---
title: Systems and traversal
description: Write systems with EcsSystem and Each, and opt traversals into partitioning.
sidebar:
  order: 5
---

## `[EcsSystem]`

`[EcsSystem]` marks a synchronous static `void` method. Supported parameters are scoped [named queries](/guides/queries) and the capabilities listed in [Resources and local state](/guides/resources), [Commands](/guides/commands), and [Messages](/guides/messages).

Private methods are callable through generated invokers inside their partial owner. Overloads, generic methods, async methods, iterators, and ref returns are rejected (diagnostic [`AV005`](/reference/diagnostics)).

## `[Each]`

```csharp
public static partial class MotionKernel
{
    [Each(Partitioned = true), Without<Frozen>]
    public static void Move(scoped ref Position position, scoped in Velocity velocity)
    {
        position = position with { X = position.X + velocity.X };
    }
}
```

`[Each]` calls the method once per matching entity.

- Use `scoped ref T` for writes and `scoped in T` for reads.
- An optional `Entity` parameter supplies identity.
- Membership and change-filter attributes apply to the method.
- Components passed by value are rejected.

## Partitioning

`[Each]` executes one traversal by default. `[Each(Partitioned = true)]` opts into splitting matched chunks across the world's execution budget, which matters once [parallel execution](/guides/execution) is enabled.

Partitioned methods accept component refs, `Entity`, and readonly unmanaged resources, including optional resources without owner-thread affinity. Locals, commands, messages, managed resources, mutable resources, and named queries are excluded. An unsafe partition request is reported as [`AV010`](/reference/diagnostics).

## Descriptors

The generator exposes each method as `Owner.MethodName_Descriptor`. Runtime composition accepts these descriptors through `BoundSchedule.Bind`. See [Schedules](/guides/schedules#runtime-composition).