Java web servers 间是如何实现 session 同步的
Java web servers 间是如何实现 session 同步的
有一个多月的时间没有更新博客了，今天终于忙里偷闲，可以把近期的收获总结一下。
本文是关于Java web servers 之间是如何实现 session 同步的，其实其他技术栈也面临同样的问题需要解决，而且大部分场景下已经有了成熟的解决方案，其实就应用开发本身，大部分人不太会关注这个问题，因为我们大部分人写代码的时候只需要考虑单节点场景，其他同步部分由服务器端负责实现，但是作为一个刨根问底的人，可能这个问题本身已经能够吸引人的了。
那么，为了解决这个问题，有哪些关键点呢，下面的几点可能是我们绕不开的，
1. 如何保证可靠传输呢，也就是说发送端确认接收节点收到了session数据
2. 一个节点如何知道他自己有哪些伙伴，他需要把session数据发给谁呢
3. 长消息如何发送呢，如何保证数据安全传输
写到这里，大家可能脑海中已经出现了可靠传输，IP多播，数据分包，加密解密，数据一致性保证，对的，就是这些技术，但是应用这些底层技术完成应用，确实需要不是一般程序员可以负担起的时间和经历。笔者也不打算展开来讲所有的技术细节，经过简单的研究，笔者发现了一个写的比较好的开源框架，可以完成所有相关的功能，下面就基于这个开源框架谈谈session同步是如何做到的。示例代码和效果如下，当我在第一张面板上写下tea的时候，在其他所用同一个组的面板上也会显示出同样的字样，同样的效果，JBoss cluster 和JBoss Cache都是基于此开源框架进行的实现，此开源框架的名字是 JGroups 。
1 public class Draw extends ReceiverAdapter implements ActionListener, ChannelListener { 2 protected String cluster_name="draw"; 3 private JChannel channel=null; 4 private int member_size=1; 5 private JFrame mainFrame=null; 6 private JPanel sub_panel=null; 7 private DrawPanel panel=null; 8 private JButton clear_button, leave_button; 9 private final Random random=new Random(System.currentTimeMillis()); 10 private final Font default_font=new Font("Helvetica",Font.PLAIN,12); 11 private final Color draw_color=selectColor(); 12 private static final Color background_color=Color.white; 13 boolean no_channel=false; 14 boolean jmx; 15 private boolean use_state=false; 16 private long state_timeout=5000; 17 private boolean use_unicasts=false; 18 protected boolean send_own_state_on_merge=true; 19 private final List<Address> members=new ArrayList<>(); 20 21 22 public Draw(String props, boolean no_channel, boolean jmx, boolean use_state, long state_timeout, 23 boolean use_unicasts, String name, boolean send_own_state_on_merge, AddressGenerator gen) throws Exception { 24 this.no_channel=no_channel; 25 this.jmx=jmx; 26 this.use_state=use_state; 27 this.state_timeout=state_timeout; 28 this.use_unicasts=use_unicasts; 29 if(no_channel) 30 return; 31 32 channel=new JChannel(props).addAddressGenerator(gen).setName(name); 33 channel.setReceiver(this).addChannelListener(this); 34 this.send_own_state_on_merge=send_own_state_on_merge; 35 } 36 37 public Draw(JChannel channel) throws Exception { 38 this.channel=channel; 39 channel.setReceiver(this); 40 channel.addChannelListener(this); 41 } 42 43 44 public Draw(JChannel channel, boolean use_state, long state_timeout) throws Exception { 45 this.channel=channel; 46 channel.setReceiver(this); 47 channel.addChannelListener(this); 48 this.use_state=use_state; 49 this.state_timeout=state_timeout; 50 } 51 52 53 public String getClusterName() { 54 return cluster_name; 55 } 56 57 public void setClusterName(String clustername) { 58 if(clustername != null) 59 this.cluster_name=clustername; 60 } 61 62 63 public static void main(String[] args) { 64 Draw draw=null; 65 String props=null; 66 boolean no_channel=false; 67 boolean jmx=true; 68 boolean use_state=false; 69 String group_name=null; 70 long state_timeout=5000; 71 boolean use_unicasts=false; 72 String name=null; 73 boolean send_own_state_on_merge=true; 74 AddressGenerator generator=null; 75 76 for(int i=0; i < args.length; i++) { 77 if("-help".equals(args[i])) { 78 help(); 79 return; 80 } 81 if("-props".equals(args[i])) { 82 props=args[++i]; 83 continue; 84 } 85 if("-no_channel".equals(args[i])) { 86 no_channel=true; 87 continue; 88 } 89 if("-jmx".equals(args[i])) { 90 jmx=Boolean.parseBoolean(args[++i]); 91 continue; 92 } 93 if("-clustername".equals(args[i])) { 94 group_name=args[++i]; 95 continue; 96 } 97 if("-state".equals(args[i])) { 98 use_state=true; 99 continue; 100 } 