volve a large number of servers (or will have failed so miserably that no one either can or wants to remember the game or world). Using multiple servers is a basic mechanism for scaling the server component of a game to the levels that are being seen in the online world today. World of Warcraft has reported more than five million subscribers with hundreds of thousands active at any one time. Second Life reports usage within an order of magnitude of World of Warcraft, and there is some evidence that sites such as Webkinz or Club Penguin are even more popular. A single server is not able to handle such load, no matter how efficient the representation. Even if a single server could deal with this load, such a server would be far too expensive for the smaller loads that are encountered ( sometimes by the same games or worlds) at times of low demand (or in parts of the product’s life cycle when demand has decreased).

Having multiple servers means that part of building the game is deciding how to partition the load over these servers. Two techniques are commonly used in both online games and virtual worlds. Sometimes only one of the two techniques is used, sometimes both, depending on the nature of the game or world.

The first technique is to exploit the geography of the game or world, decomposing the game into different areas, each of which can be mapped to a hosting server. For example, an island in Second Life corresponds to a physical server running the code for the shared reality of the world. Similarly, different areas of the World of Warcraft universe are hosted on different physical machines. Anyone who is in the area will connect to the same server, and interactions among the players on that server can be localized (and optimized). Actions happening in a different part of the world are not likely to affect those in this part of the world, so the communication traffic between servers can be kept small.

The second technique is known as sharding. A shard is a copy of a part of the game or virtual world. Different shards reside on different servers, and players who are assigned to one shard can interact with the world and other players in the shard, but will not see (or

With the possible
exception of
the highest end
of scientific
computing, no other
kind of software has
ridden the advances
of moore’s Law as
aggressively as
game or virtual-
world programs.

be able to interact with) players or objects in other shards. Shards not only allow more players to be supported in the world, but also permit independent explorations into the world by different sets of players. Thus, when a new quest or mission is added to a game, it will often be replicated with multiple shards so that more than one player (or group of players) can experience the quest or mission in its original state.

Although sharding and geographic decomposition allow multiple servers to be used to handle the load on a single game or world, they do present the developer with significant challenges. By creating noninteracting copies of parts of a world, shards isolate the players in different shards from each other. This means that players who want to share their experience of the world or game need to become aware of the different shards that are being offered, and arrange to be placed in the same shard. As the number of players who want to be in the same shard increases (some guilds, or groups of players who cooperatively play in a single game over an extended period of time, have hundreds of members), the difficulty of coordinating placement into shards increases and interferes with the experience of the world. While shards allow scale, they do so at the price of player interaction.

Geographic decomposition does not limit player interaction, but does require that the designers of the game be able to predict the size of a geographic area that will be the correct unit of decomposition. If one geographic area becomes very popular, play on that area will slow down as the server associated with the area is overloaded. If a geographic area is less popular than originally predicted, computer hardware (and money) will be wasted on that section because not enough players are there. Since the geographic decomposition is hard-wired into the code of the game or world, changing the decomposition in response to observed user behavior requires rewriting part of the game or world itself. This takes time, can introduce bugs, and is very costly. While this is being done, gameplay can be adversely affected. In extreme cases, this can have a major financial impact. When World of Warcraft was in-