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How to send unread messages notifications with Server Sent Events in Spring Boot?

I'm new to Spring Boot and web applications. I have to send notifications of unhandled/unread messages from a Spring Boot backend to a web client. I decided to use Server Sent Events since I think I don't need a bidirectional connection (otherwise I'd have thought of WebSockets).
I made a very simple REST controller which finds all unhandled messages in a db and sends them to the client. The problem is that it keeps sending forever all the messages, while I'd like to send a message only when it is added to the db, or when the client first connects to the server.
The behaviour I'd like to achieve is similar to a mail client or a messaging app, in which the user is notifyed not only on new messages but also of previous ones if he/she didn't mark them as read. The notification should happen only once when the client connects, not loop forever.
Here is my code:
#RestController
#CrossOrigin(origins = "*")
public class SseEmitterController {
private MessageDAO messageDAO;
private ExecutorService nonBlockingService = Executors
.newCachedThreadPool();
#Autowired
public SseEmitterController(MessageDAO messageDAO) {
this.messageDAO = messageDAO;
}
#GetMapping("/incoming_messages")
public SseEmitter handleSse() {
SseEmitter emitter = new SseEmitter();
nonBlockingService.execute(() -> {
try {
List<Message> messages = messageDAO.findByHandledFalse();
for (Message message: messages) {
emitter.send(message, MediaType.APPLICATION_JSON);
}
emitter.complete();
} catch (Exception ex) {
emitter.completeWithError(ex);
}
});
return emitter;
}
}
I know that the problem is caused by the fact that I query the db inside handleSse method, but I couldn't figure out how to do it outside.
Could you please help me?
Update October 05, 2021
I found out how to solve the problem, I didn't update the question because I didn't have the time, but since someone asked me to do so in the comments, I'm gonna explain my solution, hoping it may be helpful.
Here's my code:
The SseEmitterController is responsible for invoking the SseService on frontend's request:
#RestController
#CrossOrigin(origins = "*")
public class SseEmitterController {
private final SseService sseService;
#Autowired
SseEmitterController(SseService sseService) {
this.sseService = sseService;
}
#GetMapping("/incoming_messages")
public ResponseEntity<SseEmitter> handleSse() {
final SseEmitter emitter = new SseEmitter();
sseService.addEmitter(emitter);
emitter.onCompletion(() -> sseService.removeEmitter(emitter));
emitter.onTimeout(() -> sseService.removeEmitter(emitter));
return new ResponseEntity<>(emitter, HttpStatus.OK);
}
}
The SseService is called on a new message arrival (from another part of the application) and sends the notification (actually a server sent event) to the frontend (which previously called the endpoint in the controller above.
The service is called like so: sseService.sendHelpRequestNotification(helpRequest);
#Service
public class SseService {
private final List<SseEmitter> emitters = new CopyOnWriteArrayList<>();
public void addEmitter(final SseEmitter emitter) {
emitters.add(emitter);
}
public void removeEmitter(final SseEmitter emitter) {
emitters.remove(emitter);
}
public void sendMessagesNotification(Message message) {
List<SseEmitter> sseEmitterListToRemove = new ArrayList<>();
emitters.forEach((SseEmitter emitter) -> {
try {
emitter.send(message, MediaType.APPLICATION_JSON);
} catch (Exception e) {
sseEmitterListToRemove.add(emitter);
}
});
emitters.removeAll(sseEmitterListToRemove);
}
}
And finally there is another controller to get all previous unhandled messages (not involving server sent events):
#GetMapping(value = "/unhandled_help_requests")
public ResponseEntity<List<HelpRequest>> getUnhandledMessages() {
List<Message> resultSet = messageDAO.findByHandledFalse(Sort.by("date").and(Sort.by("time")));
return new ResponseEntity<>(resultSet, HttpStatus.OK);
}
So, to sum it up: the frontend calls the SseEmitterController to listen for new SSEs. These SSEs are created and sent whenever a new message arrives to the backend, via the SseService. Finally, to get all unhandled (for whatever reason) messages, there is a specific old fashioned controller.