101 if("-timeout".equals(args[i])) { 102 state_timeout=Long.parseLong(args[++i]); 103 continue; 104 } 105 if("-bind_addr".equals(args[i])) { 106 System.setProperty("jgroups.bind_addr", args[++i]); 107 continue; 108 } 109 if("-use_unicasts".equals(args[i])) { 110 use_unicasts=true; 111 continue; 112 } 113 if("-name".equals(args[i])) { 114 name=args[++i]; 115 continue; 116 } 117 if("-send_own_state_on_merge".equals(args[i])) { 118 send_own_state_on_merge=Boolean.getBoolean(args[++i]); 119 continue; 120 } 121 if("-uuid".equals(args[i])) { 122 generator=new OneTimeAddressGenerator(Long.valueOf(args[++i])); 123 continue; 124 } 125 126 help(); 127 return; 128 } 129 130 try { 131 draw=new Draw(props, no_channel, jmx, use_state, state_timeout, use_unicasts, name, 132 send_own_state_on_merge, generator); 133 if(group_name != null) 134 draw.setClusterName(group_name); 135 draw.go(); 136 } 137 catch(Throwable e) { 138 e.printStackTrace(System.err); 139 System.exit(0); 140 } 141 } 142 143 144 static void help() { 145 System.out.println("\nDraw [-help] [-no_channel] [-props <protocol stack definition>]" + 146 " [-clustername <name>] [-state] [-timeout <state timeout>] [-use_unicasts] " + 147 "[-bind_addr <addr>] [-jmx <true | false>] [-name <logical name>] [-send_own_state_on_merge true|false] " + 148 "[-uuid <UUID>]"); 149 System.out.println("-no_channel: doesn't use JGroups at all, any drawing will be relected on the " + 150 "whiteboard directly"); 151 System.out.println("-props: argument can be an old-style protocol stack specification, or it can be " + 152 "a URL. In the latter case, the protocol specification will be read from the URL\n"); 153 } 154 155 156 private Color selectColor() { 157 int red=Math.abs(random.nextInt() % 255); 158 int green=Math.abs(random.nextInt() % 255); 159 int blue=Math.abs(random.nextInt() % 255); 160 return new Color(red, green, blue); 161 } 162 163 164 private void sendToAll(byte[] buf) throws Exception { 165 for(Address mbr: members) 166 channel.send(new Message(mbr, buf)); 167 } 168 169 170 public void go() throws Exception { 171 if(!no_channel && !use_state) 172 channel.connect(cluster_name); 173 mainFrame=new JFrame(); 174 mainFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE); 175 panel=new DrawPanel(use_state); 176 panel.setBackground(background_color); 177 sub_panel=new JPanel(); 178 mainFrame.getContentPane().add("Center", panel); 179 clear_button=new JButton("Clear"); 180 clear_button.setFont(default_font); 181 clear_button.addActionListener(this); 182 leave_button=new JButton("Leave"); 183 leave_button.setFont(default_font); 184 leave_button.addActionListener(this); 185 sub_panel.add("South", clear_button); 186 sub_panel.add("South", leave_button); 187 mainFrame.getContentPane().add("South", sub_panel); 188 mainFrame.setBackground(background_color); 189 clear_button.setForeground(Color.blue); 190 leave_button.setForeground(Color.blue); 191 mainFrame.pack(); 192 mainFrame.setLocation(15, 25); 193 mainFrame.setBounds(new Rectangle(250, 250)); 194 195 if(!no_channel && use_state) { 196 channel.connect(cluster_name, null, state_timeout); 197 } 198 mainFrame.setVisible(true); 199 setTitle(); 200 } 201 202 203 204 205 void setTitle(String title) { 206 String tmp=""; 207 if(no_channel) { 208 mainFrame.setTitle(" Draw Demo "); 209 return; 210 } 211 if(title != null) { 212 mainFrame.setTitle(title); 213 } 214 else { 215 if(channel.getAddress() != null) 216 tmp+=channel.getAddress(); 217 tmp+=" (" + member_size + ")"; 218 mainFrame.setTitle(tmp); 219 } 220 } 221 222 void setTitle() { 223 setTitle(null); 224 } 225 226 public