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 。
public class Draw extends ReceiverAdapter implements ActionListener, ChannelListener {
protected String cluster_name="draw";
private JChannel channel=null;
private int member_size=1;
private JFrame mainFrame=null;
private JPanel sub_panel=null;
private DrawPanel panel=null;
private JButton clear_button, leave_button;
private final Random random=new Random(System.currentTimeMillis());
private final Font default_font=new Font("Helvetica",Font.PLAIN,12);
private final Color draw_color=selectColor();
private static final Color background_color=Color.white;
boolean no_channel=false;
boolean jmx;
private boolean use_state=false;
private long state_timeout=5000;
private boolean use_unicasts=false;
protected boolean send_own_state_on_merge=true;
private final List<Address> members=new ArrayList<>();
public Draw(String props, boolean no_channel, boolean jmx, boolean use_state, long state_timeout,
boolean use_unicasts, String name, boolean send_own_state_on_merge, AddressGenerator gen) throws Exception {
this.no_channel=no_channel;
this.jmx=jmx;
this.use_state=use_state;
this.state_timeout=state_timeout;
this.use_unicasts=use_unicasts;
if(no_channel)
return;
channel=new JChannel(props).addAddressGenerator(gen).setName(name);
channel.setReceiver(this).addChannelListener(this);
this.send_own_state_on_merge=send_own_state_on_merge;
}
public Draw(JChannel channel) throws Exception {
this.channel=channel;
channel.setReceiver(this);
channel.addChannelListener(this);
}
public Draw(JChannel channel, boolean use_state, long state_timeout) throws Exception {
this.channel=channel;
channel.setReceiver(this);
channel.addChannelListener(this);
this.use_state=use_state;
this.state_timeout=state_timeout;
}
public String getClusterName() {
return cluster_name;
}
public void setClusterName(String clustername) {
if(clustername != null)
this.cluster_name=clustername;
}
public static void main(String[] args) {
Draw draw=null;
String props=null;
boolean no_channel=false;
boolean jmx=true;
boolean use_state=false;
String group_name=null;
long state_timeout=5000;
boolean use_unicasts=false;
String name=null;
boolean send_own_state_on_merge=true;
AddressGenerator generator=null;
for(int i=0; i < args.length; i++) {
if("-help".equals(args[i])) {
help();
return;
}
if("-props".equals(args[i])) {
props=args[++i];
continue;
}
if("-no_channel".equals(args[i])) {
no_channel=true;
continue;
}
if("-jmx".equals(args[i])) {
jmx=Boolean.parseBoolean(args[++i]);
continue;
}
if("-clustername".equals(args[i])) {
group_name=args[++i];
continue;
}
if("-state".equals(args[i])) {
use_state=true;
continue;
}
if("-timeout".equals(args[i])) {
state_timeout=Long.parseLong(args[++i]);
continue;
}
if("-bind_addr".equals(args[i])) {
System.setProperty("jgroups.bind_addr", args[++i]);
continue;
}
if("-use_unicasts".equals(args[i])) {
use_unicasts=true;
continue;
}
if("-name".equals(args[i])) {
name=args[++i];
continue;
}
if("-send_own_state_on_merge".equals(args[i])) {
send_own_state_on_merge=Boolean.getBoolean(args[++i]);
continue;
}
if("-uuid".equals(args[i])) {
generator=new OneTimeAddressGenerator(Long.valueOf(args[++i]));
continue;
}
help();
return;
}
try {
draw=new Draw(props, no_channel, jmx, use_state, state_timeout, use_unicasts, name,
send_own_state_on_merge, generator);
if(group_name != null)
draw.setClusterName(group_name);
draw.go();
}
catch(Throwable e) {
e.printStackTrace(System.err);
System.exit(0);
}
}
static void help() {
System.out.println("\nDraw [-help] [-no_channel] [-props <protocol stack definition>]" +
" [-clustername <name>] [-state] [-timeout <state timeout>] [-use_unicasts] " +
