INT-3616: Zookeeper LockRegistry

JIRA: https://jira.spring.io/browse/INT-3616

Zookeeper implementation of `LockRegistry`, for example to support
clustered aggregators (when using external message group stores).

INT-3616: Polishing; PR Comments

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/lock/ZookeeperLockRegistry.java

@@ -0,0 +1,262 @@
+/*
+ * Copyright 2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.integration.zookeeper.lock;
+
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.Lock;
+
+import org.apache.curator.framework.CuratorFramework;
+import org.apache.curator.framework.recipes.locks.InterProcessMutex;
+
+import org.springframework.integration.support.locks.ExpirableLockRegistry;
+import org.springframework.messaging.MessagingException;
+import org.springframework.util.Assert;
+
+/**
+ * {@link ExpirableLockRegistry} implementation using Zookeeper, or more specifically,
+ * Curator {@link InterProcessMutex}.
+ *
+ * @author Gary Russell
+ * @since 4.2
+ *
+ */
+public class ZookeeperLockRegistry implements ExpirableLockRegistry {
+
+	private static final String DEFAULT_ROOT = "/SpringIntegration-LockRegistry";
+
+	private final CuratorFramework client;
+
+	private final KeyToPathStrategy keyToPath;
+
+	private final Map<String, ZkLock> locks = new HashMap<String, ZkLock>();
+
+	private final boolean trackingTime;
+
+	/**
+	 * Construct a lock registry using the default {@link KeyToPathStrategy} which
+	 * simple appends the key to '/SpringIntegration-LockRegistry/'.
+	 * @param client the {@link CuratorFramework}.
+	 */
+	public ZookeeperLockRegistry(CuratorFramework client) {
+		this(client, DEFAULT_ROOT);
+	}
+
+	/**
+	 * Construct a lock registry using the default {@link KeyToPathStrategy} which
+	 * simple appends the key to {@code '<root>/'}.
+	 * @param client the {@link CuratorFramework}.
+	 * @param path the path root (no trailing /).
+	 */
+	public ZookeeperLockRegistry(CuratorFramework client, String root) {
+		this(client, new DefaultKeyToPathStrategy(root));
+	}
+
+	/**
+	 * Construct a lock registry using the supplied {@link KeyToPathStrategy}.
+	 * @param client the {@link CuratorFramework}.
+	 * @param keyToPath the implementation of {@link KeyToPathStrategy}.
+	 */
+	public ZookeeperLockRegistry(CuratorFramework client, KeyToPathStrategy keyToPath) {
+		Assert.notNull(client, "'client' cannot be null");
+		Assert.notNull(client, "'keyToPath' cannot be null");
+		this.client = client;
+		this.keyToPath = keyToPath;
+		this.trackingTime = !keyToPath.bounded();
+	}
+
+	@Override
+	public Lock obtain(Object lockKey) {
+		Assert.isInstanceOf(String.class, lockKey);
+		String path = this.keyToPath.pathFor((String) lockKey);
+		ZkLock lock = this.locks.get(path);
+		if (lock == null) {
+			synchronized (this.locks) {
+				lock = this.locks.get(path);
+				if (lock == null) {
+					lock = new ZkLock(this.client, path);
+					this.locks.put(path, lock);
+				}
+				if (this.trackingTime) {
+					lock.setLastUsed(System.currentTimeMillis());
+				}
+			}
+		}
+		return lock;
+	}
+
+	/**
+	 * Remove locks last acquired more than 'age' ago that are not currently locked.
+	 * Expiry is not supported if the {@link KeyToPathStrategy} is bounded (returns a finite
+	 * number of paths). With such a {@link KeyToPathStrategy}, the overhead of tracking when
+	 * a lock is obtained is avoided.
+	 * @param age the time since the lock was last obtained.
+	 */
+	@Override
+	public void expireUnusedOlderThan(long age) {
+		if (!this.trackingTime) {
+			throw new IllegalStateException("Ths KeyToPathStrategy is bounded; expiry is not supported");
+		}
+		synchronized(this.locks) {
+			Iterator<Entry<String, ZkLock>> iterator = this.locks.entrySet().iterator();
+			long now = System.currentTimeMillis();
+			while(iterator.hasNext()) {
+				Entry<String, ZkLock> entry = iterator.next();
