---
title: Messages and removal records
description: Send messages between systems and observe component removals.
sidebar:
  order: 9
---

## Registering a message

An unmanaged `[Message]` value is registered through an included system parameter, `IncludeMessage<T>`, or `WorldBuilder.AddMessage<T>()`.

## Writers and readers

- `scoped MessageWriter<T>` sends values with `Send(in value)`.
- `scoped MessageReader<T>` enumerates unread retained values and exposes `MissedCount`.

Readers own independent cursors. Writers conflict with readers and with other writers of the same message type. An explicitly ordered reader sees a writer's messages in the same schedule execution.

## Retention

`world.SendMessage(in value)` sends while the world is idle. `world.AdvanceMessages()` rotates retention while idle. The current and previous rotations remain available, so a message expires on the second rotation after it was sent.

:::note
Schedules never rotate buffers automatically. Call `world.AdvanceMessages()` once per outer update.
:::

A reader that falls behind sees skipped records in `MissedCount`.

## Removal records

`scoped RemovedComponents<T>` enumerates `Removal` records with `Entity` and `Tick`. It has the same retention and missed-count behavior as messages. Recording for a component begins when its removal reader is bound.