跟着实例学习ZooKeeper的用法： 缓存

转自：http://ifeve.com/zookeeper-path-cache/

可以利用ZooKeeper在集群的各个节点之间缓存数据。 每个节点都可以得到最新的缓存的数据。 Curator提供了三种类型的缓存方式：Path Cache,Node Cache 和Tree Cache。

Path Cache

Path Cache用来监控一个ZNode的子节点. 当一个子节点增加， 更新，删除时， Path Cache会改变它的状态， 会包含最新的子节点， 子节点的数据和状态。 这也正如它的名字表示的那样， 那监控path。

实际使用时会涉及到四个类：

• PathChildrenCache
• PathChildrenCacheEvent
• PathChildrenCacheListener
• ChildData

通过下面的构造函数创建Path Cache:

public PathChildrenCache(CuratorFramework client, String path, boolean cacheData)

想使用cache，必须调用它的start方法，不用之后调用close方法。 start有两个， 其中一个可以传入StartMode，用来为初始的cache设置暖场方式(warm)：

1. NORMAL: 初始时为空。
2. BUILD_INITIAL_CACHE: 在这个方法返回之前调用rebuild()。
3. POST_INITIALIZED_EVENT: 当Cache初始化数据后发送一个PathChildrenCacheEvent.Type#INITIALIZED事件

public void addListener(PathChildrenCacheListener listener)可以增加listener监听缓存的改变。

getCurrentData()方法返回一个List<ChildData>对象，可以遍历所有的子节点。

这个例子摘自官方的例子， 实现了一个控制台的方式操作缓存。 它提供了三个命令， 你可以在控制台中输入。

• set 用来新增或者更新一个子节点的值， 也就是更新一个缓存对象
• remove 是删除一个缓存对象
• list 列出所有的缓存对象

另外还提供了一个help命令提供帮助。

设置/更新、移除其实是使用client (CuratorFramework)来操作, 不通过PathChildrenCache操作：

client.setData().forPath(path, bytes);client.create().creatingParentsIfNeeded().forPath(path, bytes);client.delete().forPath(path);

而查询缓存使用下面的方法：

for (ChildData data : cache.getCurrentData()) {    System.out.println(data.getPath() + " = " + new String(data.getData()));}

Node Cache

Path Cache用来监控一个ZNode. 当节点的数据修改或者删除时，Node Cache能更新它的状态包含最新的改变。

涉及到下面的三个类：

• NodeCache
• NodeCacheListener
• ChildData

想使用cache，依然要调用它的start方法，不用之后调用close方法。

getCurrentData()将得到节点当前的状态，通过它的状态可以得到当前的值。 可以使用public void addListener(NodeCacheListener listener)监控节点状态的改变。

