La mentalidad ECS

Bevy is built on the Entity Component System (ECS) pattern. Instead of deep inheritance hierarchies, you compose behavior from small, independent pieces of data.

This data-oriented design keeps memory layout cache-friendly and lets the engine parallelize work across CPU cores almost for free.

An entity is nothing but a lightweight identifier — a generational index. It owns no data and has no behavior on its own.

Think of it as a key in a database row. Components attached to that key give the entity meaning, like "Player" or "Bullet".

Components are plain Rust structs that derive Component. They carry state but contain no logic.

Keep them small and focused. A position, a velocity, and a health value are three separate components rather than one monolithic struct.

Systems are ordinary functions that operate on components. They are where all your game logic lives.

Bevy inspects each system's parameters to figure out what data it touches, then schedules systems that don't conflict to run in parallel.

A Query declaratively requests entities that have a specific set of components. Bevy returns only matching entities.

You can request shared access with &T or exclusive access with &mut T. This borrow information drives the parallel scheduler.

The second type parameter of a Query is a filter. Use With and Without to narrow results without borrowing that component's data.

This is ideal for tag components — empty structs that mark an entity's role.

Some data is unique to the whole game — a score, the elapsed time, or game settings. These live as resources rather than components.

Access them in systems with Res<T> for read access or ResMut<T> for write access.

Everything is wired together in the App. You add plugins, insert resources, and register systems against a schedule.

DefaultPlugins brings windowing, rendering, input, and time so you can focus on gameplay.

Systems run inside schedules. Startup runs once when the app launches — perfect for spawning the world.

Update runs every frame and is where movement, input, and logic belong. Bevy also offers FixedUpdate for deterministic physics steps.

Want a flying enemy that can be poisoned? Just attach Flying and Poisoned components. No new class, no inheritance chain.

Behavior emerges from which systems match which component combinations. This makes features additive and refactors cheap.

Because each system declares its data access, Bevy's scheduler knows which systems conflict. Non-conflicting systems run on different threads automatically.

You write single-threaded-looking code, and the engine extracts parallelism from your borrow declarations. No locks, no manual threading.

Test your understanding of ECS roles.

Entities are IDs, components are data, systems are logic, and resources are global state. The App ties them together through schedules.

By composing small components and letting systems match them, you get flexible, parallel, cache-friendly games. Next we'll spawn entities and make them move.