Entity component system
This article is written like a personal reflection, personal essay, or argumentative essay that states a Wikipedia editor's personal feelings or presents an original argument about a topic. (October 2013)
Entity–component–system (ECS) is a software architectural pattern that is mostly used in video game development. ECS follows the principle of composition over inheritance, meaning that every entity is defined not by a "type" assigned to it, but by named sets of data, called "components", that are associated to it, which allows greater flexibility. In an ECS, processes called "systems" determine which entities have certain desired components, and act on all of those entities, using the information contained in the components. For example a physics system may query for entities having mass, velocity and position components, and then iterate over all of them and do physics calculations. The behavior of an entity can therefore 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 are often combined with data-oriented design techniques. Although an entity is associated with its components, those components are typically not necessarily stored in proximity to each other in physical memory. It is notable that an ECS resembles the design of databases and, as such, can be referred to as an in memory database.
In 2007, the team working on Operation Flashpoint: Dragon Rising experimented with ECS designs, including those inspired by Bilas/Dungeon Siege, and Adam Martin later wrote a detailed account of ECS design, including definitions of core terminology and concepts. In particular, Martin's work popularized the ideas of "systems" as a first-class element, "entities as IDs", "components as raw data", and "code stored in systems, not in components or entities".
In 2015, Apple Inc. introduced GameplayKit, an API framework for iOS, macOS and tvOS game development that includes an implementation of ECS. Although it is agnostic to the graphics engine used for rendering a game, it includes convenience support for integrating with Apple's SpriteKit, SceneKit and the Xcode Scene Editor.
In October 2018 the company Unity released its famous megacity demo that utilized a tech stack built on an ECS. It had 100,000 audio sources, one for every car, every neon sign, and more – creating a huge, complex soundscape.
Martin's terminology, in use today:
- Entity: The entity is a general-purpose object. Usually, it only consists of a unique id. They "tag every coarse gameobject as a separate item". Implementations typically use a plain integer for this.
- Component: the raw data for one aspect of the object, and how it interacts with the world. "Labels the Entity as possessing this particular aspect". Implementations typically use structs, classes, or associative arrays.
- System: "Each System runs continuously (as though each System had its own private thread) and performs global actions on every Entity that possesses a Component or Components that match that Systems query."
Suppose there is a drawing function. This would be a "System" that iterates through all entities that have both a physical and a visible component, and draws them. The visible component could typically have some information about how an entity should look (e.g. human, monster, sparks flying around, flying arrow), and use the physical component to know where to draw it. Another system could be collision detection. It would iterate through all entities that have a physical component, as it would not care how the entity is drawn. This system would then, for instance, detect arrows that collide with monsters, and generate an event when that happens. It should not need to understand what an arrow is, and what it means when another object is hit by an arrow. Yet another component could be health data, and a system that manages health. Health components would be attached to the human and monster entities, but not to arrow entities. The health management system would subscribe to the event generated from collisions and update health accordingly. This system could also periodically iterate through all entities with the health component, and regenerate health.
An entity only consists of an ID for accessing components. The idea is to have no game code (behavior) inside of the components. The components don't have to be located physically together with the entity, but should be easy to find and access using the entity. It is a common practice to use a unique ID for each entity. This is not a requirement, but it has several advantages:
- The entity can be referred using the ID instead of a pointer. This is more robust, as it would allow for the entity to be destroyed without leaving dangling pointers.
- It helps for saving state externally. When the state is loaded again, there is no need for pointers to be reconstructed.
- Data can be shuffled around in memory as needed.
- Entity ids can be used when communicating over a network to uniquely identify the entity.
Some of these advantages can also be achieved using smart pointers.
The normal way to transmit data between systems is to store the data in components, and then have each system access the component sequentially. For example, the position of an object can be updated regularly. This position is then used by other systems. If there are a lot of different infrequent events, a lot of "flags" will be needed in one or more components. Systems will then have to monitor these "flags" every iteration, which can become inefficient. A solution could be to use the observer pattern. All systems that depend on an event subscribe to it. The action from the event will thus only be executed once, when it happens, and no polling is needed.
The ECS architecture handles dependencies in a very safe and simple way. Since components are simple data buckets, they have no dependencies. Each system will typically query the components an entity must have for the system to operate on it. For example, a render system might register the model, transform, and drawable components. It will then check each entity for those components, and if the entity has them all the system will perform its logic on that entity. If not, the entity is simply skipped, with no need for complex dependency trees. However this can be a place for bugs to hide, since propagating values from one system to another through components may be very hard to debug. ECS may be used where uncoupled data needs to be bound to a given lifetime.
The ECS architecture uses composition, rather than inheritance trees. An entity will be typically made up of an ID and a list of components that are attached to it. Any type of game object can be created by adding the correct components to an entity. This can also allow the developer to easily add features of one type of object to another, without any dependency issues. For example, a player entity could have a "bullet" component added to it, and then it would meet the requirements to be manipulated by some "bulletHandler" system, which could result in that player doing damage to things by running into them.
In the original talk at GDC  Scott Bilas compares C++ object system and his new custom component system. This is consistent with a traditional use of this term in general systems engineering with Common Lisp Object System and type system as examples. Therefore, the ideas of "Systems" as a first-class element is a personal opinion essay. Overall, ECS is a mixed personal reflection of orthogonal well-established ideas in general computer science and programming language theory. For example, components can be seen as a mixin idiom in various programming languages. Alternatively, components are just a small case under the general delegation (object-oriented programming) approach and meta-object protocol. I.e. any complete component object system can be expressed with templates and empathy model within The Orlando Treaty vision of object-oriented programming,
- Martin, Adam. "Entity Systems are the Future of MMOG Development". Archived from the original on 26 December 2013. Retrieved 25 December 2013.
- Martin, Adam. "Entity Systems are the Future of MMOG Development Part 2". Archived from the original on 26 December 2013. Retrieved 25 December 2013.
- "Introducing GameplayKit - WWDC 2015 - Videos". Archived from the original on 2017-10-06. Retrieved 2017-10-06.
- "SanderMertens - Overview". GitHub. Retrieved 2021-09-06.
- "Unity unleashes Megacity demo - millions of objects in a huge cyberpunk world". MCV/DEVELOP. 2018-10-24. Retrieved 2021-06-24.
- "Entity Systems Wiki". Archived from the original on 31 December 2019. Retrieved 31 December 2019.
- Bilas, Scott. "A Data-Driven Game Object System" (PDF). Archived (PDF) from the original on 18 September 2013. Retrieved 25 December 2013.
- Lynn Andrea Stein, Henry Liberman, David Ungar: A shared view of sharing: The Treaty of Orlando. In: Won Kim, Frederick H. Lochovsky (Eds.): Object-Oriented Concepts, Databases, and Applications ACM Press, New York 1989, ch. 3, pp. 31–48 ISBN 0-201-14410-7 (online Archived 2016-10-07 at the Wayback Machine)