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。