"[-bind_addr <addr>] [-jmx <true | false>] [-name <logical name>] [-send_own_state_on_merge true|false] " +
"[-uuid <UUID>]");
System.out.println("-no_channel: doesn't use JGroups at all, any drawing will be relected on the " +
"whiteboard directly");
System.out.println("-props: argument can be an old-style protocol stack specification, or it can be " +
"a URL. In the latter case, the protocol specification will be read from the URL\n");
}
private Color selectColor() {
int red=Math.abs(random.nextInt() % 255);
int green=Math.abs(random.nextInt() % 255);
int blue=Math.abs(random.nextInt() % 255);
return new Color(red, green, blue);
}
private void sendToAll(byte[] buf) throws Exception {
for(Address mbr: members)
channel.send(new Message(mbr, buf));
}
public void go() throws Exception {
if(!no_channel && !use_state)
channel.connect(cluster_name);
mainFrame=new JFrame();
mainFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
panel=new DrawPanel(use_state);
panel.setBackground(background_color);
sub_panel=new JPanel();
mainFrame.getContentPane().add("Center", panel);
clear_button=new JButton("Clear");
clear_button.setFont(default_font);
clear_button.addActionListener(this);
leave_button=new JButton("Leave");
leave_button.setFont(default_font);
leave_button.addActionListener(this);
sub_panel.add("South", clear_button);
sub_panel.add("South", leave_button);
mainFrame.getContentPane().add("South", sub_panel);
mainFrame.setBackground(background_color);
clear_button.setForeground(Color.blue);
leave_button.setForeground(Color.blue);
mainFrame.pack();
mainFrame.setLocation(15, 25);
mainFrame.setBounds(new Rectangle(250, 250));
if(!no_channel && use_state) {
channel.connect(cluster_name, null, state_timeout);
}
mainFrame.setVisible(true);
setTitle();
}
void setTitle(String title) {
String tmp="";
if(no_channel) {
mainFrame.setTitle(" Draw Demo ");
return;
}
if(title != null) {
mainFrame.setTitle(title);
}
else {
if(channel.getAddress() != null)
tmp+=channel.getAddress();
tmp+=" (" + member_size + ")";
mainFrame.setTitle(tmp);
}
}
void setTitle() {
setTitle(null);
}
public void receive(Message msg) {
byte[] buf=msg.getRawBuffer();
if(buf == null) {
System.err.printf("%s: received null buffer from %s, headers: %s\n", channel.getAddress(), msg.src(), msg.printHeaders());
return;
}
try {
DrawCommand comm=Util.streamableFromByteBuffer(DrawCommand.class, buf, msg.getOffset(), msg.getLength());
switch(comm.mode) {
case DrawCommand.DRAW:
if(panel != null)
panel.drawPoint(comm);
break;
case DrawCommand.CLEAR:
clearPanel();
break;
default:
System.err.println("***** received invalid draw command " + comm.mode);
break;
}
}
catch(Exception e) {
e.printStackTrace();
}
}
public void viewAccepted(View v) {
member_size=v.size();
if(mainFrame != null)
setTitle();
members.clear();
members.addAll(v.getMembers());
if(v instanceof MergeView) {
System.out.println("** " + v);
// This is an example of a simple merge function, which fetches the state from the coordinator
// on a merge and overwrites all of its own state
if(use_state && !members.isEmpty()) {
Address coord=members.get(0);
Address local_addr=channel.getAddress();
if(local_addr != null && !local_addr.equals(coord)) {
try {
// make a copy of our state first
Map<Point,Color> copy=null;
if(send_own_state_on_merge) {
synchronized(panel.state) {
copy=new LinkedHashMap<>(panel.state);
}
}
System.out.println("fetching state from " + coord);
channel.getState(coord, 5000);
if(copy != null)
sendOwnState(copy); // multicast my own state so everybody else has it too
}
catch(Exception e) {
e.printStackTrace();
}
}
}
}
else
System.out.println("** View=" + v);
}
public void getState(OutputStream ostream) throws Exception {
panel.writeState(ostream);
}