我们依然使用上面的例子框架来演示Node Cache。

package com.colobu.zkrecipe.cache;import java.io.BufferedReader;import java.io.InputStreamReader;import java.util.Arrays;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.cache.NodeCache;import org.apache.curator.framework.recipes.cache.NodeCacheListener;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.test.TestingServer;import org.apache.curator.utils.CloseableUtils;import org.apache.zookeeper.KeeperException;public class NodeCacheExample {    private static final String PATH = "/example/nodeCache";    public static void main(String[] args) throws Exception {        TestingServer server = new TestingServer();        CuratorFramework client = null;        NodeCache cache = null;        try {            client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));            client.start();            cache = new NodeCache(client, PATH);            cache.start();            processCommands(client, cache);        } finally {            CloseableUtils.closeQuietly(cache);            CloseableUtils.closeQuietly(client);            CloseableUtils.closeQuietly(server);        }    }    private static void addListener(final NodeCache cache) {        // a PathChildrenCacheListener is optional. Here, it's used just to log        // changes        NodeCacheListener listener = new NodeCacheListener() {            @Override            public void nodeChanged() throws Exception {                if (cache.getCurrentData() != null)                    System.out.println("Node changed: " + cache.getCurrentData().getPath() + ", value: " + new String(cache.getCurrentData().getData()));            }        };        cache.getListenable().addListener(listener);    }    private static void processCommands(CuratorFramework client, NodeCache cache) throws Exception {        printHelp();        try {            addListener(cache);            BufferedReader in = new BufferedReader(new InputStreamReader(System.in));            boolean done = false;            while (!done) {                System.out.print("> ");                String line = in.readLine();                if (line == null) {                    break;                }                String command = line.trim();                String[] parts = command.split("\\s");                if (parts.length == 0) {                    continue;                }                String operation = parts[0];                String args[] = Arrays.copyOfRange(parts, 1, parts.length);                if (operation.equalsIgnoreCase("help") || operation.equalsIgnoreCase("?")) {                    printHelp();                } else if (operation.equalsIgnoreCase("q") || operation.equalsIgnoreCase("quit")) {                    done = true;                } else if (operation.equals("set")) {                    setValue(client, command, args);                } else if (operation.equals("remove")) {                    remove(client);                } else if (operation.equals("show")) {                    show(cache);                }                Thread.sleep(1000); // just to allow the console output to catch                                    // up            }        } catch (Exception ex) {            ex.printStackTrace();        } finally {        }    }    private static void show(NodeCache cache) {        if (cache.getCurrentData() != null)            System.out.println(cache.getCurrentData().getPath() + " = " + new String(cache.getCurrentData().getData()));        else            System.out.println("cache don't set a value");    }    private static void remove(CuratorFramework client) throws Exception {        try {            client.delete().forPath(PATH);        } catch (KeeperException.NoNodeException e) {            // ignore        }    }    private static void setValue(CuratorFramework client, String command, String[] args) throws Exception {        if (args.length != 1) {            System.err.println("syntax error (expected set <value>): " + command);            return;        }        byte[] bytes = args[0].getBytes();        try {            client.setData().forPath(PATH, bytes);        } catch (KeeperException.NoNodeException e) {            client.create().creatingParentsIfNeeded().forPath(PATH, bytes);        }    }    private static void printHelp() {        System.out.println("An example of using PathChildrenCache. This example is driven by entering commands at the prompt:\n");        System.out.println("set <value>: Adds or updates a node with the given name");        System.out.println("remove: Deletes the node with the given name");        System.out.println("show: Display the node's value in the cache");        System.out.println("quit: Quit the example");        System.out.println();    }}

操作和上面的Path cache类似， 只是getCurrentData()返回的类型不同

Tree Node

这种类型的即可以监控节点的状态，还监控节点的子节点的状态， 类似上面两种cache的组合。 这也就是Tree的概念。 它监控整个树中节点的状态。 涉及到下面四个类。

• TreeCache
• TreeCacheListener
• TreeCacheEvent
• ChildData

而关键的TreeCache的构造函数为

public TreeCache(CuratorFramework client, String path, boolean cacheData)

想使用cache，依然要调用它的start方法，不用之后调用close方法。

getCurrentChildren()返回cache的状态，类型为Map<String, ChildData>。 而getCurrentData()返回监控的path的数据。

