Cettia Java Server 1.1.0 released

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We are happy to announce that the 1.1.0 release of Cettia Java Server is now generally available. In this release, various useful features have been added which make a real-time web application development much easier. Here are the key highlights of Cettia Java Server 1.1.

  • A predicate to match sockets.
    • Server find(ServerSocketPredicate predicate, SerializableAction<ServerSocket> action) in Server
    • Sentence find(ServerSocketPredicate predicate) in Server
  • A sentence accepts a socket action.
    • Sentence execute(SerializableAction<ServerSocket> action) in Sentence
  • A socket’s attributes.
    • <T> T get(String name) in ServerSocket
    • ServerSocket set(String name, Object value) in ServerSocket
    • ServerSocket remove(String name) in ServerSocket
  • A socket identifier.
    • String id() in ServerSocket

Before walking through each feature in a summary of the starter kit, I would like to thank Ralph who has created and maintained cettia-demo for sharing his ideas that have driven the development of 1.1. As always, Cettia is an open source projects for the community by the community. Feel free to join the community and share your thoughts.

The following is a summary of the Cettia starter kit to help you get started quickly. In the summary, comments starting with ## refer to a title of a related chapter in the tutorial, Building Real-Time Web Applications With Cettia, where you can find a detailed explanation. You may want to highlight the ##.

Maven dependencies.

<!-- ## Setting Up the Project -->
<!-- To write a Cettia application -->
<!-- To run a Cettia application on Servlet 3 and Java WebSocket API 1 -->
<!-- Besides them, you can also use Spring WebFlux, Spring MVC, Grizzly, Vert.x, Netty, and so on -->

A class to play with the Cettia server. Import statements, verbose try-catch blocks, empty methods, etc. are skipped for brevity.

public class CettiaConfigListener implements ServletContextListener {
  public void contextInitialized(ServletContextEvent event) {
    // Cettia part
    // If you don't want to form a cluster,
    // replace the following line with `Server server = new DefaultServer();`
    ClusteredServer server = new ClusteredServer();
    HttpTransportServer httpAction = new HttpTransportServer().ontransport(server);
    WebSocketTransportServer wsAction = new WebSocketTransportServer().ontransport(server);

    // If a client opens a socket, the server creates and passes a ServerSocket to socket handlers
    server.onsocket((ServerSocket socket) -> {
      // ## Socket Lifecycle
      Action<Void> logState = v -> System.out.println(socket + " " + socket.state());

      // ## Sending and Receiving Events
      // An `echo` event handler where any received echo event is sent back
      socket.on("echo", data -> socket.send("echo", data));

      // ## Attributes and Tags
      // Attributes and tags are contexts to store the socket state in the form of Map and Set
      String username = findParam(socket.uri(), "username");
      if (username == null) {
        // Attaches a tag to the socket
      } else {
        // Associates an attribute with the the socket
        socket.set("username", username);

      // ## Working with Sockets
      // A `chat` event handler to send a given chat event to every socket in every server in the cluster
      socket.on("chat", data -> server.all().send("chat", data));

      // ## Finder Methods and Sentence
      if (username != null) {
        // A myself event handler to send a given myself event to sockets whose username is the same
        socket.on("myself", data -> {
          // Find sockets by the attribute and deal with them directly
          server.find(s -> username.equals(s.get("username"))).execute(s -> s.send("myself"));

        // How to allow only one socket per username
        boolean onlyOneSocket = Boolean.parseBoolean(findParam(socket.uri(), "onlyOneSocket"));
        if (onlyOneSocket) {
          // Finds sockets whose username is the same except this socket
          String me = socket.id();
          server.find(s -> username.equals(s.get("username")) && !me.equals(s.id()))
          // Sends a `signout` event to prevent reconnection and closes a connection
          // As of 1.2, it can be written more concisely
          // `server.byAttr("username", username).exclude(socket).send("signout").close();`

      // ## Recovering Missed Events
      Queue<Object[]> queue = new ConcurrentLinkedQueue<>();
      // Caches events that fail to send due to disconnection
      socket.oncache(args -> queue.offer(args));
      // Sends cached events on the next connection
      socket.onopen(v -> {
        while (socket.state() == ServerSocket.State.OPENED && !queue.isEmpty()) {
          Object[] args = queue.poll();
          socket.send((String) args[0], args[1], (Action<?>) args[2], (Action<?>) args[3]);
      // If the client fails to connect within 1 minute after disconnection,
      // You may want to consider notifying the user of finally missed events, like push notifications
      socket.ondelete(v -> queue.forEach(args -> {
        System.out.println(socket + " missed event - name: " + args[0] + ", data: " + args[1]);

    // ## Working with Sockets
    // To deal with sockets, inject the server wherever you want
    ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
    // Sends a welcome event to sockets representing user not signed in every 5 seconds
    executor.scheduleAtFixedRate(() -> server.byTag("nonmember").send("welcome"), 0, 5, SECONDS);

