Bevy ECS composition

When to use

Load this when gameplay data should be split across small components and systems, instead of one type that owns everything. The Quick Start ECS page is the source for the names below.

ECS shape

• Components are Rust structs that derive Component. The ECS page splits Person and Name so more than one kind of entity can carry a name.
• Entities are spawned with Commands in a Startup system. The page spawns a tuple of components: commands.spawn((Person, Name(...))).
• Systems are functions. Query<&Name, With<Person>> reads names on entities that also have Person. Query<&mut Name, With<Person>> writes. Register with add_systems.
• Schedule: Startup runs once. Update runs afterward. Systems run in parallel unless you call .chain(). The page chains update_people before greet_people and leaves an unrelated system outside the chain.

Use the tuple spawn the ECS page shows. Check the ECS docs for a Bundle trait before writing a bundle macro.

Coming from Godot

Godot idea	Bevy idiom
Has-a parts on one object	Several Component structs on one entity
Behavior script	A system function over a Query
Spawn at start	Commands in a Startup system
Ordered steps	.chain() on the systems that depend on each other
Parent/child scene tree	Check the hierarchy docs before writing a parent/child API.

Scope

• Check the ECS docs for parent-child hierarchy, required-component derives, and bundle helper traits before writing them.
• Check the ECS docs for change detection and system sets before writing them. Order that matters uses .chain() only.
• Editor composition and Control-tree apps stay on the Godot composition-apps skill.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A Godot node script
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No Godot node, scene, or component-script types. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No Bundle API that this file did not open. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No third-party crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

docs.rs links use the unversioned latest permalink. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Migration guides - Unversioned hop index.
• bevy crate - Unversioned API index.
• Quick Start: ECS - Component, Query, With, Commands, Startup, Update, and .chain().
• Apps - App holds the World and the Schedule.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Who owns which piece of state.
• bevy-project-foundations - Crate and App shell.

Complements

• bevy-data-resources - App-wide data that should not be a component.
• bevy-events-messaging - Cross-system notices that are not a shared component.
• godot-composition - Equivalent Godot skill. Do not import Godot types.

Downstream / consumers

• bevy-character-movement - Movement components and systems on the fixed timestep.

Master

• bevy-master - Bevy router.