want to spend much time discussing the current approaches, but I'll
include something about them if only for completeness. I expect pragmatism
and games companies desiring to get games out by Christmas 'this year'
rather than 'five years hence' are the essential reasons why massive
multiplayer games are not yet really with us. And why sell a hundred
player game if a ten player game sells just as well?
Techniques. All multiplayer games based on networked computers
will adopt an approach that is either about communicating input, or
communicating the game model. A combination of these two can also be
used where with some effort the 'need to know' relationships between
players have been computed, typically based on their proximity with
each other in the game world.
in the following lists 'user input' can be communicated in its raw form
or can be processed into higher level forms (possibly within the context
of the game model).
to Peer. This approach is where each computer runs the game
model, but communicates with its peers in order to synchronise and ensure
consistency. Three variations I can think of off the cuff:
computer publishes its user's input to all others
input is collected by a central point and then republished to subscribers
- A master
model collects input and publishes model deltas
multiplayer games adopt one of these approaches.
This approach is where only a single server runs the game model
and all user input is fed into this. Each client performs only enough
computation to present the user's view of the scene. However, it may
be that there is little difference between the client software that
models the local scene and the server software that models the entire
game model, i.e. it's only the nature of the data that's different.
Another selection of flavors that this approach may come in:
download compressed frames (MPEG say) or scene graph deltas (high
running scene modellers download scene changes
running scene modellers periodically download whole scene states followed
by regular updates
Multiple servers run the game model, replication being co-ordinated
by a super server. Sometimes replication is not performed, instead each
server runs an independent portion of the game model and services only
those players currently located in that portion.
games such as the RPGs, namely: Ultima Online, EverQuest,
Meridian 59, Asheron's Call, and Diablo, will probably
be using client/server or multi-server approaches.
System. The distributed systems approach is where a system of
replication is used which doesn't make any distinction between participating
computers. In effect, all computers are simultaneously behaving as peer,
client, and server, depending upon their relationship with each other.
Furthermore these relationships may be continuously changing, and the
proportion of the model that each computer stores and processes will
also change according to its need and capacity.
is not necessarily any particular computer that needs to be ultimate
control or in full possession of the entire game model at any one moment.
A true distributed system is a fully collaborative effort, shared among
all. However, it's still important that at some point, this system presents
itself to the game application as though it existed upon a single computer
- this is termed 'transparency'.
to plug into your search engines are: InVerse (Sandeep Singhal), Bamboo
(Kent Watsen), and HLA - High Level Architecture (Judith Dahmann). Don't
think that these are all similar, or even paragons of distributed systems,
rather just take them as a tiny sample of the variety of approaches
that can be adopted, including assorted elements of brute force, pragmatism,
be exploring these or any other particular approaches. I'll examine
the essential tasks of any distributed system that is to form the basis
of a massive multiplayer games engine. If you want real-world facts
in the meantime, checking out the above might be somewhere to start.
Today - Here Tomorrow
are only just beginning to appreciate, is how differently one has to
approach the design and implementation of global scale systems. The
Web is one of the best known and, along with e-mail, represent effective
systems. This is due in large part to the fact that they work whatever
the size of the network and number of users that they must support (OK,
with a bit of tweaking over the years). Global scale systems are unlikely
to succeed if they are designed for a specific level of demand or resource
availability. This is despite the attraction of developing a system
optimised for a specific environment and usage, given it will tend to
outperform a system that can adapt to any environment. The point to
note is that the latter only needs to be written once.
system has to reach a global scale, you don't want to rewrite it each
time it has to cope with another order of magnitude in scale. The system
has to look after itself.
being a high-speed, natural selection environment (for companies if
nothing else) will tend to select systems that get adopted most rapidly.
A system that works as well for a million as well as for a hundred players
and doesn't require another 100 database specialists on the development
team, or investment in individual servers all over the planet, will
leapfrog one that does.
develop a system that grinds to a halt once 10,000 players participate,
then while you're fixing the problem, you'll be overtaken by one that
grinds to a halt at 100,000. Both will be overtaken by a system that
scales to any number. And any system that requires significant expenditure
upon infrastructure in linear proportion to its scale will become unaffordable.
imagine having to cough up a few grand for a new server every time you
sell another thousand copies of your game. No, don't bother, plenty
of companies think this is reasonable, so I'm wasting bytes trying to
convince you otherwise. I'll just leave it that you have to start designing
systems like viruses or worms, i.e. totally autonomous and self-organizing.
You want to write a game and that's it. No hosting deals with ISPs,
or administrating a load of servers. Beware the vested interests that
will try to maintain the multi-server approach - there's a lot of money
perspective, if your game engine isn't scalable it will be as future-proof
as storing a year in two digits. Yup, 'Scalable Systems' is the next
paradigm shift after 'Object Orientation', and funnily enough it's also