How to write a http proxy using netty

I want to write a simple program using netty to proxy http request send by browser.
I think it can be divided into 3 steps
get request send by browser
send it to the website
receive data from website and send it back to the browser.
Question:
How to translate url into host and port when I using Bootstrap.connect(host, port);
When I using HttpServerResponseHandler.connect and ChannelHandlerContext.writeAndFlush(httpMessage); to send data to website, how can I get the response data from the website and send it back to the browser?
It's my first day studying netty, so please try to answer as easy as possible. Thank you very much.
public class Server {
public static void main(String[] args) throws InterruptedException {
final int port = 8888;
// copy from https://github.com/netty/netty/wiki/User-guide-for-4.x
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.childHandler(new ChannelInitializer<SocketChannel>() {
#Override
public void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new HttpRequestDecoder(), new HttpServerRequestHandler());
}
})
.option(ChannelOption.SO_BACKLOG, 128)
.childOption(ChannelOption.SO_KEEPALIVE, true);
// Bind and start to accept incoming connections.
ChannelFuture f = b.bind(port).sync();
// Wait until the server socket is closed.
// In this example, this does not happen, but you can do that to gracefully
// shut down your server.
f.channel().closeFuture().sync();
} finally {
workerGroup.shutdownGracefully();
bossGroup.shutdownGracefully();
}
}
}
public class HttpServerRequestHandler extends ChannelInboundHandlerAdapter {
#Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
// step 1 get data from browser
if (msg instanceof LastHttpContent) {
ctx.close();
return;
}
DefaultHttpRequest httpMessage = (DefaultHttpRequest) msg;
System.out.println("浏览器请求====================");
System.out.println(msg);
System.out.println();
doWork(ctx, httpMessage);
}
#Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
cause.printStackTrace();
ctx.close();
}
private void doWork(ChannelHandlerContext ctx, final DefaultHttpRequest msg) {
// step 2 send data to website
// translate url into host and port
String host = msg.uri();
int port = 80;
if (host.startsWith("https://")) {
host = host.replaceFirst("https://", "");
port = 443;
} else if (host.startsWith("http://")) {
host = host.replaceFirst("http://", "");
port = 80;
}
if (host.contains(":443")) {
host = host.replace(":443", "");
port = 443;
}
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
Bootstrap b = new Bootstrap();
b.group(workerGroup);
b.channel(NioSocketChannel.class);
//b.option(ChannelOption.AUTO_READ, true);
b.handler(new ChannelInitializer<SocketChannel>() {
#Override
public void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new HttpServerResponseHandler(msg), new HttpRequestEncoder());
}
});
// question 1
ChannelFuture f = b.connect(host, port).sync();
//ChannelFuture f = b.connect("www.baidu.com", 443).sync();
f.channel().closeFuture().sync();
} catch (Exception e) {
e.printStackTrace();
} finally {
workerGroup.shutdownGracefully();
}
}
}
public class HttpServerResponseHandler extends ChannelOutboundHandlerAdapter {
private Object httpMessage;
public HttpServerResponseHandler(Object o) {
this.httpMessage = o;
}
#Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {
System.out.println("网页请求结果=========================");
System.out.println(httpMessage);
System.out.println();
}
#Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { // (4)
cause.printStackTrace();
ctx.close();
}
#Override
public void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress,
SocketAddress localAddress, ChannelPromise promise) throws Exception {
System.out.println("connect !!!!!!!!!!!");
// question 2
ctx.writeAndFlush(httpMessage);
}
}

Coincidentally, I've also been working on a Netty proxy server for the purposes of learning. I've a fully working code that you can find on my GitHub, but I'll answer your questions here. Netty also has an official proxy server example here but unlike my code, they don't have unit tests.
(FYI, My code is in Kotlin).
Core Idea:
When creating a proxy server, you need a server to accept client requests, as well as a client for the remote than you are proxying. You created the server, but not the client. It is best to reuse the EventLoop created by the server rather than creating a new one for the client. Each event loop runs on a dedicated thread, so creating more would produce additional threads, necessitating context switching when exchanging data between the accepted Channel and the client Channel.
How to translate url into host and port
To keep things simple, I've used a HttpObjectAggregator that aggregates an HttpMessage and its following HttpContents into a single FullHttpRequest or FullHttpResponse (depending on if it used to handle requests or responses). Setting the URL is trivial: just call FullHttpRequest.setUri.
To get the host and port, call Channel.remoteAddress() on the client channel and cast the resulting SocketAddress to an InetSocketAddress, from which you can get the host and port. Don't forget to similarly reset the Host header if present.
how can I get the response data
After establishing a client channel (that you're missing), you need to make a request on that channel. The client channel has a handler with a reference to the original server channel. Once the handler receives a response, it writes it to the server channel.

Vert.x how to pass/get messages from REST to message bus?

