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DESCRIPTION
Il protocollo WebSocket è ormai diventato un protocollo standard (RFC 6455), e ormai quasi tutti i browsers lo supportano. A differenza di HTTP permette una vera comunicazione bidirezionale tra client e server, permettendo di realizzare applicazioni molto scalabili. In questa sessione daremo uno sguardo al protocollo WebSocket e esamineremo come scrivere una applicazione web utilizzando WebSocket.
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3Simone Bordet
Simone Bordet
Senior Engineer/Architect at Intalio/WebtideSupport for Jetty and CometD open source projects
Open Source Contributor Jetty, CometD, MX4J, Foxtrot, LiveTribe, JBoss, Larex
JVM tuning expert
Working on Web Network ProtocolsHTTP, WebSocket, SPDY, etc.
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Agenda
What is WebSocket
WebSocket Browser API
WebSocket Server API
Demo
Q&A
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WebSocket
WebSocket is the HTML 5 standard protocol for bidirectional web communicationHTTP is always client-initiatedFinally, standard server-push
Work began in 2009, ended in 2012Excruciatingly painfulBut at the end, a good standard protocol
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WebSocket Protocol
The WebSocket protocol is made of 2 partsWebSocket upgradeWebSocket data exchange
WebSocket upgrade It's plain old HTTP with an “Upgrade” headerWebSocket runs on port 80Could be a trouble for inspecting HTTP proxies that
do not support WebSocket
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WebSocket Upgrade
REQUEST
GET / HTTP/1.1
Host: localhost:8080
Origin: http://localhost:8080
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Key: SbdIETLKHQ1TNBLeZFZS0g==
Sec-WebSocket-Version: 13
RESPONSE
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Accept: y4yXRUolfnFfo3Jc5HFqRHNgx2A=
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WebSocket Protocol
WebSocket data exchangeSequence of WebSocket “frames”
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7+-+-+-+-+-------+-+-------------+-------------------------------+|F|R|R|R| opcode|M| Payload len | Extended payload length ||I|S|S|S| (4) |A| (7) | (16/64) ||N|V|V|V| |S| | (if payload len==126/127) || |1|2|3| |K| | |+-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - +| Extended payload length continued, if payload len == 127 |+ - - - - - - - - - - - - - - - +-------------------------------+| |Masking-key, if MASK set to 1 |+-------------------------------+-------------------------------+| Masking-key (continued) | Payload Data |+-------------------------------- - - - - - - - - - - - - - - - +: Payload Data continued ... :+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +| Payload Data continued |+---------------------------------------------------------------+
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WebSocket Protocol
Very compactSmallest data frame has only 2 bytes overhead
Typical first byte is 0x81 for a text frame0x82 for a binary frame0x88 for a close frame
WebSocket “close” frameSent by the application to signal end of dataMust be replied before closing TCP connection
Prevents loss of data
Browsers MUST mask framesBut servers are not required to
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WebSocket Browser API
interface WebSocket {
WebSocket(DOMString url, optional (DOMString or DOMString[]) protocols)]
attribute Function? onopen;
attribute Function? onerror;
attribute Function? onclose;
attribute Function? onmessage;
void send(DOMString data);
void close(optional unsigned short code, optional DOMString reason);
attribute DOMString binaryType; // “blob” or “arraybuffer”
void send(ArrayBuffer data);
void send(Blob data);
};
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WebSocket Browser API Usage
var ws = new window.WebSocket(“ws://localhost:8080/ws”);
ws.onopen = function() {
ws.send(“Hello, World”);
};
ws.onclose = function(event) {
// Server closed
}
ws.onmessage = function(event) {
var data = event.data;
window.console.info(“Server says: ” + data);
};
ws.send(“Hello, World”);ws.send(“Hello, World”);
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WebSocket Server API
No standard API for WebSocket webapps yet
Servlet 3.0 addressed asynchronous HTTPServlet 3.1 (JSR 340) to address this
Expected Q1 2013
For now, rely on Jetty API Java API for both client and server
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WebSocket Jetty API
public class MyWebSocketServlet extends WebSocketServlet {
public WebSocket doWebSocketConnect(HttpServletRequest r, String p) {
return new MyWebSocket();
}
}
public class MyWebSocket implements WebSocket.OnTextMessage {
public void onOpen(Connection connection) {
}
public void onMessage(String data) {
}
public void onClose(int closeCode, String message) {
}
}
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Conclusions
WebSocket programming is easy !
However, may not be widely deployedOld browsersOld internet proxies
WebSocket is low-levelA framework on top gives much more productivityCometD
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References
WebSocket Protocolhttp://www.ietf.org/rfc/rfc6455.txt
JSR 340 (Servlet 3.1)http://jcp.org/en/jsr/detail?id=340
WebSocket APIhttp://dev.w3.org/html5/websockets/
Jettyhttp://eclipse.org/jetty
CometDhttp://cometd.org
