Irreversible Events
Events whose effect on the game state cannot be undone.
All events in a game affect the game state but many may be countered by another events, e.g. a movement in one direction may be countered by a movement in the opposite direction. Game state changes that cannot be revert back in this way are the effects of Irreversible Events. These type of events guarantee that the gameplay progresses according to a certain design and may be used to clearly indicate changes in what is the current focus of the gameplay to players and, if the player can choose to do the actions that cause the events, can give players more meaningful choices.
Examples
The Ko (and meta Ko) rule in Go is used to make it impossible for the game to run into infinite loops and without these some events in the game would be reversible. Building pyramids in Amun-Re are Irreversible Events since they remain until the game ends even across the transition between the old and new kingdom which removes all other tokens from the gameplay area.
Pushing a box into a corner in Sokoban is an Irreversible Event since the box cannot be moved from there, as the boxes can only be pushed and not pulled.
Using the pattern
Typical reasons for considering having Irreversible Events in games is to increase Tension, make Reversibility impossible, and make it possible for Exceptional Events to occur during gameplay. To really enforce that events are irreversible in games they need to have Dedicated Game Facilitators, including Game Masters (but the latter may also make this assumption invalid). Other games may have them in the rules but these may not be followed.
Even though Real-Time Games make it impossible to undo actions (without resorting to Save-Load Cycles or Game Time Manipulation) this does not automatically create Irreversible Events since later action may cancel the effect of earlier ones. Making events into Irreversible Events can be done by making sure that all other events in the whole game design do not affect a specific part of the game state (other events may affect that part of the game state as long as all affect it in the same way or direction). Other ways of making Irreversible Events are to make actions use Non-Renewable Resources (in which case the action rather that the consequences of the action may be Irreversible Events) or create events that give players information. Designed Surprises are thus a form of Irreversible Events, and since they are a form of Trans-Game Information, they are even Irreversible Events across game sessions.
Irreversible Events can be instantiated in many different forms. Alarms, Controllers, and Switches are game elements that can be used to enforce Irreversible Events, in the first case by not being able to be activated more than once or having effect that remain even when they are activated again, and in the second case by not being able to be deactivated once activated (or vice versa). The Area Control gained by placing settlements in Settlers of Catan are Irreversible Events since they cannot be lost later in the game. A typical example of a class of Irreversible Events is Leaps of Faith, which forces players to dedicate themselves to a certain movement in the game. One-Way Travel is another type of movement that easily can be an Irreversible Event if one cannot come back to the old space - the most common example of this is probably being moved to one Level after completing another with no possibility of returning. The pattern Shrinking Game Worlds makes use of Irreversible Events to ensure that Game Worlds become smaller as gameplay progresses. Making the Transfer of Control of Resources into Irreversible Events is part of creating Non-Renewable Resources; choices regarding Character Development or Upgrades are examples of this (the latter is one way to modulate Sockets).
Using Irreversible Events in conjunction with Loyalty or the creation of Enemies is a way of creating parts of Predetermined Story Structures. Revealing oneself as a Traitor or committing a noticeable Betrayal are both examples of Irreversible Events since they make players change their allegiances (and even if they later change back this will very likely permanently lowered their reliability). These latter types of events, which reveal aspects of what kind of Character a player is portraying, can be explicitly designed as Character Defining Actions and by performing them they become Irreversible Events. In this way, Irreversible Events can modulate how a Character's Open Destiny evolves during gameplay.
Irreversible Events can be created to be the consequences of actions but the actions themselves do not have to be irreversible - their effect may be since partly or completely prevented if they are Interruptible Actions. Likewise, actions that cause Ultra-Powerful Events do not have to be Irreversible Events since other Ultra-Powerful Events may counter the event after it is completed.
One of the possibilities of Irreversible Events is that they can function as Closure Points since after they have occurred one can know that some parts of the game state are irrelevant for future gameplay.
