ECS features in detail
Note: The main content of this page has migrated to the Unity Data-Oriented reference. ECS related features are listed below in alphabetical order, with a short description and links to further information about it. This page is not an exhaustive list and can be added to over time as ECS, and its related documentation expands. If you spot something that is out-of-date or broken links, then make sure to let us know in the forums or as an issue in the repository.
EntityCommandBufferSystem
When using jobs, you must request command buffers from an entity command buffer system on the main thread, and pass them to the jobs. When the EntityCommandBufferSystem updates, the command buffers playback on the main thread in the order they were created. This extra step is required so that memory management can be centralized and determinism of the generated entities and components can be guaranteed.
For more information, see the EntityCommandBuffer reference page.
Chunk
A Chunk contains the ComponentData for each entity. All entities in one Chunk follow the same memory layout. When iterating over components, memory access of components within a Chunk is always completely linear, with no waste loaded into cache lines. This is a hard guarantee.
For more information, see the Chunk reference page.
ComponentDataFromEntity
If you need to access ComponentData on another entity, the only stable way of referencing that component data is via the entity ID. EntityManager provides a simple get & set ComponentData API for it. However, you can't use the EntityManager in a C# job. ComponentDataFromEntity gives you a simple API that you can also safely use in a job.
For more information, see the ComponentDataFromEntity reference page.
EntityQuery
The EntityQuery is the foundation class on top of which all iteration methods are built foreach, IJobForEach, etc.). Essentially a EntityQuery is constructed with a set of required components and or subtractive components. EntityQuery lets you extract individual arrays of entities based on their components.
For more information, see the EntityQuery reference page.
Entity
An entity is an ID. You can think of it as a super lightweight GameObject that does not even have a name by default.
You can add and remove components from entities at runtime. entity ID's are stable. They are the only stable way to store a reference to another component or entity.
For more information, see the Entity reference page.
EntityArchetype
An EntityArchetype is a unique array of ComponentType structs. The EntityManager uses EntityArchetypestructs to group all entities using the same ComponentType structs into Chunks.
For more information, see the EntityArchetype reference page.
EntityCommandBuffer
The EntityCommandBuffer abstraction allows you to queue up changes (from either a job or from the main thread) so that they can take effect later on the main thread.
For more information, see the EntityCommandBuffer reference page.
EntityManager
EntityManager is where you find APIs to create entities, check if an entity is still alive, instantiate entities and add or remove components.
For more information, see the EntityManager reference page.
ExclusiveEntityTransaction
ExclusiveEntityTransaction is an API to create & destroy entities from a job. The purpose is to enable procedural generation scenarios where instantiation on a big scale must happen in jobs. As the name implies, it is exclusive to any other access to the EntityManager.
For more information, see the ExclusiveEntityTransaction reference page.
IComponentData
IComponentData is a pure ECS-style component, meaning that it defines no behavior, only data. IComponentData is a struct rather than a class, meaning that it is copied by value instead of by reference by default.
For more information, see the ComponentData reference page - see "IComponentData."
JobComponentSystem (Automatic job dependency management)
Managing dependencies is hard, which is why the JobComponentSystem does it automatically for you. The rules are simple: jobs from different systems can read from IComponentData of the same type in parallel. If one of the jobs is writing to the data, then they can't run in parallel and will be scheduled with a dependency between the jobs.
For more information, see the JobComponentSystem reference page.
Shared ComponentData
ISharedComponentData is useful when many entities have something in common, for example in the Boid demo we instantiate many entities from the same Prefab, and thus the RenderMesh between many Boid entities is the same.
For more information, see the SharedComponentData reference page.
SystemStateComponentData
The purpose of SystemStateComponentData is to allow you to track resources internal to a system and have the opportunity to appropriately create and destroy those resources as needed without relying on individual callbacks.
For more information, see the SystemStateComponent reference page.
System update order
In ECS all systems are updated on the main thread. Systems update based on a set of constraints and an optimization pass, which tries to order the systems in a way so that the time between scheduling a job and waiting for it is as long as possible.
For more information, see the System update order reference page.
World
A World owns both an EntityManager and a set of ComponentSystems. You can create as many World objects as you like. Commonly you would create a simulation World and rendering or presentation World.
For more information, see the World reference page.