一、心跳机制简介
在分布式系统中,分布在不同主机上的节点需要检测其他节点的状态,如服务器节点需要检测从节点是否失效。为了检测对方节点的有效性,每隔固定时间就发送一个固定信息给对方,对方回复一个固定信息,如果长时间没有收到对方的回复,则断开与对方的连接。
发包方既可以是服务端,也可以是客户端,这要看具体实现。因为是每隔固定时间发送一次,类似心跳,所以发送的固定信息称为心跳包。心跳包一般为比较小的包,可根据具体实现。心跳包主要应用于长连接的保持与短线链接。
一般而言,应该客户端主动向服务器发送心跳包,因为服务器向客户端发送心跳包会影响服务器的性能。
二、心跳机制实现方式
心跳机制有两种实现方式,一种基于TCP自带的心跳包,TCP的SO_KEEPALIVE选项可以,系统默认的默认跳帧频率为2小时,超过2小时后,本地的TCP 实现会发送一个数据包给远程的 Socket. 如果远程Socket 没有发回响应, TCP实现就会持续尝试 11 分钟, 直到接收到响应为止。 否则就会自动断开Socket连接。但TCP自带的心跳包无法检测比较敏感地知道对方的状态,默认2小时的空闲时间,对于大多数的应用而言太长了。可以手工开启KeepAlive功能并设置合理的KeepAlive参数。
另一种在应用层自己进行实现,基本步骤如下:
Client使用定时器,不断发送心跳;
Server收到心跳后,回复一个包;
Server为每个Client启动超时定时器,如果在指定时间内没有收到Client的心跳包,则Client失效。
三、Java实现心跳机制
这里基于Java实现的简单RPC框架实现心跳机制。Java实现代码如下所示:
心跳客户端类:
public class HeartbeatClient implements Runnable { private String serverIP = "127.0.0.1"; private int serverPort = 8089; private String nodeID = UUID.randomUUID().toString(); private boolean isRunning = true; // 最近的心跳时间 private long lastHeartbeat; // 心跳间隔时间 private long heartBeatInterval = 10 * 1000; public void run() { try { while (isRunning) { HeartbeatHandler handler = RPClient.getRemoteProxyObj(HeartbeatHandler.class, new InetSocketAddress(serverIP, serverPort)); long startTime = System.currentTimeMillis(); // 是否达到发送心跳的周期时间 if (startTime - lastHeartbeat > heartBeatInterval) { System.out.println("send a heart beat"); lastHeartbeat = startTime; HeartbeatEntity entity = new HeartbeatEntity(); entity.setTime(startTime); entity.setNodeID(nodeID); // 向服务器发送心跳,并返回需要执行的命令 Cmder cmds = handler.sendHeartBeat(entity); if (!processCommand(cmds)) continue; } } } catch (Exception e) { e.printStackTrace(); } } private boolean processCommand(Cmder cmds) { // ... return true; } }
心跳包实体类:
public class HeartbeatEntity implements Serializable { private long time; private String nodeID; private String error; private Map<String, Object> info = new HashMap<String, Object>(); public String getNodeID() { return nodeID; } public void setNodeID(String nodeID) { this.nodeID = nodeID; } public String getError() { return error; } public void setError(String error) { this.error = error; } public Map<String, Object> getInfo() { return info; } public void setInfo(Map<String, Object> info) { this.info = info; } public long getTime() { return time; } public void setTime(long time) { this.time = time; } }
服务器接受心跳包返回的命令对象类:
public class Cmder implements Serializable { private String nodeID; private String error; private Map<String, Object> info = new HashMap<String, Object>(); public String getNodeID() { return nodeID; } public void setNodeID(String nodeID) { this.nodeID = nodeID; } public String getError() { return error; } public void setError(String error) { this.error = error; } public Map<String, Object> getInfo() { return info; } public void setInfo(Map<String, Object> info) { this.info = info; } }
RPC服务注册中心:
public class ServiceCenter { private ExecutorService executor = Executors.newFixedThreadPool(20); private final ConcurrentHashMap<String, Class> serviceRegistry = new ConcurrentHashMap<String, Class>(); private AtomicBoolean isRunning = new AtomicBoolean(true); // 服务器监听端口 private int port = 8089; // 心跳监听器 HeartbeatLinstener linstener; // 单例模式 private static class SingleHolder { private static final ServiceCenter INSTANCE = new ServiceCenter(); } private ServiceCenter() { } public static ServiceCenter getInstance() { return SingleHolder.INSTANCE; } public void register(Class serviceInterface, Class impl) { System.out.println("regeist service " + serviceInterface.getName()); serviceRegistry.put(serviceInterface.getName(), impl); } public void start() throws IOException { ServerSocket server = new ServerSocket(); server.bind(new InetSocketAddress(port)); System.out.println("start server"); linstener = HeartbeatLinstener.getInstance(); System.out.println("start listen heart beat"); try { while (true) { // 1.监听客户端的TCP连接,接到TCP连接后将其封装成task,由线程池执行 executor.execute(new ServiceTask(server.accept())); } } finally { server.close(); } } public void stop() { isRunning.set(false); executor.shutdown(); } public boolean isRunning() { return isRunning.get(); } public int getPort() { return port; } public void settPort(int port) { this.port = port; } public ConcurrentHashMap<String, Class> getServiceRegistry() { return serviceRegistry; } private class ServiceTask implements Runnable { Socket clent = null; public ServiceTask(Socket client) { this.clent = client; } public void run() { ObjectInputStream input = null; ObjectOutputStream output = null; try { // 2.