public void setState(InputStream istream) throws Exception {
panel.readState(istream);
}
/* --------------- Callbacks --------------- */
public void clearPanel() {
if(panel != null)
panel.clear();
}
public void sendClearPanelMsg() {
DrawCommand comm=new DrawCommand(DrawCommand.CLEAR);
try {
byte[] buf=Util.streamableToByteBuffer(comm);
if(use_unicasts)
sendToAll(buf);
else
channel.send(new Message(null, buf));
}
catch(Exception ex) {
System.err.println(ex);
}
}
public void actionPerformed(ActionEvent e) {
String command=e.getActionCommand();
switch(command) {
case "Clear":
if(no_channel) {
clearPanel();
return;
}
sendClearPanelMsg();
break;
case "Leave":
stop();
break;
default:
System.out.println("Unknown action");
break;
}
}
public void stop() {
if(!no_channel) {
try {
channel.close();
}
catch(Exception ex) {
System.err.println(ex);
}
}
mainFrame.setVisible(false);
mainFrame.dispose();
}
protected void sendOwnState(final Map<Point,Color> copy) {
if(copy == null)
return;
for(Point point: copy.keySet()) {
// we don't need the color: it is our draw_color anyway
DrawCommand comm=new DrawCommand(DrawCommand.DRAW, point.x, point.y, draw_color.getRGB());
try {
byte[] buf=Util.streamableToByteBuffer(comm);
if(use_unicasts)
sendToAll(buf);
else
channel.send(new Message(null, buf));
}
catch(Exception ex) {
System.err.println(ex);
}
}
}
/* ------------------------------ ChannelListener interface -------------------------- */
public void channelConnected(JChannel channel) {
if(jmx) {
Util.registerChannel(channel, "jgroups");
}
}
public void channelDisconnected(JChannel channel) {
if(jmx) {
MBeanServer server=Util.getMBeanServer();
if(server != null) {
try {
JmxConfigurator.unregisterChannel(channel, server, cluster_name);
}
catch(Exception e) {
e.printStackTrace();
}
}
}
}
public void channelClosed(JChannel channel) {
}
/* --------------------------- End of ChannelListener interface ---------------------- */
protected class DrawPanel extends JPanel implements MouseMotionListener {
protected final Dimension preferred_size=new Dimension(235, 170);
protected Image img; // for drawing pixels
protected Dimension d, imgsize;
protected Graphics gr;
protected final Map<Point,Color> state;
public DrawPanel(boolean use_state) {
if(use_state)
state=new LinkedHashMap<>();
else
state=null;
createOffscreenImage(false);
addMouseMotionListener(this);
addComponentListener(new ComponentAdapter() {
public void componentResized(ComponentEvent e) {
if(getWidth() <= 0 || getHeight() <= 0) return;
createOffscreenImage(false);
}
});
}
public void writeState(OutputStream outstream) throws IOException {
if(state == null)
return;
synchronized(state) {
DataOutputStream dos=new DataOutputStream(new BufferedOutputStream(outstream));
// DataOutputStream dos=new DataOutputStream(outstream);
dos.writeInt(state.size());
for(Map.Entry<Point,Color> entry: state.entrySet()) {
Point point=entry.getKey();
Color col=entry.getValue();
dos.writeInt(point.x);
dos.writeInt(point.y);
dos.writeInt(col.getRGB());
}
dos.flush();
System.out.println("wrote " + state.size() + " elements");
}
}
public void readState(InputStream instream) throws IOException {
DataInputStream in=new DataInputStream(new BufferedInputStream(instream));
Map<Point,Color> new_state=new LinkedHashMap<>();
int num=in.readInt();
for(int i=0; i < num; i++) {
Point point=new Point(in.readInt(), in.readInt());
Color col=new Color(in.readInt());
new_state.put(point, col);
}
synchronized(state) {
state.clear();
state.putAll(new_state);
System.out.println("read " + state.size() + " elements");
createOffscreenImage(true);
}
}
void createOffscreenImage(boolean discard_image) {
d=getSize();
if(discard_image) {
img=null;
imgsize=null;
}
