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Replies

A reply named per request has no fixed channel, so the document points its address at the reply-to header of the request.

A responder answers on the response topic its request carries. The crate maps that property onto the reply-to header, and a publish transform with Destination = Names reads it there. The document is built before any request arrives, so it cannot name the topic. The operation's reply.address.location is $message.header#/reply-to instead: a client reads the address of its answer from the header of its own request. The mount-site name, reply("devices/dev42/pong"), is the reply channel the document reports, and where a request without a response topic is answered. A reply that the type or the mount site names reports that channel alone.

Example​

The operation receive_devices_dev42_ping reports "location": "$message.header#/reply-to" in its reply address.

examples/mqtt_asyncapi.rs
/// Answers on the topic the request names in its response topic, the `reply-to` header.
struct ToResponseTopic;

impl<C, Options> PublishTransform<ForReply<C>, Options> for ToResponseTopic {
    type Destination = Names;

    fn apply(
        &self,
        out: &mut Outgoing<'_>,
        _options: &mut Option<Options>,
        cx: &PublishContext<'_, C>,
    ) {
        if let Some(topic) = cx.headers().get_str("reply-to") {
            out.set_name(topic.to_owned());
        }
    }
}

/// A request without a response topic is answered on `devices/dev42/pong`.
#[subscriber(MqttTopic::new("devices/dev42/ping"), reply("devices/dev42/pong"))]
async fn answer(ping: &Telemetry) -> Pong {
    Pong {
        device: ping.device.clone(),
    }
}