将客户端发送的码流反序列化成对象,反射调用服务实现者,获取执行结果 input = new ObjectInputStream(clent.getInputStream()); String serviceName = input.readUTF(); String methodName = input.readUTF(); Class<?>[] parameterTypes = (Class<?>[]) input.readObject(); Object[] arguments = (Object[]) input.readObject(); Class serviceClass = serviceRegistry.get(serviceName); if (serviceClass == null) { throw new ClassNotFoundException(serviceName + " not found"); } Method method = serviceClass.getMethod(methodName, parameterTypes); Object result = method.invoke(serviceClass.newInstance(), arguments); // 3.将执行结果反序列化,通过socket发送给客户端 output = new ObjectOutputStream(clent.getOutputStream()); output.writeObject(result); } catch (Exception e) { e.printStackTrace(); } finally { if (output != null) { try { output.close(); } catch (IOException e) { e.printStackTrace(); } } if (input != null) { try { input.close(); } catch (IOException e) { e.printStackTrace(); } } if (clent != null) { try { clent.close(); } catch (IOException e) { e.printStackTrace(); } } } } } }
心跳监听类:
package com.cang.heartbeat; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.lang.reflect.Method; import java.net.InetSocketAddress; import java.net.ServerSocket; import java.net.Socket; import java.util.Iterator; import java.util.Map; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.atomic.AtomicBoolean; /** * 心跳监听保存信息 * * @author cang * @create_time 2016-09-28 11:40 */ public class HeartbeatLinstener { private ExecutorService executor = Executors.newFixedThreadPool(20); private final ConcurrentHashMap<String, Object> nodes = new ConcurrentHashMap<String, Object>(); private final ConcurrentHashMap<String, Long> nodeStatus = new ConcurrentHashMap<String, Long>(); private long timeout = 10 * 1000; // 服务器监听端口 private int port = 8089; // 单例模式 private static class SingleHolder { private static final HeartbeatLinstener INSTANCE = new HeartbeatLinstener(); } private HeartbeatLinstener() { } public static HeartbeatLinstener getInstance() { return SingleHolder.INSTANCE; } public ConcurrentHashMap<String, Object> getNodes() { return nodes; } public void registerNode(String nodeId, Object nodeInfo) { nodes.put(nodeId, nodeInfo); nodeStatus.put(nodeId, System.currentTimeMillis()); } public void removeNode(String nodeID) { if (nodes.containsKey(nodeID)) { nodes.remove(nodeID); } } // 检测节点是否有效 public boolean checkNodeValid(String key) { if (!nodes.containsKey(key) || !nodeStatus.containsKey(key)) return false; if ((System.currentTimeMillis() - nodeStatus.get(key)) > timeout) return false; return true; } // 删除所有失效节点 public void removeInValidNode() { Iterator<Map.Entry<String, Long>> it = nodeStatus.entrySet().iterator(); while (it.hasNext()) { Map.Entry<String, Long> e = it.next(); if ((System.currentTimeMillis() - nodeStatus.get(e.getKey())) > timeout) { nodes.remove(e.getKey()); } } } }
心跳处理类接口:
public interface HeartbeatHandler { public Cmder sendHeartBeat(HeartbeatEntity info); }
心跳处理实现类:
public class HeartbeatHandlerImpl implements HeartbeatHandler { public Cmder sendHeartBeat(HeartbeatEntity info) { HeartbeatLinstener linstener = HeartbeatLinstener.getInstance(); // 添加节点 if (!linstener.checkNodeValid(info.getNodeID())) { linstener.registerNode(info.getNodeID(), info); } // 其他操作 Cmder cmder = new Cmder(); cmder.setNodeID(info.getNodeID()); // ... System.out.println("current all the nodes: "); Map<String, Object> nodes = linstener.getNodes(); for (Map.Entry e : nodes.entrySet()) { System.out.println(e.getKey() + " : " + e.getValue()); } System.out.println("hadle a heartbeat"); return cmder; } }
测试类:
public class HeartbeatTest { public static void main(String[] args) { new Thread(new Runnable() { public void run() { try { ServiceCenter serviceServer = ServiceCenter.getInstance(); serviceServer.register(HeartbeatHandler.class, HeartbeatHandlerImpl.class); serviceServer.start(); } catch (IOException e) { e.printStackTrace(); } } }).start(); Thread client1 = new Thread(new HeartbeatClient()); client1.start(); Thread client2 = new Thread(new HeartbeatClient()); client2.start(); } }
四、总结
上面的代码还有很多不足的地方,希望有空能进行改善:
- 配置为硬编码;
- 命令类Cmder没有实际实现,返回的Cmder对象没有实际进行处理;
其他小问题就暂时不管了,希望以后能重写上面的代码。
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