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Socket.IO Java Server

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ScaleCube Socket.IO is a lightweight implementation of Socket.IO Java server based on Netty framework. It implements subset of Socket.IO protocol and optimized for high throughput and low latency real-time messaging. It is designed to support requirements of most demanding modern applications such as online gaming, financial trading, social and advertising platforms.

Socket.IO protocol provides WebSocket transport with fallback options to other transports such as XHR-Polling in case if client does not support or unable to establish WebSocket connection (e.g. due to proxy or firewall restrictions). It supports reconnection mechanism based on exponential backoff algorithm and heartbeat-based detection of disconnections.

ScaleCube Socket.IO is a lightweight embeddable library with minimum dependencies for the Java VM. Major use case is to provide an implementation of transport layer for API Gateway pattern in microservices architecture. Mobile and web clients use Socket.IO transport to communicate with application microservices.

Supported transport protocols:

  • WebSocket
  • Flash Socket
  • XHR-Polling
  • JSONP-Polling

Supports 0.7+ up to 0.9.16 versions of Socket.IO-client.


Tested on VM: CentOS, 4vCPU, 2GB RAM, Java 7

Client Sessions:

  • 10,000 long-polling sessions on single node
  • 50,000 WebSocket sessions on single node


  • 4,000 requests per second per single channel
  • 80,000 requests per second total

Getting Started

Start echo Socket.IO server on default port (8080) as simple as:

SocketIOServer echoServer = SocketIOServer.newInstance();
echoServer.setListener(new SocketIOAdapter() {
  public void onMessage(Session session, ByteBuf message) {

Start Socket.IO server on port 5000 which prints to console all received messages and connected/disconnected events:

SocketIOServer logServer = SocketIOServer.newInstance(5000 /*port*/);
logServer.setListener(new SocketIOListener() {
  public void onConnect(Session session) {
    System.out.println("Connected: " + session);  
  public void onMessage(Session session, ByteBuf message) {
    System.out.println("Received: " + message.toString(CharsetUtil.UTF_8));
  public void onDisconnect(Session session) {
    System.out.println("Disconnected: " + session);  

Received message has type of Netty's ByteBuffer. Since the popular use case are proxy-like applications it allows to resend received payload without full decoding it. If byte buffer will be sent to another Netty channel it will be released automatically, otherwise it is required to manually release buffer.

To start Socket.IO server with SSL/TLS support you need to provide in server config either JDK's SSLContext or Netty's SslContext which may be backed by OpenSSL implementation:

// Server config
SSLContext sslContext = ... // your server's SSLContext 
ServerConfiguration configWithSsl = ServerConfiguration.builder()
// Start server
SocketIOServer sslServer = SocketIOServer.newInstance(configWithSsl);
sslServer.setListener(new SocketIOAdapter() { /* listener code */ });

To play with your Socket.IO server you may use our demo client.

For more examples and demo client application, see Socket.IO Examples.


Binaries and dependency information for Maven can be found at

Change history and version numbers at

Maven dependency:


All dependencies are made optional in order to allow change to compatible version used by your project. So following or compatible versions of Netty 4.1.x and slf4j-api 1.7.x modules should be added to your project:


Server configuration

  • port

    Port on which Socket.IO server will be started. Default value is 8080.

  • sslContext

    SSL context which is used to run secure socket. If it's set to null server runs without SSL. Default value is null.

  • transports

    A string with list of allowed transport methods separated by comma. Default value is "websocket,flashsocket,xhr-polling,jsonp-polling".

  • heartbeatTimeout

    The timeout in seconds for the client when it should send a new heart beat to the server. This value is sent to the client after a successful handshake. The default value is 60.

  • closeTimeout

    The timeout in seconds for the client, when it closes the connection it still X amounts of seconds to do re open of the connection. This value is sent to the client after a successful handshake. Default value is 60.

  • heartbeatInterval

    The timeout in seconds for the server, we should receive a heartbeat from the client within this interval. This should be less than the heartbeat timeout. Default value is 25.

  • eventExecutorEnabled

    Flag which defines if listener will be executed, true - different thread, false - io-thread. Default is true.

  • eventExecutorThreadNumber

    Event executor thread number, if eventExecutorEnabled flag set to true. Default value is Runtime.getRuntime().availableProcessors() x 2.

  • maxWebSocketFrameSize

    Maximum allowable web socket frame payload length. Setting this value to your application's requirement may reduce denial of service attacks using long data frames. Default is 65536.

  • alwaysSecureWebSocketLocation

    Flag which if set to true will always return secure web socket location protocol ("wss://") even when connection is established over plain socket. It is used as a workaround related to case when SSL is offloaded to Load Balancer, but it doesn't modify web socket location. By default it is false.

  • remoteAddressHeader

    The HTTP header name which is used as a session remote address. It is a workaround related to case when Load Balancer modify client address with its address. This header is supposed to be set by Load Balancer. If it is set to null then this header is not used. Default value is null.

  • epollEnabled

    Flag which defines if Linux native epoll transport will be used if available. Default is true.

  • httpCompressionEnabled

    Flag which defines if HTTP compression is enabled. Default is false.

  • websocketCompressionEnabled

    Flag which defines if websocket compression is enabled. Default is false.

Bugs and Feedback

For bugs, questions and discussions please use the GitHub Issues.


Apache License, Version 2.0


Socket.IO Java Server based on Netty. was created to meet gaming performance requirements. battle tested and in use by Playtech Microservices API Gateway.








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