    // ## Plugging Into the Web Framework
    // Cettia is designed to run on any web framework seamlessly on the JVM
    // Note how `httpAction` and `wsAction` are plugged into Servlet and Java API for Websocket
    ServletContext context = event.getServletContext();
    AsityServlet asityServlet = new AsityServlet().onhttp(/* ㅇㅅㅇ */ httpAction);
    ServletRegistration.Dynamic reg = context.addServlet(AsityServlet.class.getName(), asityServlet);

    ServerContainer container = (ServerContainer) context.getAttribute(ServerContainer.class.getName());
    ServerEndpointConfig.Configurator configurator = new ServerEndpointConfig.Configurator() {
      public <T> T getEndpointInstance(Class<T> endpointClass) {
        AsityServerEndpoint asityServerEndpoint = new AsityServerEndpoint();
        asityServerEndpoint.onwebsocket(/* ㅇㅅㅇ */ wsAction);
        return endpointClass.cast(asityServerEndpoint);
    container.addEndpoint(ServerEndpointConfig.Builder.create(AsityServerEndpoint.class, "/cettia")

    // ## Scaling a Cettia Application
    // Any publish-subscribe messaging system can be used to scale a Cettia application horizontally,
    // and it doesn’t require any modification in the existing application.
    HazelcastInstance hazelcast = HazelcastInstanceFactory.newHazelcastInstance(new Config());
    ITopic<Map<String, Object>> topic = hazelcast.getTopic("cettia");
    // It publishes messages given by the server
    server.onpublish(message -> topic.publish(message));
    // It relays published messages to the server
    topic.addMessageListener(message -> server.messageAction().on(message.getMessageObject()));

Here’s an example with the Spring WebFlux 5 to show Cettia’s framework-agnostic nature. Consult Asity’s Run Anywhere section for how to plug a Cettia application into other various frameworks.

public class CettiaServer {
  public RouterFunction<ServerResponse> httpMapping(HttpTransportServer httpAction) {
    AsityHandlerFunction asityHandlerFunction = new AsityHandlerFunction();
    asityHandlerFunction.onhttp(/* ㅇㅅㅇ */ httpAction);

    RequestPredicate isNotWebSocket = headers(h -> !"websocket".equalsIgnoreCase(h.asHttpHeaders().getUpgrade()));
    return route(path("/cettia").and(isNotWebSocket), asityHandlerFunction);

  public HandlerMapping wsMapping(WebSocketTransportServer wsAction) {
    AsityWebSocketHandler asityWebSocketHandler = new AsityWebSocketHandler();
    asityWebSocketHandler.onwebsocket(/* ㅇㅅㅇ */ wsAction);
    Map<String, WebSocketHandler> map = new LinkedHashMap<>();
    map.put("/cettia", asityWebSocketHandler);

    SimpleUrlHandlerMapping mapping = new SimpleUrlHandlerMapping();

    return mapping;

You need minimal HTML to load the cettia object. Also, if you have a cettia-client npm module installed, you should be able to load the cettia object with require("cettia-client/cettia-bundler"); and require("cettia-client"); in Webpack and Node, respectively.

 <!DOCTYPE html>
 <script src="https://unpkg.com/cettia-client@1.0.1/cettia-browser.min.js"></script>

Below is the JavaScript code to play with the cettia object. Open the above page and its developer console in several browsers, run the script, and watch results on the fly.

// ## Opening a Socket
// Manipulates the below params object to play with the server implementation
var params = {
  username: "flowersinthesand",
  onlyOneSocket: true
// Let's assume that each key and value are already encoding safe
var query = Object.keys(params).filter(k => params[k]).map(k => `${k}=${params[k]}`).join("&");
var socket = cettia.open("/cettia?" + query);

// ## Socket Lifecycle
var logState = () => console.log(socket.state());
socket.on("connecting", logState).on("open", logState).on("close", logState);
socket.on("waiting", (delay, attempts) => console.log(socket.state(), delay, attempts));

// ## Sending and Receiving Events
["echo", "chat", "myself", "welcome"].forEach(event => {
  socket.on(event, data => console.log(event, data));
socket.on("signout", () => {
  console.log("signout", "You've been signed out since you signed in on another device");
  // It prevents reconnection

// This open event handler registered through `once` is called at maximum once
socket.once("open", () => {
  // Sends an echo event to be returned
  socket.send("echo", "Hello world");
  // Sends a chat event to be broadcast to every sockets in the server
  socket.send("chat", {text: "I'm a text"});
  // Sends an event to sockets whose username is the same
  // with composite data consisting of text data and binary data
  socket.send("myself", {text: "I'm a text", binary: new TextEncoder().encode("I'm a binary")});

The full source code for the starter kit is available at the repository, https://github.com/cettia/cettia-starter-kit. If you want to dig deeper, we strongly recommend to read an introductory tutorial to Cettia, Building Real-Time Web Applications With Cettia. It explains the reason behind key design decisions that the Cettia team have made in the Cettia, as well as various patterns and features required to build real-time oriented applications without compromise with Cettia.

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