void receive(Message msg) { 227 byte[] buf=msg.getRawBuffer(); 228 if(buf == null) { 229 System.err.printf("%s: received null buffer from %s, headers: %s\n", channel.getAddress(), msg.src(), msg.printHeaders()); 230 return; 231 } 232 233 try { 234 DrawCommand comm=Util.streamableFromByteBuffer(DrawCommand.class, buf, msg.getOffset(), msg.getLength()); 235 switch(comm.mode) { 236 case DrawCommand.DRAW: 237 if(panel != null) 238 panel.drawPoint(comm); 239 break; 240 case DrawCommand.CLEAR: 241 clearPanel(); 242 break; 243 default: 244 System.err.println("***** received invalid draw command " + comm.mode); 245 break; 246 } 247 } 248 catch(Exception e) { 249 e.printStackTrace(); 250 } 251 } 252 253 public void viewAccepted(View v) { 254 member_size=v.size(); 255 if(mainFrame != null) 256 setTitle(); 257 members.clear(); 258 members.addAll(v.getMembers()); 259 260 if(v instanceof MergeView) { 261 System.out.println("** " + v); 262 263 // This is an example of a simple merge function, which fetches the state from the coordinator 264 // on a merge and overwrites all of its own state 265 if(use_state && !members.isEmpty()) { 266 Address coord=members.get(0); 267 Address local_addr=channel.getAddress(); 268 if(local_addr != null && !local_addr.equals(coord)) { 269 try { 270 271 // make a copy of our state first 272 Map<Point,Color> copy=null; 273 if(send_own_state_on_merge) { 274 synchronized(panel.state) { 275 copy=new LinkedHashMap<>(panel.state); 276 } 277 } 278 System.out.println("fetching state from " + coord); 279 channel.getState(coord, 5000); 280 if(copy != null) 281 sendOwnState(copy); // multicast my own state so everybody else has it too 282 } 283 catch(Exception e) { 284 e.printStackTrace(); 285 } 286 } 287 } 288 } 289 else 290 System.out.println("** View=" + v); 291 } 292 293 294 public void getState(OutputStream ostream) throws Exception { 295 panel.writeState(ostream); 296 } 297 298 public void setState(InputStream istream) throws Exception { 299 panel.readState(istream); 300 } 301 302 /* --------------- Callbacks --------------- */ 303 304 305 306 public void clearPanel() { 307 if(panel != null) 308 panel.clear(); 309 } 310 311 public void sendClearPanelMsg() { 312 DrawCommand comm=new DrawCommand(DrawCommand.CLEAR); 313 try { 314 byte[] buf=Util.streamableToByteBuffer(comm); 315 if(use_unicasts) 316 sendToAll(buf); 317 else 318 channel.send(new Message(null, buf)); 319 } 320 catch(Exception ex) { 321 System.err.println(ex); 322 } 323 } 324 325 326 public void actionPerformed(ActionEvent e) { 327 String command=e.getActionCommand(); 328 switch(command) { 329 case "Clear": 330 if(no_channel) { 331 clearPanel(); 332 return; 333 } 334 sendClearPanelMsg(); 335 break; 336 case "Leave": 337 stop(); 338 break; 339 default: 340 System.out.println("Unknown action"); 341 break; 342 } 343 } 344 345 346 public void stop() { 347 if(!no_channel) { 348 try { 349 channel.close(); 350 } 351 catch(Exception ex) { 352 System.err.println(ex); 353 } 354 } 355 mainFrame.setVisible(false); 356 mainFrame.dispose(); 357 } 358 359 protected void sendOwnState(final Map<Point,Color> copy) { 360 if(copy == null) 361 return; 362 for(Point point: copy.keySet()) { 363 // we don't need the color: it is our draw_color anyway 364 DrawCommand comm=new DrawCommand(DrawCommand.DRAW, point.x, point.y, draw_color.getRGB()); 365 try { 366 byte[] buf=Util.streamableToByteBuffer(comm); 367 if(use_unicasts) 368 sendToAll(buf); 369 else 370 channel.send(new Message(null, buf)); 371 } 372 catch(Exception ex) { 373 System.err.println(ex); 374 } 375 } 376 } 377 378 379 /* ------------------------------ ChannelListener interface -------------------------- */ 380 381 public void channelConnected(JChannel channel) { 382 if(jmx) { 383 Util.registerChannel(channel, "jgroups"); 384 } 385 } 386 387 public void channelDisconnected(JChannel channel) { 388 if(jmx) { 389 MBeanServer server=Util.getMBeanServer(); 390 if(server != null) { 391 try { 392 JmxConfigurator.unregisterChannel(channel, server, cluster_name); 393 } 394 catch(Exception e) { 395 e.printStackTrace(); 396 } 397 } 398 } 399 } 400 401 public void channelClosed(JChannel channel) { 402 403 } 404 405 406 /* --------------------------- End of ChannelListener interface ---------------------- */ 407 408 409 410 protected class DrawPanel extends JPanel implements MouseMotionListener { 411 protected final Dimension preferred_size=new Dimension(235, 170); 412 protected Image img; // for drawing pixels 413 protected Dimension d, imgsize; 414 protected Graphics gr; 415 protected final Map<Point,Color> state; 416 417 418 public DrawPanel(boolean use_state) { 419 if(use_state) 420 state=new LinkedHashMap<>(); 421 else 422 state=null; 423 createOffscreenImage(false); 424 addMouseMotionListener(this); 425 addComponentListener(new ComponentAdapter() { 426 public void componentResized(ComponentEvent e) { 427 if(getWidth() <= 0 || getHeight() <= 0) return; 428 createOffscreenImage(false); 429 } 430 }); 431 } 432 433 434 public void writeState(OutputStream outstream) throws IOException { 435 if(state == null) 436 return; 437 synchronized(state) { 438 DataOutputStream dos=new DataOutputStream(new BufferedOutputStream(outstream)); 439 // DataOutputStream dos=new DataOutputStream(outstream); 440 dos.writeInt(state.size()); 441 for(Map.Entry<Point,Color> entry: state.entrySet()) { 442 Point point=entry.getKey(); 443 Color col=entry.getValue(); 444 dos.writeInt(point.x); 445 dos.writeInt(point.y); 446 dos.writeInt(col.getRGB()); 447 } 448 dos.flush(); 449 System.out.println("wrote " + state.size() + " elements"); 450 } 451 } 452 453 454 public void readState(InputStream instream) throws IOException { 455 DataInputStream in=new DataInputStream(new BufferedInputStream(instream)); 456 Map<Point,Color> new_state=new LinkedHashMap<>(); 457 int num=in.readInt(); 458 for(int i=0; i < num; i++) { 459 Point point=new Point(in.readInt(), in.readInt()); 460 Color col=new Color(in.readInt()); 461 new_state.put(point, col); 462 } 463 464 synchronized(state) { 465 state.clear(); 466 state.putAll(new_state); 467 System.out.println("read " + state.size() + " elements"); 468 createOffscreenImage(true); 469 } 470 } 471 472 473 void createOffscreenImage(boolean discard_image) { 474 d=getSize(); 475 if(discard_image) { 476 img=null; 477 imgsize=null; 478 } 479 if(img == null || imgsize == null || imgsize.width != d.width || imgsize.height != d.height) { 480 img=createImage(d.width, d.height); 481 if(img != null) { 482 gr=img.getGraphics(); 483 if(gr != null && state != null) { 484 drawState(); 485 } 486 } 487 imgsize=d; 488 } 489 repaint(); 490 } 491 492 493 /* ---------------------- MouseMotionListener interface------------------------- */ 494 495 public void mouseMoved(MouseEvent e) {} 496 497 public void mouseDragged(MouseEvent e) { 498 int x=e.getX(), y=e.getY(); 499 DrawCommand comm=new DrawCommand(DrawCommand.DRAW, x, y, draw_color.getRGB()); 500 501 if(no_channel) { 502 drawPoint(comm); 503 return; 504 } 505 506 try { 507 byte[] buf=Util.streamableToByteBuffer(comm); 508 if(use_unicasts) 509 sendToAll(buf); 510 else 511 channel.send(new Message(null, buf)); 512 } 513 catch(Exception ex) { 514 System.err.println(ex); 