A persistent world or persistent state world (PSW) is a virtual world which, by the definition by Richard Bartle, "continues to exist and develop internally even when there are no people interacting with it". The first virtual worlds were text-based and often called MUDs, but the term is frequently used in relation to massively multiplayer online role-playing games (MMORPGs) and pervasive games. Examples of persistent worlds that exist in video games include Battle Dawn, EVE Online, and Realms of Trinity. A persistent world can be achieved by developing and maintaining a single or dynamic instance state of the game world that is shared and viewed by all players around the clock. The persistence of a world can be subdivided into "game persistence", "world persistence" and "data persistence". Data persistence ensures that any world data is not lost in the event of computer system failure. World persistence means the world continues to exist and is available to players when they want to access it. Game persistence refers to the persistence of game events within the world (a Groundhog Day MUD is a virtual world where the entire (game) world is reset periodically). When referring to a "persistent world", world and game persistence are sometimes used interchangeably. The persistence criterion is the trait that separates virtual worlds from other types of video games. Pervasive games The real world is persistent. The game world of a pervasive game takes place in the real world and so pervasive games are also persistent. In other words, pervasive games share the persistence trait with virtual worlds. An example of a pervasive game that makes heavy use of a virtual world is Can You See Me Now?, where street runners exist in a virtual world while simultaneously running around the real physical world; the game was persistent during play, as well as the virtual world. Simulated persistence To give the illusion that the game world is always available, persistence can be simulated. This can be achieved by scheduling when players are allowed to play, around times when the world is offline, or as in the Animal Crossing series, having the game generate events that could have happened during the period of inactivity. Aside from virtual worlds, the simulation of a persistent world is also possible in single player games. In Noctis, players are advised to turn off the game while refueling because it takes so long. In