public void addListener(TreeCacheListener listener)可以增加listener来监控状态的改变。

例子依然采用和上面的例子类似， 尤其和Path Cache类似。

package com.colobu.zkrecipe.cache;import java.io.BufferedReader;import java.io.InputStreamReader;import java.util.Arrays;import java.util.Map;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.cache.ChildData;import org.apache.curator.framework.recipes.cache.TreeCache;import org.apache.curator.framework.recipes.cache.TreeCacheEvent;import org.apache.curator.framework.recipes.cache.TreeCacheListener;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.test.TestingServer;import org.apache.curator.utils.CloseableUtils;import org.apache.curator.utils.ZKPaths;import org.apache.zookeeper.KeeperException;public class TreeCacheExample {    private static final String PATH = "/example/treeCache";    public static void main(String[] args) throws Exception {        TestingServer server = new TestingServer();        CuratorFramework client = null;        TreeCache cache = null;        try {            client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));            client.start();            cache = new TreeCache(client, PATH);            cache.start();            processCommands(client, cache);        } finally {            CloseableUtils.closeQuietly(cache);            CloseableUtils.closeQuietly(client);            CloseableUtils.closeQuietly(server);        }    }    private static void addListener(final TreeCache cache) {        TreeCacheListener listener = new TreeCacheListener() {            @Override            public void childEvent(CuratorFramework client, TreeCacheEvent event) throws Exception {                switch (event.getType()) {                case NODE_ADDED: {                    System.out.println("TreeNode added: " + ZKPaths.getNodeFromPath(event.getData().getPath()) + ", value: "                            + new String(event.getData().getData()));                    break;                }                case NODE_UPDATED: {                    System.out.println("TreeNode changed: " + ZKPaths.getNodeFromPath(event.getData().getPath()) + ", value: "                            + new String(event.getData().getData()));                    break;                }                case NODE_REMOVED: {                    System.out.println("TreeNode removed: " + ZKPaths.getNodeFromPath(event.getData().getPath()));                    break;                }                default:                    System.out.println("Other event: " + event.getType().name());                }            }        };        cache.getListenable().addListener(listener);    }    private static void processCommands(CuratorFramework client, TreeCache cache) throws Exception {        // More scaffolding that does a simple command line processor        printHelp();        try {            addListener(cache);            BufferedReader in = new BufferedReader(new InputStreamReader(System.in));            boolean done = false;            while (!done) {                System.out.print("> ");                String line = in.readLine();                if (line == null) {                    break;                }                String command = line.trim();                String[] parts = command.split("\\s");                if (parts.length == 0) {                    continue;                }                String operation = parts[0];                String args[] = Arrays.copyOfRange(parts, 1, parts.length);                if (operation.equalsIgnoreCase("help") || operation.equalsIgnoreCase("?")) {                    printHelp();                } else if (operation.equalsIgnoreCase("q") || operation.equalsIgnoreCase("quit")) {                    done = true;                } else if (operation.equals("set")) {                    setValue(client, command, args);                } else if (operation.equals("remove")) {                    remove(client, command, args);                } else if (operation.equals("list")) {                    list(cache);                }                Thread.sleep(1000); // just to allow the console output to catch                                    // up            }        } finally {        }    }    private static void list(TreeCache cache) {        if (cache.getCurrentChildren(PATH).size() == 0) {            System.out.println("* empty *");        } else {                        for (Map.Entry<String, ChildData> entry : cache.getCurrentChildren(PATH).entrySet()) {                System.out.println(entry.getKey() + " = " + new String(entry.getValue().getData()));            }        }    }    private static void remove(CuratorFramework client, String command, String[] args) throws Exception {        if (args.length != 1) {            System.err.println("syntax error (expected remove <path>): " + command);            return;        }        String name = args[0];        if (name.contains("/")) {            System.err.println("Invalid node name" + name);            return;        }        String path = ZKPaths.makePath(PATH, name);        try {            client.delete().forPath(path);        } catch (KeeperException.NoNodeException e) {            // ignore        }    }    private static void setValue(CuratorFramework client, String command, String[] args) throws Exception {        if (args.length != 2) {            System.err.println("syntax error (expected set <path> <value>): " + command);            return;        }        String name = args[0];        if (name.contains("/")) {            System.err.println("Invalid node name" + name);            return;        }        String path = ZKPaths.makePath(PATH, name);        byte[] bytes = args[1].getBytes();        try {            client.setData().forPath(path, bytes);        } catch (KeeperException.NoNodeException e) {            client.create().creatingParentsIfNeeded().forPath(path, bytes);        }    }    private static void printHelp() {        System.out.println("An example of using PathChildrenCache. This example is driven by entering commands at the prompt:\n");        System.out.println("set <name> <value>: Adds or updates a node with the given name");        System.out.println("remove <name>: Deletes the node with the given name");        System.out.println("list: List the nodes/values in the cache");        System.out.println("quit: Quit the example");        System.out.println();    }}