+				ZkLock lock = entry.getValue();
+				if (now - lock.getLastUsed() > age
+						&& !lock.isAcquiredInThisProcess()) {
+					iterator.remove();
+				}
+			}
+		}
+	}
+
+	/**
+	 * Strategy to convert a lock key (e.g. aggregation correlation id) to a
+	 * Zookeeper path.
+	 *
+	 */
+	public interface KeyToPathStrategy {
+
+		/**
+		 * Return the path for the key.
+		 * @param key the key.
+		 * @return the path.
+		 */
+		String pathFor(String key);
+
+		/**
+		 * @return true if this strategy returns a bounded number of locks, removing
+		 * the need for removing LRU locks.
+		 */
+		boolean bounded();
+
+	}
+
+	private static class DefaultKeyToPathStrategy implements KeyToPathStrategy {
+
+		private final String root;
+
+		public DefaultKeyToPathStrategy(String rootPath) {
+			Assert.notNull(rootPath, "'rootPath' cannot be null");
+			if (!rootPath.endsWith("/")) {
+				this.root = rootPath + "/";
+			}
+			else {
+				this.root = rootPath;
+			}
+		}
+
+		@Override
+		public String pathFor(String key) {
+			return root + key;
+		}
+
+		@Override
+		public boolean bounded() {
+			return false;
+		}
+
+	}
+
+	private static class ZkLock implements Lock {
+
+		private final InterProcessMutex mutex;
+
+		private final String path;
+
+		private long lastUsed;
+
+		public ZkLock(CuratorFramework client, String path) {
+			this.mutex = new InterProcessMutex(client, path);
+			this.path = path;
+		}
+
+		public long getLastUsed() {
+			return this.lastUsed;
+		}
+
+		public void setLastUsed(long lastUsed) {
+			this.lastUsed = lastUsed;
+		}
+
+		@Override
+		public void lock() {
+			try {
+				this.mutex.acquire();
+			}
+			catch (Exception e) {
+				throw new RuntimeException("Failed to aquire mutex at " + this.path, e);
+			}
+		}
+
+		@Override
+		public void lockInterruptibly() throws InterruptedException {
+			boolean locked = false;
+			// this is a bit ugly, but...
+			while (!locked) {
+				locked = tryLock(1, TimeUnit.SECONDS);
+			}
+
+		}
+
+		@Override
+		public boolean tryLock() {
+			try {
+				return tryLock(0, TimeUnit.MICROSECONDS);
+			}
+			catch (InterruptedException e) {
+				Thread.currentThread().interrupt();
+				return false;
+			}
+		}
+
+		@Override
+		public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
+			try {
+				return this.mutex.acquire(time, unit);
+			}
+			catch (Exception e) {
+				throw new MessagingException("Failed to aquire mutex at " + this.path, e);
+			}
+		}
+
+		@Override
+		public void unlock() {
+			try {
+				this.mutex.release();
+			}
+			catch (Exception e) {
+				throw new MessagingException("Failed to release mutex at " + this.path, e);
+			}
+		}
+
+		@Override
+		public Condition newCondition() {
+			throw new UnsupportedOperationException("Conditions are not supported");
+		}
+
+		public boolean isAcquiredInThisProcess() {
+			return this.mutex.isAcquiredInThisProcess();
+		}
+
+	}
+
+}

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/metadata/ZookeeperMetadataStore.java

@@ -42,6 +42,7 @@ import org.springframework.util.Assert;
  * the names of which are stored as keys.
  *
  * @author Marius Bogoevici
+ * @since 4.2
  */
 public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLifecycle {
 
@@ -320,9 +321,9 @@ public class ZookeeperMetadataStore implements ListenableMetadataStore, SmartLif
 
 	private static class LocalChildData {
 
-		private String value;
+		private final String value;
 
-		private int version;
+		private final int version;
 
 		public LocalChildData(String value, int version) {
 			this.value = value;

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/metadata/ZookeeperMetadataStoreException.java

@@ -18,6 +18,7 @@ package org.springframework.integration.zookeeper.metadata;
 
 /**
  * @author Marius Bogoevici
+ * @since 4.2
  */
 @SuppressWarnings("serial")
 public class ZookeeperMetadataStoreException extends RuntimeException {