if(img == null || imgsize == null || imgsize.width != d.width || imgsize.height != d.height) {
img=createImage(d.width, d.height);
if(img != null) {
gr=img.getGraphics();
if(gr != null && state != null) {
drawState();
}
}
imgsize=d;
}
repaint();
}
/* ---------------------- MouseMotionListener interface------------------------- */
public void mouseMoved(MouseEvent e) {}
public void mouseDragged(MouseEvent e) {
int x=e.getX(), y=e.getY();
DrawCommand comm=new DrawCommand(DrawCommand.DRAW, x, y, draw_color.getRGB());
if(no_channel) {
drawPoint(comm);
return;
}
try {
byte[] buf=Util.streamableToByteBuffer(comm);
if(use_unicasts)
sendToAll(buf);
else
channel.send(new Message(null, buf));
}
catch(Exception ex) {
System.err.println(ex);
}
}
/* ------------------- End of MouseMotionListener interface --------------------- */
/**
* Adds pixel to queue and calls repaint() whenever we have MAX_ITEMS pixels in the queue
* or when MAX_TIME msecs have elapsed (whichever comes first). The advantage compared to just calling
* repaint() after adding a pixel to the queue is that repaint() can most often draw multiple points
* at the same time.
*/
public void drawPoint(DrawCommand c) {
if(c == null || gr == null) return;
Color col=new Color(c.rgb);
gr.setColor(col);
gr.fillOval(c.x, c.y, 10, 10);
repaint();
if(state != null) {
synchronized(state) {
state.put(new Point(c.x, c.y), col);
}
}
}
public void clear() {
if(gr == null) return;
gr.clearRect(0, 0, getSize().width, getSize().height);
repaint();
if(state != null) {
synchronized(state) {
state.clear();
}
}
}
/** Draw the entire panel from the state */
public void drawState() {
// clear();
Map.Entry entry;
Point pt;
Color col;
synchronized(state) {
for(Iterator it=state.entrySet().iterator(); it.hasNext();) {
entry=(Map.Entry)it.next();
pt=(Point)entry.getKey();
col=(Color)entry.getValue();
gr.setColor(col);
gr.fillOval(pt.x, pt.y, 10, 10);
}
}
repaint();
}
public Dimension getPreferredSize() {
return preferred_size;
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
if(img != null) {
g.drawImage(img, 0, 0, null);
}
}
}
}
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。
public class SimpleChat extends ReceiverAdapter {
JChannel channel;
String user_name=System.getProperty("user.name", "n/a");
final List<String> state=new LinkedList<>();
public void viewAccepted(View new_view) {
System.out.println("** view: " + new_view);
}
public void receive(Message msg) {
String line=msg.getSrc() + ": " + msg.getObject();
System.out.println(line);
synchronized(state) {
state.add(line);
}
}
public void getState(OutputStream output) throws Exception {
synchronized(state) {
Util.objectToStream(state, new DataOutputStream(output));
}
}
@SuppressWarnings("unchecked")
public void setState(InputStream input) throws Exception {
List<String> list=Util.objectFromStream(new DataInputStream(input));
synchronized(state) {
state.clear();
state.addAll(list);
}
System.out.println("received state (" + list.size() + " messages in chat history):");
list.forEach(System.out::println);
}
private void start() throws Exception {
channel=new JChannel().setReceiver(this);
channel.connect("ChatCluster");
channel.getState(null, 10000);
eventLoop();
channel.close();
}
private void eventLoop() {
BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
while(true) {
try {
System.out.print("> "); System.out.flush();
String line=in.readLine().toLowerCase();
if(line.startsWith("quit") || line.startsWith("exit")) {
break;
}
line="[" + user_name + "] " + line;
Message msg=new Message(null, line);
channel.send(msg);
}
catch(Exception e) {
}
}
}
public static void main(String[] args) throws Exception {
new SimpleChat().start();
}
}
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。