request

Caryatid basic request-response example

This example demonstrates a request-response model running on top of the pub-sub message bus. The request is still routed in the same way as any other message, but the response is correlated with it to provide a asynchronous RPC-style model as you might use in REST or gRPC. For simplicity, the messages are just JSON objects.

How to run it

$ cd examples/request
$ cargo run

Points of interest

The Requester uses message_bus.request() to send a Hello, world! message and awaits a response, which it then unpacks and logs:

            match message_bus.request(&topic, test_message.clone()).await {
                Ok(response) => { info!("Got response: {:?}", response); },
                Err(e) => { error!("Got error: {e}"); }
            }

It also sends a request on a bogus topic to demonstrate the timeout if there is no response. To make the test run quicker, this is set in the configuration to 1 second - by default it is 5:

[message-correlator]
timeout = 1

The Responder defines an async handler function which adds a response property to the message and returns it:

    async fn handler(message: Arc<Value>) -> Arc<Value> {
        info!("Handler received {:?}", message);

        let mut message = (*message).clone();

        if let Some(obj) = message.as_object_mut() {
            obj.insert("response".to_string(),
                       Value::String("Loud and clear".to_string()));
        }

        info!("Responding with {:?}", message);
        Arc::new(message)
    }

The handler function is registered on the topic with message_bus.handle():

        context.message_bus.handle(&topic, Self::handler)?;