Entity–component–system (ECS) is an architectural pattern that is mostly used in game development. ECS follows the composition over inheritance principle that allows greater flexibility in defining entities where every object in a game's scene is an entity (e.g. enemies, bullets, vehicles, etc.). Every entity consists of one or more components which contains data or state. Therefore, the behavior of an entity can be changed at runtime by systems that add, remove or mutate components. This eliminates the ambiguity problems of deep and wide inheritance hierarchies that are difficult to understand, maintain and extend. Common ECS approaches are highly compatible and often combined with data-oriented design techniques.