Irreversible Events can exist in weaker forms. One case is in games with the possibility of Saving, since this makes it impossible to have totally Irreversible Events if players are willing to make use of Save-Load Cycles. This can be countered to a certain extent by causing actions to have Delayed Effects (c.f. the Witcher series) at the cost of Predictable Consequences. Another way of having weakly Irreversible Events is if there are certain Privileged Abilities that can reverse the effect of the events; one example of this relates to cursed Equipment in that equipping them are Irreversible Events unless some remove curse effect can be activated. Equipment in World of Warcraft that become "Soulbound" when equipped is another example - this makes it impossible to later use them in Trading and by doing so avoids one form of Purchasable Game Advantages in the game.
Consequences
Having Irreversible Events in games opens up for having Exceptional Events since events that can be reversed are unlikely to seem exceptional since they can occur time and time again. Further, every Irreversible Event that a player understand to be irreversible is a closure. This can indicate progress or failure clearly and provide clear Value of Effort for successful actions; for example, cases where Construction actions are Irreversible Events, these can provide an increased Value of Effort to the Construction since players do not need to worry about them being destroyed. However, the presence of the pattern may negatively affect Experimenting since there is no Reversibility, and make Puzzle Solving more difficult. Rather, the presence of Irreversible Events promote Stimulated Planning and may cause Analysis Paralysis in Multiplayer Games that are also Turn-Based Games. When linked to player actions, these aspects of the pattern can cause Tension in players; both from knowing that their actions may not be able to be reversed and from others wishing one to make the actions quickly. Since many possible player actions are in the form of Abilities, the use of Irreversible Events on player actions is likely to modify Abilities.
Although One-Way Travel is a form of Irreversible Events, the pattern can also appear due to Obstacles being introduced as Irreversible Events. Irreversible Events that affect Game Worlds, e.g. changes in Reconfigurable Game Worlds that cannot be undone, give rise to Persistent Game World Changes.
Irreversible Events are required to meaningfully progress Predetermined Story Structures and can be used to provide a condensed history of gameplay in a game session without losing any events that affected the final outcome. Leaving a Level and not being able to come back to it is an example of how Predetermined Story Structures can be maintained by Irreversible Events. An example of how Irreversible Events can advance the Predetermined Story Structures in Real-Time Games without relying on Levels is to move the places where Spawning occurs (examples of this can be found in the Team Fortress series of games).
The synthesis of having a Freedom of Choice between several Irreversible Events is one way to create Internal Conflicts.
Quite naturally, Irreversible Events are incompatible with Free Game Element Manipulation although this may break game rules. Given that Game Masters decide what the rules are, these may whenever they wish reverse events while simultaneously change the rules (they may of course enforce the effect of events as they wish also).
Relations
Can Instantiate
Closure Points, Enemies, Exceptional Events, Internal Conflicts, Predetermined Story Structures, Persistent Game World Changes, Shrinking Game Worlds, Stimulated Planning, Tension, Value of Effort
with Construction
with Obstacles
with Multiplayer Games and Turn-Based Games
Can Modulate
Abilities, Alarms, Area Control, Character Development, Construction, Game Worlds, Levels, Loyalty, Open Destiny, Puzzle Solving, Sockets, Spawning, Transfer of Control, Ultra-Powerful Events
Can Be Instantiated By
Betrayal, Character Defining Actions, Controllers, Dedicated Game Facilitators, Equipment, Game Masters, Leaps of Faith, Non-Renewable Resources, One-Way Travel, Switches, Surprises, Traitors, Trans-Game Information Upgrades, Upgrading
Can Be Modulated By
Interruptible Actions, Privileged Abilities
Possible Closure Effects
-
Potentially Conflicting With
Experimenting, Free Game Element Manipulation, Game Masters, Reversibility, Save-Load Cycles
Predictable Consequences when Irreversible Events is used together with Delayed Effects
Purchasable Game Advantages when the pattern is used together with Equipment
History
An rewrite of the original pattern named Irreversible Actions in the book 'Patterns in Game Design' (Björk & Holopainen, 2004).
References
Björk, S. & Holopainen, J. (2004) Patterns in Game Design. Charles River Media. ISBN1-58450-354-8.
