just assume that the back-of-the-napkin math limits us to some given latency in the tens of milliseconds for an average trip time. (I'm being very fuzzy deliberately.)

The bottom line here is that if I want to have realtime interactions with someone on the opposite side of the country, I'm going to have to wait an almost perceptible amount of time for my signals to reach them, and then an almost perceptible amount of time for their response to come back to me. Put together, this means that even in the purely optimal case where I can talk directly to other players, I might notice latency, or "lag", if they are far enough away.

So what about if I can't talk directly to my peers, but need a server?


Network Architecture
This is where things get truly fun. MMOs are traditionally client-server based, meaning that for me to talk to player B, I talk to my server first, who then relays my interactions to player B on my behalf. This is why server performance matters so much; if the server is busy and takes 60 milliseconds to forward my message to player B, I just made the speed-of-light problem that much worse.

But suppose I need to not just poke player B, but also demonstrate to the other 60 people in the immediate area that I did in fact poke player B. Now my server has to send 60 more messages. Each of those messages takes time to send - we don't have the luxury of sending multiple messages in parallel.

This means that the network that handles the game (i.e. typically a datacenter setup) has to be able to successfully blast out 60 of those messages to a potentially geographically diverse set of people, while maintaining minimal latency. Remember, again, every millisecond we pile on to the message trip time is a millisecond the player might end up feeling as lag.

Now, routing internet traffic is a pretty well-handled problem these days (in most of the world). Figuring out how to get our messages to each of our 62 players in a timely fashion is thankfully not that hard. What is hard is the fact that access to that level of internet beefiness is not cheap and not easy to attain. Internet Service Providers (ISPs) have peer agreements with datacenter installations, which govern how much traffic can come and go from the datacenter, and what quality of service it receives - i.e. how much latency and how much bandwidth will be involved.

A lot of people like to suggest "the cloud" as a solution here. The problem is that cloud providers generally can't compete with the quality of service that a good peering contract will give you. Yes, cloud services are great and still improving fast, but as of today, a dedicated datacenter is a better way to host a latency-sensitive game.

Datacenters, and peering agreements, are expensive propositions. This is back to the content volume problem, though - money can solve it. So let's look at other network related headaches.


The N^2 problem
Suppose we have a client-server model where 10 players are gathered to do whatever. Player 0 suddenly decides to jump up into the air. Now, players 1-9 need to see that, and player 0 needs confirmation that his character is jumping. So that's a total of 10 messages to relay.

Now suppose all the players jump simultaneously. Each player generates 10 messages, as we saw a second ago. Total up the messages for all the players: we must now relay 100 messages - 10 squared. This is generally referred to as "an N-squared problem."

N^2 problems occur all over the place in networked game construction. They have implications on many aspects of implementing an MMO: physics, game rule simulation, network throughput, security (think DDoS problems) and so on. There are ways to solve and/or mitigate the pain, but generally, every system that is affected by N^2 needs to solve the problem a little bit differently.

This means an interesting thing for software architecture: what solves your graphics N^2 may not solve your physics N^2, and both of those might fail to work with combat rules, and even then you need to figure out the network throughput issues, and so on and so forth. Suddenly you have a dozen systems that behave all slightly differently that have to be stitched together into a harmonious whole. That's not always easy, and in fact, it's one of the things that makes implementing a good MMO very difficult.


Nonlinear scaling
The final general principle I'd like to cover is that scaling is rarely linear. Let's say you make a multiplayer game. With 5 players, your server is doing great - 10% CPU usage. With 10 players, 18% CPU usage. With 20 players, 32% CPU usage. If you're following the trend, this sounds awesome! We can probably hit 60 players before running out of CPU! Except you add the 40th player and the CPU pegs out at 100%. What went wrong?

Scaling is rarely linear! We extrapolated that we could triple our 20-player numbers and get 60 players comfortably, but this extrapolation turns out to be flawed. What most people don't realize is that nonlinear scaling is everywhere in MMOs.

Take our N^2 scaling problem from earlier, even. It looks kinda linear if you don't look hard enough, but once you throw on the numbers, it bites hard. Believe it or not, there are worse ways to scale than N^2.

Network traffic, memory usage, CPU usage, database transaction volume, communication between back-end server systems... all these can scale in nonlinear ways. And when they do, you want to know about it long before it turns into a player-facing outage. This means testing, and load testing in particular. Good load tests are hard to do in general, but stress-testing an entire MMO infrastructure and proving it robust is particularly difficult. It makes the difference between a good launch and a bad launch, between profitable operation and burning money.


Parting Thoughts
There are many more challenges to making an MMO a successful business: security, legal issues, attracting a big enough player base, maintaining the player base (aka retention), monetization models, community management, and so on. Even for the issues I've touched on above, I've barely scratched the surface.

I hope this underscores the fact that MMOs are hard. Not impossible, certainly not impossible. Just very, very hard. It takes a diverse collection of skills and expertise to do them right, and sadly that mix just doesn't happen that often. I hope this writeup isn't depressing to aspiring MMO-makers; instead, I hope it gives you a start at understanding what exactly you will need in order to be successful.
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This is a great read, a lot of the things you highlighted here apply to more than just MMO's aswell.

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Nice read.

 

It seems to me like most of your concerns with MMO(*) development is focused around scale (Massive) and networking (Multiplayer Online) and the combination of them, which is logical since that is what seperates this genre from its non-MMO counterparts.

 

Knowing this, would you recommend focusing on the non-MMO parts of the game (llke the roleplaying parts in a MMORPG) at early stages of development (or prototyping) and wait with the MMO parts until later?

 

I can see the value in doing so to get up and running with prototyping ASAP, but to me, this seems like a very dangerous approach. Being used to working in an agile manner, I see the value of focusing on the bigger risks early, and leaving the MMO parts till later would be the opposite of that. For example, you might invest time in a game mechanic which seems promising, only to find out it just doesn't work in a MMO environment.

 

I guess there has to be some nice middleground between attacking big risks early while staying productive (which I have found is even more important when developing as a hobby vs. doing it at work.)

 

Any thoughts/recommendations on this?

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