I want to pass messages to bus via REST, and get it back. But I cant correctly setup the message bus receiver, it throws java.lang.IllegalStateException: Response has already been written. In real life message bus should receive messages from different sources and pass a message to another target. Therefore we just need to publish the message to the bus. But how to correctly read messages and handle all of them? For example from a REST interface: read that messages!
My simple app start:
public static void main(String[] args) {
Vertx vertx = Vertx.vertx();
vertx.deployVerticle(new RESTVerticle());
vertx.deployVerticle(new Receiver());
EventBus eventBus = vertx.eventBus();
eventBus.registerDefaultCodec(MessageDTO.class, new CustomMessageCodec());
}
REST part
public class RESTVerticle extends AbstractVerticle {
private EventBus eventBus = null;
#Override
public void start() throws Exception {
Router router = Router.router(vertx);
eventBus = vertx.eventBus();
router.route().handler(BodyHandler.create());
router.route().handler(CorsHandler.create("*")
.allowedMethod(HttpMethod.GET)
.allowedHeader("Content-Type"));
router.post("/api/message").handler(this::publishToEventBus);
// router.get("/api/messagelist").handler(this::getMessagesFromBus);
router.route("/*").handler(StaticHandler.create());
vertx.createHttpServer().requestHandler(router::accept).listen(9999);
System.out.println("Service running at 0.0.0.0:9999");
}
private void publishToEventBus(RoutingContext routingContext) {
System.out.println("routingContext.getBodyAsString() " + routingContext.getBodyAsString());
final MessageDTO message = Json.decodeValue(routingContext.getBodyAsString(),
MessageDTO.class);
HttpServerResponse response = routingContext.response();
response.setStatusCode(201)
.putHeader("content-type", "application/json; charset=utf-8")
.end(Json.encodePrettily(message));
eventBus.publish("messagesBus", message);
}
And the Receiver: I move it to a different class, but it does not help
public class Receiver extends AbstractVerticle {
#Override
public void start() throws Exception {
EventBus eventBus = vertx.eventBus();
Router router = Router.router(vertx);
router.route().handler(BodyHandler.create());
router.route().handler(CorsHandler.create("*")
.allowedMethod(HttpMethod.GET)
.allowedHeader("Content-Type"));
router.get("/api/messagelist").handler(this::getMessagesFromBus);
router.route("/*").handler(StaticHandler.create());
vertx.createHttpServer().requestHandler(router::accept).listen(9998);
System.out.println("Service Receiver running at 0.0.0.0:9998");
private void getMessagesFromBus(RoutingContext routingContext) {
EventBus eventBus = vertx.eventBus();
eventBus.consumer("messagesBus", message -> {
MessageDTO customMessage = (MessageDTO) message.body();
HttpServerResponse response = routingContext.response();
System.out.println("Receiver ->>>>>>>> " + customMessage);
if (customMessage != null) {
response.putHeader("content-type", "application/json; charset=utf-8")
.end(Json.encodePrettily(customMessage));
}
response.closed();
});
}
So if i post message to REST and handler publish it to the bus, when I am runtime get http://localhost:9998/api/messagelist it is return json, but second time it trow exception
java.lang.IllegalStateException: Response has already been written
at io.vertx.core.http.impl.HttpServerResponseImpl.checkWritten(HttpServerResponseImpl.java:561)
at io.vertx.core.http.impl.HttpServerResponseImpl.putHeader(HttpServerResponseImpl.java:154)
at io.vertx.core.http.impl.HttpServerResponseImpl.putHeader(HttpServerResponseImpl.java:52)
at com.project.backend.Receiver.lambda$getMessagesFromBus$0(Receiver.java:55)
at io.vertx.core.eventbus.impl.HandlerRegistration.handleMessage(HandlerRegistration.java:207)
at io.vertx.core.eventbus.impl.HandlerRegistration.handle(HandlerRegistration.java:201)
at io.vertx.core.eventbus.impl.EventBusImpl.lambda$deliverToHandler$127(EventBusImpl.java:498)
at io.vertx.core.impl.ContextImpl.lambda$wrapTask$18(ContextImpl.java:335)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:358)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:357)
at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:112)
at java.lang.Thread.run(Thread.java:745)
Receiver ->>>>>>>> Message{username=Aaaewfewf2d, message=41414wefwef2d2}
How to correctly get all messages from the receiver? Or if the bus received messages, should I immediately store them to the db? Can a message bus keep messages and not lost them?
Thanks

Each hit in the entry point "/api/messagelist" creates one new consumer with the request routing context.
The first request will create the consumer and reply to the request. When the second message was published, that consumer will receive the message and will reply to the previous request (instance) and this was closed.
I think that you misunderstood the event bus purpose and I really recommend you to read the documentation.
http://vertx.io/docs/vertx-core/java/#event_bus

I did not had the chance to test your code but it seems that the publish operation is throwing an exception and vertx will try to send back an error message. However you already replied and ended the connection.
Now the error might be from your codec but due to the asynchronous nature of vertx you only see it at a later stage and mangled with the internal error handler.