515 } 516 } 517 518 /* ------------------- End of MouseMotionListener interface --------------------- */ 519 520 521 /** 522 * Adds pixel to queue and calls repaint() whenever we have MAX_ITEMS pixels in the queue 523 * or when MAX_TIME msecs have elapsed (whichever comes first). The advantage compared to just calling 524 * repaint() after adding a pixel to the queue is that repaint() can most often draw multiple points 525 * at the same time. 526 */ 527 public void drawPoint(DrawCommand c) { 528 if(c == null || gr == null) return; 529 Color col=new Color(c.rgb); 530 gr.setColor(col); 531 gr.fillOval(c.x, c.y, 10, 10); 532 repaint(); 533 if(state != null) { 534 synchronized(state) { 535 state.put(new Point(c.x, c.y), col); 536 } 537 } 538 } 539 540 541 542 public void clear() { 543 if(gr == null) return; 544 gr.clearRect(0, 0, getSize().width, getSize().height); 545 repaint(); 546 if(state != null) { 547 synchronized(state) { 548 state.clear(); 549 } 550 } 551 } 552 553 554 /** Draw the entire panel from the state */ 555 public void drawState() { 556 // clear(); 557 Map.Entry entry; 558 Point pt; 559 Color col; 560 synchronized(state) { 561 for(Iterator it=state.entrySet().iterator(); it.hasNext();) { 562 entry=(Map.Entry)it.next(); 563 pt=(Point)entry.getKey(); 564 col=(Color)entry.getValue(); 565 gr.setColor(col); 566 gr.fillOval(pt.x, pt.y, 10, 10); 567 568 } 569 } 570 repaint(); 571 } 572 573 574 public Dimension getPreferredSize() { 575 return preferred_size; 576 } 577 578 579 public void paintComponent(Graphics g) { 580 super.paintComponent(g); 581 if(img != null) { 582 g.drawImage(img, 0, 0, null); 583 } 584 } 585 586 } 587 588 }
我们甚至可以通过如下短短的几行代码写一个简易的聊天程序，这样，一个人发送的消息，组内所有成员都可以收到，并且可以同步聊天记录，同时组内节点可以感知道其他节点的加入，关闭，甚至意外退出。
1 public class SimpleChat extends ReceiverAdapter { 2 JChannel channel; 3 String user_name=System.getProperty("user.name", "n/a"); 4 final List<String> state=new LinkedList<>(); 5 6 public void viewAccepted(View new_view) { 7 System.out.println("** view: " + new_view); 8 } 9 10 public void receive(Message msg) { 11 String line=msg.getSrc() + ": " + msg.getObject(); 12 System.out.println(line); 13 synchronized(state) { 14 state.add(line); 15 } 16 } 17 18 public void getState(OutputStream output) throws Exception { 19 synchronized(state) { 20 Util.objectToStream(state, new DataOutputStream(output)); 21 } 22 } 23 24 @SuppressWarnings("unchecked") 25 public void setState(InputStream input) throws Exception { 26 List<String> list=Util.objectFromStream(new DataInputStream(input)); 27 synchronized(state) { 28 state.clear(); 29 state.addAll(list); 30 } 31 System.out.println("received state (" + list.size() + " messages in chat history):"); 32 list.forEach(System.out::println); 33 } 34 35 36 private void start() throws Exception { 37 channel=new JChannel().setReceiver(this); 38 channel.connect("ChatCluster"); 39 channel.getState(null, 10000); 40 eventLoop(); 41 channel.close(); 42 } 43 44 private void eventLoop() { 45 BufferedReader in=new BufferedReader(new InputStreamReader(System.in)); 46 while(true) { 47 try { 48 System.out.print("> "); System.out.flush(); 49 String line=in.readLine().toLowerCase(); 50 if(line.startsWith("quit") || line.startsWith("exit")) { 51 break; 52 } 53 line="[" + user_name + "] " + line; 54 Message msg=new Message(null, line); 55 channel.send(msg); 56 } 57 catch(Exception e) { 58 } 59 } 60 } 61 62 63 public static void main(String[] args) throws Exception { 64 new SimpleChat().start(); 65 } 66 }
总结
本文通过两个简单的示例展示了JGroups的用法，说明了 Java web servers 间是实现 session 同步的基本原理，大家如果对更多的细节感兴趣，可以和笔者进行沟通，笔者可以在下次的文章中加入更多的细节。
