Getting Started With RSocket: Servers Calling Clients

Engineering | Ben Wilcock | May 12, 2020 | ...

Reading Time: about 7 minutes. Coding Time: about 20 minutes.

If you've been following my series on RSocket, you've heard me refer to "clients and servers" many times. But, with RSocket, the line between client and server is blurry. With Rsocket, servers can send messages to clients, and clients can respond to these requests in the same way a server would.

In fact, the RSocket docs don't use the terms 'client' or 'server.' The docs use the terms 'requester' and 'responder' instead. In RSocket, any component can act as a requester, and any component can act as a responder or even both at the same time. In RSocket, all this back-and-forth communication between requesters and responders takes place over a single 'bi-directional' connection.

Today, you're going to take advantage of these features by programming your rsocket-client to respond to requests coming from the server. In the server-side code, you'll listen out for client connection events, and when a connection event happens, you'll store the new client in a list of connected clients. You'll also ask every client to stream telemetry messages back to the server for as long as their connection is alive.

If you've been following the series, you can code along by following the directions below. The code is also available to download from GitHub.

Step 1: Update Spring Boot and RSocket

First, some housekeeping. Spring Boot and the RSocket Java library were both updated recently. These updates include the usual round of bug fixes, enhancements, and deprecations, so it's in our interest to upgrade.

Working in the <parent> section of the Maven pom.xml, change the spring-boot-starter-parent to version 2.3.0.RELEASE as shown below. You'll need to do this twice, as you have two separate projects — rsocket-client and rsocket-server.

  <relativePath/> <!-- lookup parent from repository -->

To refresh your code, run the following command in the root folder of both projects:

./mvnw clean compile

Now you can move on to the coding task.

Step 2: Add A Request-Response Message Handler To The Client

The client needs to be able to respond to messages coming from the server. In the rsocket-client project's, add a new inner class called ClientHandler as follows:

class ClientHandler {
 public Flux<String> statusUpdate(String status) {"Connection {}", status);
   return Flux.interval(Duration.ofSeconds(5)).map(index -> String.valueOf(Runtime.getRuntime().freeMemory()));

This class contains a single method called statusUpdate() decorated with a @MessageMapping annotation like the ones in the rsocket-server project. The client uses this class and this method to capture and respond to requests coming in from the server. The response itself is a stream of messages. Every 5 seconds, the client tells the server its current free memory. Think of this as client telemetry data.

For this to work, you must 'register' this class with your RSocket connection. You'll do this in the next section.

Step 3: Register The ClientHandler In The Client’s Constructor

Before the client can respond to messages coming from the server, it must register the ClientHandler with the RSocket connection. The revised constructor code is listed below. Notice the change to the constructor's method signature to add the RSocketStrategies variable. Spring supplies this variable to your constructor. Replace your old constructor code, with the new code listed below.

public RSocketShellClient(RSocketRequester.Builder rsocketRequesterBuilder, RSocketStrategies strategies) {
 // (1)
 String client = UUID.randomUUID().toString();"Connecting using client ID: {}", client);
 // (2)
 SocketAcceptor responder = RSocketMessageHandler.responder(strategies, new ClientHandler());
 // (3) 
 this.rsocketRequester = rsocketRequesterBuilder
 .rsocketConnector(connector -> connector.acceptor(responder))
 .connectTcp("localhost", 7000)
 // (4)
 .doOnError(error -> log.warn("Connection CLOSED"))
 .doFinally(consumer ->"Client DISCONNECTED"))

In the code above, you first generate and store a unique ID that identifies this client instance (1). Next, you create a new SocketAcceptor using the RSocket strategies plus a new ClientHandler instance (2). Then use the RSocketRequesterBuilder to register the new SocketAcceptor (3). And finally, make sure that disconnection is handled gracefully by handling the RSocket onClose() events (4).

That's it for the client-side code. Let's move on to the server-side changes.

Step 4: Remember Clients On The Server

The first thing to do in the rsocket-server project is to create a collection of RSocketRequester clients by adding a class-level variable to the class as follows:

private final List<RSocketRequester> CLIENTS = new ArrayList<>();

Next, add a connection handler by adding a new method like this:

 void connectShellClientAndAskForTelemetry
(RSocketRequester requester, @Payload String client) {
 // The code for the method will go HERE

The @ConnectMapping annotation lets you listen to client connection events as they happen. Using this event, you can schedule two pieces of work. The first is to add each new client to the CLIENTS list. The second is to call out to each client and start their telemetry streams.

