Document message interceptors

Docs for rabbitmq/rabbitmq-server#13760

Author: ansd

docs/message-interceptors.md

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+---
+title: Intercepting Messages
+displayed_sidebar: docsSidebar
+---
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+
+# Intercepting Messages
+
+## Overview
+
+RabbitMQ provides a generic mechanism to intercept messages on the broker.
+Interception can occur at two stages:
+1. **Incoming messages** – intercepted when a message enters RabbitMQ, just before it is routed to [queues](./queues).
+2. **Outgoing messages** – intercepted when RabbitMQ delivers a message to a client, just before it is [converted](./conversions) to the target protocol.
+
+Interceptors are executed by one of the following [Erlang processes](https://www.erlang.org/doc/system/ref_man_processes.html):
+* AMQP 1.0 session
+* AMQP 0.9.1 [channel](./channels)
+* MQTT connection
+
+Messages sent over the [RabbitMQ Streams protocol](https://github.com/rabbitmq/rabbitmq-server/blob/main/deps/rabbitmq_stream/docs/PROTOCOL.adoc) are not intercepted.
+
+A message interceptor is an [Erlang module](https://www.erlang.org/doc/system/modules.html) that implements the [rabbit_msg_interceptor](https://github.com/rabbitmq/rabbitmq-server/blob/main/deps/rabbit/src/rabbit_msg_interceptor.erl) behaviour.
+What the interceptor does is entirely up to its implementation - it can validate message metadata, add annotations, or perform arbitrary side effects.
+
+Custom interceptors can be developed and integrated via [plugins](./plugins).
+
+RabbitMQ ships with several built-in message interceptors.
+Below are examples of how to configure them using the [rabbitmq.conf](./configure#config-file) file.
+
+## Incoming Message Interceptors
+
+### Timestamp
+
+This interceptor adds a timestamp to each incoming message:
+
+```ini
+message_interceptors.incoming.set_header_timestamp.overwrite = true
+```
+
+* AMQP 1.0 and Streams clients receive a message annotation: `x-opt-rabbitmq-received-time` (timestamp in milliseconds since January 1, 1970 UTC).
+* AMQP 0.9.1 clients receive:
+    * `timestamp_in_ms` header (milliseconds) for compatibility with the former [Message Timestamp Plugin](https://github.com/rabbitmq/rabbitmq-message-timestamp)
+    * `timestamp` property (seconds)
+
+To preserve an existing `timestamp_in_ms` header, set `overwrite` to `false`:
+```ini
+message_interceptors.incoming.set_header_timestamp.overwrite = false
+```
+
+### Routing Node
+
+This interceptor adds a message annotation `x-routed-by` indicating which RabbitMQ [node](./clustering#node-names) received and routed the message:
+```ini
+message_interceptors.incoming.set_header_routing_node.overwrite = true
+```
+
+Set `overwrite` to `false` to preserve an existing value:
+```ini
+message_interceptors.incoming.set_header_routing_node.overwrite = false
+```
+
+### MQTT client ID
+
+If the [MQTT plugin](./mqtt) is enabled, RabbitMQ can annotate incoming messages with the [client ID](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901059) of the publishing MQTT client.
+This is done by adding a message annotation with the key `x-opt-mqtt-client-id`.
+
+```init
+mqtt.message_interceptors.incoming.set_client_id_annotation.enabled = true
+```
+
+This annotation is visible to AMQP 1.0, AMQP 0.9.1, and Streams consumers.
+However, MQTT clients will not receive this annotation, as the MQTT spec does not allow arbitrary broker-added annotations.
+
+## Outgoing Message Interceptors
+
+### Timestamp
+
+This interceptor timestamps messages when they are sent to clients:
+
+```ini
+message_interceptors.outgoing.timestamp.enabled = true
+```
+
+The annotation key is `x-opt-rabbitmq-sent-time`, and its value is a timestamp in milliseconds since January 1, 1970 UTC.

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