How do MMO servers handle a large concentrated number of players?

Question

I have a pretty good understanding of how to scale servers but there's one thing I can't quite be sure about is how to manage a lot of players in a concentrated area.

If I have a server which is using multiple threads and processes as long as you limit the amount of players that can join a single game instance then this can scale indefinitely by adding more servers and placing it behind a load balancer.

But MMO games don't work that way because it's broken down into zones that anyone can connect to. So if a massive amount of players all decided to connect to the same zone it would put all of the stress on 1 server.

If you tried to spread the load across multiple servers then you'd run into problems of having to share information between the servers (Such as if player X was connected to sever 1 then all of his movements would also have to be passed on to the players connected to server 2 since server 1 and 2 are managing the load for the same zone).

So I was wondering how do you go about solving this problem? Do you just keep on ramping up the specs for the server on heavily populated zones? Or is there a fast and efficient way for multiple servers to manage the load for the same zone and efficiently share data between the two servers?

Answer 1

Are you already familiar with technologies that exist like redis? Server architecture is definitely a complicated subject and it's good that you're planning it out now, but you might be worrying without cause and underestimating the amount of throughput possible with a well-written scalable backend.

In my experience, it's better to have servers set up to handle their own zones, but written in a way so that in the future, it's easy to modify a particularly busy zone to run multiple load-balanced servers. At that point, each server on the zone should be handling around 1000-2000 concurrent connections, and propagating player data using something like redis between the nodes.

Another common solution I've seen (particularly on popular minecraft servers) is to tweak the actual game design to limit the amount of players on a single server. To do this, have multiple load-balanced servers for the same zone, but allow the player to see what server they're connected to, and only render player data within that server. The result is multiple "realities", where each server independently has a different crowd of players in the same apparent area, but this reduces server stress and gives the flexibility of allowing the player to switch freely between servers if they want. If you do allow them to switch (in order to meet with a friend or something), impose a maximum player count per server, and you'll have a hard limit of how much each server should ever have to handle.

You should at least get a working game and run some simulation tests with a few hundred or thousand client connections to get actual performance specs before worrying where you should go from there. It's possible that there might not be an issue at all.

Also, before you concern yourself with the massive numbers of players you're asking about, consider that your game might not ever be that popular. I mean no offense, it's just relatively rare to develop a "hit." So instead, focus on more important issues like good game design, so that you'll be able to worry about this later on.