Server sending a greeting message with websocket and netty -> causing exception

I have a websocket server using Netty (4.0.17) that answers requests from a JavaScript client.
The communication works fine but I have an exception when I try to immediately send a greeting message when a client connects.
My code looks like that :
public class LiveOthelloWSHandler extends SimpleChannelInboundHandler<TextWebSocketFrame> {
#Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
super.channelActive(ctx);
ChannelFuture f = ctx.channel().writeAndFlush(new TextWebSocketFrame("(gameID=0)[LiveOthelloServer="+ VERSION_NUMBER + "]\n"));
}
// ...
#Override
protected void channelRead0(ChannelHandlerContext ctx, TextWebSocketFrame frame) throws Exception {
final String request = frame.text();
Channel thisChannel = ctx.channel();
// Do something with request
// Write back
thisChannel.writeAndFlush(new TextWebSocketFrame(response + "\n"));
}
}
The channelRead0() is ok, the client sends messages and the server answers back without any issue.
What doesn't work is the "greetings" part. I would like to send a welcoming message to the client (the string using VERSION_NUMBER in the ChannelActive() method) but I always get an exception :
java.lang.UnsupportedOperationException: unsupported message type: TextWebSocketFrame
I guess this is maybe because the channelActive() gets invoked as soon as the connection is established but before the websocket handshake is complete. How can I wait for the handshake to be finished and then send the greeting message (without the client having sent any request yet)?
For information, my initialization is:
#Override
public void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline pipeline = ch.pipeline();
pipeline.addLast(
new HttpRequestDecoder(),
new HttpObjectAggregator(65536),
new HttpResponseEncoder(),
new WebSocketServerProtocolHandler("/websocket"),
myLiveOthelloWSHandler);

Just RTFM...
http://netty.io/4.0/api/io/netty/handler/codec/http/websocketx/WebSocketServerProtocolHandler.html
Best way to detect handshake is to override ChannelInboundHandler.userEventTriggered...
So I just had to add:
#Override
public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
super.userEventTriggered(ctx, evt);
if (evt == WebSocketServerProtocolHandler.ServerHandshakeStateEvent.HANDSHAKE_COMPLETE) {
ChannelFuture f = ctx.channel().writeAndFlush(new TextWebSocketFrame("(gameID=0)[LiveOthelloServer="+ VERSION_NUMBER + "]\n"));
}
}

Simple way to use Netty to build an http proxy server?

I'm new to Netty, and am looking at using it to make a simple http proxy server that receives requests from a client, forwards the requests to another server, and then copies the response back to the response for the original request. One extra requirement is that I be able to support a timeout, so that if the proxied server takes too long to respond the proxy will respond by itself and close the connection to the proxied server.
I've already implemented such an application using Jetty, but with Jetty I need to use too many threads to keep inbound requests from getting blocked (this is a lightweight app that uses very little memory or cpu, but the latency of the proxied server is high enough that bursts in traffic cause either queueing in the proxy server, or require too many threads).
According to my understanding, I can use Netty to build a pipeline in which each stage performs a small amount of computation, then releases it's thread and waits until data is ready for the next stage in the pipeline to be executed.
My question is, is there a simple example of such an application? What I have so far is a simple modification of the server code for the basic Netty tutorial, but it lacks all support for a client. I saw the netty client tutorial, but am not sure how to mix code from the two to create a simple proxy app.
public static void main(String[] args) throws Exception {
ChannelFactory factory =
new NioServerSocketChannelFactory(
Executors.newCachedThreadPool(),
Executors.newCachedThreadPool());
ServerBootstrap bootstrap = new ServerBootstrap(factory);
bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
public ChannelPipeline getPipeline() {
return Channels.pipeline(
new HttpRequestDecoder(),
new HttpResponseEncoder(),
/*
* Is there something I can put here to make a
* request to another server asynchronously and
* copy the result to the response inside
* MySimpleChannelHandler?
*/
new MySimpleChannelHandler()
);
}
});
bootstrap.setOption("child.tcpNoDelay", true);
bootstrap.setOption("child.keepAlive", true);
bootstrap.bind(new InetSocketAddress(8080));
}
private static class MySimpleChannelHandler extends SimpleChannelHandler {
#Override
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
HttpRequest request = (HttpRequest) e.getMessage();
HttpResponse response = new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
response.setContent(request.getContent());
Channel ch = e.getChannel();
ChannelFuture f = ch.write(response);
f.addListener(new ChannelFutureListener() {
public void operationComplete(ChannelFuture future) {
Channel ch = future.getChannel();
ch.close();
}
});
}
#Override
public void exceptionCaught(ChannelHandlerContext ctx, ExceptionEvent e) {
e.getCause().printStackTrace();
Channel ch = e.getChannel();
ch.close();
}
}

you would have to look at LittleProxy to see how they did it as it is written on top of Netty.