Add the following code to the method you just created:

        .onClose() // (1)
        .doFirst(() -> { 
  "Client: {} CONNECTED.", client);
            CLIENTS.add(requester); // (2)
        .doOnError(error -> { 
            log.warn("Channel to client {} CLOSED", client); // (3)
        .doFinally(consumer -> { 
  "Client {} DISCONNECTED", client); // (4)

The first thing to notice is the call to the requester.rsocket().onClose() method (1). This method returns a reactive Mono object, which contains all the callbacks you need.

The mono's doFirst() method gets called before any calls to subscribe(), but after the initial creation of the mono. Use this method to add the client's requester object to the CLIENTS list (2).

This code might feel counter-intuitive — calling onClose() while a client's connecting and then using the resulting mono to store a reference to the new client. Sometimes, event-driven API's can feel a bit odd. But think of it as the mono for this RSocket connection sending you an "I'm alive" event. You're using that creation event to trigger the storage of each client's reference in the list.

RSocket calls the mono's doOnError() method whenever there is a problem with the connection. This includes situations where the client has chosen to close the connection. You can use the error variable provided to decide what action to take. In the code above, the error simply gets logged as a warning (3).

The mono's doFinally() method is triggered when the RSocket connection has closed. This method is the ideal place to run the code that removes the client from the list CLIENTS (4).

Finally, subscribe() activates the reactive code you've added to the mono and signals that you're ready to process the events.

Step 5: Ask For Free Memory Readings From Clients

As each client connects, request a stream of telemetry readings. To do this, working once more in the connectShellClientAndAskForTelemetry() method, you need to send a request to the client-status message handler you added earlier. The code for this is as follows:

        .doOnNext(s ->"Client: {} Free Memory: {}.",client,s))

Using the requester, target the route "client-status". Send the string "OPEN" as the message data, and retrieve a Flux of type String. Each string that arrives contains the client's current free memory reading. Log this reading to the console. Finally, don't forget to subscribe() to activate your reactive code.

Step 6: Build And Run The RSocket Server

It's time to test your code. Open a terminal window and move to the rsocket-server directory and run the server as follows:

cd rsocket-server
./mvnw clean package spring-boot:run -DskipTests=true

The server starts up on localhost port 7000.

Step 7: Build And Run The RSocket Client

Open a second terminal window and move to the rsocket-client directory. From there, build and run the client as follows:

cd rsocket-client
./mvnw clean package spring-boot:run -DskipTests=true

How it Works

Once booted, you'll notice that both the client and the server components have new log entries in the console. In the rsocket-client window, you'll see log entries showing the unique ID of the client — in the form of a UUID — and an "OPEN" connection status as follows:

Connecting using client ID: 0acc1c60-4bc4-444d-bb82-eb6b510f4168
Connection OPEN
Started RsocketShellClientApplication in 1.516 seconds (JVM running for 1.814)

Wait at least 10 seconds and then type exit at the shell:> prompt. The rsocket-client now disconnects from the server and shuts down.

Now switch to the rsocket-server window. The log entries look something like this:

Started RsocketServerApplication in 0.945 seconds (JVM running for 1.248)
Client: 0acc1c60-4bc4-444d-bb82-eb6b510f4168 CONNECTED.
Client: 0acc1c60-4bc4-444d-bb82-eb6b510f4168 Free Memory: 211317712.
Channel to client 0acc1c60-4bc4-444d-bb82-eb6b510f4168 CLOSED
Client 0acc1c60-4bc4-444d-bb82-eb6b510f4168 DISCONNECTED

As the client connects, it's added to the list of clients, and the console prints its client ID and the status "CONNECTED". Then, every 5 seconds, the log shows the client's current 'free memory' reading. Finally, as the client disconnects, it's RSocket channel's status becomes "CLOSED" and the client is "DISCONNECTED."

You can stop the rsocket-server process by pressing Ctrl-C in its terminal window.

Final Thoughts

The ability to call out to clients is very powerful. It’s ideal for all kinds of scenarios, including mobile, internet-of-things, or microservices. And because all this can happen over TCP or WebSockets, you already have all the infrastructure you need without resorting to heavyweight solutions like message brokers.

You covered a lot of ground here. You learned how to turn servers into 'requesters' and clients into 'responders.' You discovered how to listen into connection events. You also learned a little bit about how to handle errors and events coming from rsocket connections. And, although in this exercise, you chose 'request-stream' as your server-client communication, you could use any of the four RSocket interaction modes, depending on your needs.

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