HTTPCLIENT-1824, HTTPCLIENT-1868: Asynchronous HTTP cache storage API; Memcached backend implementation of async HTTP cache storage

httpclient5-cache/src/main/java/org/apache/hc/client5/http/cache/HttpAsyncCacheStorage.java

@@ -0,0 +1,90 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.cache;
+
+import org.apache.hc.core5.concurrent.Cancellable;
+import org.apache.hc.core5.concurrent.FutureCallback;
+
+/**
+ * {@literal HttpAsyncCacheStorage} represents an abstract HTTP cache
+ * storage backend that can then be plugged into the  asynchronous
+ * (non-blocking ) request execution
+ * pipeline.
+ *
+ * @since 5.0
+ */
+public interface HttpAsyncCacheStorage {
+
+    Cancellable NOOP_CANCELLABLE = new Cancellable() {
+
+        @Override
+        public boolean cancel() {
+            return false;
+        }
+
+    };
+
+    /**
+     * Store a given cache entry under the given key.
+     * @param key where in the cache to store the entry
+     * @param entry cached response to store
+     * @param callback result callback
+     */
+    Cancellable putEntry(
+            String key, HttpCacheEntry entry, FutureCallback<Boolean> callback);
+
+    /**
+     * Retrieves the cache entry stored under the given key
+     * or null if no entry exists under that key.
+     * @param key cache key
+     * @param callback result callback
+     * @return an {@link HttpCacheEntry} or {@code null} if no
+     *   entry exists
+     */
+    Cancellable getEntry(
+            String key, FutureCallback<HttpCacheEntry> callback);
+
+    /**
+     * Deletes/invalidates/removes any cache entries currently
+     * stored under the given key.
+     * @param key
+     * @param callback result callback
+     */
+    Cancellable removeEntry(
+            String key, FutureCallback<Boolean> callback);
+
+    /**
+     * Atomically applies the given callback to processChallenge an existing cache
+     * entry under a given key.
+     * @param key indicates which entry to modify
+     * @param casOperation the CAS operation to perform.
+     * @param callback result callback
+     */
+    Cancellable updateEntry(
+            String key, HttpCacheCASOperation casOperation, FutureCallback<Boolean> callback);
+
+}

httpclient5-cache/src/main/java/org/apache/hc/client5/http/cache/HttpAsyncCacheStorageAdaptor.java

@@ -0,0 +1,98 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.cache;
+
+import org.apache.hc.core5.concurrent.Cancellable;
+import org.apache.hc.core5.concurrent.FutureCallback;
+import org.apache.hc.core5.util.Args;
+
+/**
+ * {@link HttpAsyncCacheStorage} implementation that emulates asynchronous
+ * behavior using an instance of classic {@link HttpCacheStorage}.
+ *
+ * @since 5.0
+ */
+public final class HttpAsyncCacheStorageAdaptor implements HttpAsyncCacheStorage {
+
+    private final HttpCacheStorage cacheStorage;
+
+    public HttpAsyncCacheStorageAdaptor(final HttpCacheStorage cacheStorage) {
+        this.cacheStorage = Args.notNull(cacheStorage, "Cache strorage");
+    }
+
+    public Cancellable putEntry(final String key, final HttpCacheEntry entry, final FutureCallback<Boolean> callback) {
+        Args.notEmpty(key, "Key");
+        Args.notNull(entry, "Cache ehtry");
+        Args.notNull(callback, "Callback");
+        try {
+            cacheStorage.putEntry(key, entry);
+            callback.completed(Boolean.TRUE);
+        } catch (final Exception ex) {
+            callback.failed(ex);
+        }
+        return NOOP_CANCELLABLE;
+    }
+
+    public Cancellable getEntry(final String key, final FutureCallback<HttpCacheEntry> callback) {
+        Args.notEmpty(key, "Key");
+        Args.notNull(callback, "Callback");
+        try {
+            final HttpCacheEntry entry = cacheStorage.getEntry(key);
+            callback.completed(entry);
+        } catch (final Exception ex) {
+            callback.failed(ex);
+        }
+        return NOOP_CANCELLABLE;
+    }
+
+    public Cancellable removeEntry(final String key, final FutureCallback<Boolean> callback) {
+        Args.notEmpty(key, "Key");
+        Args.notNull(callback, "Callback");
+        try {
+            cacheStorage.removeEntry(key);
+            callback.completed(Boolean.TRUE);
+        } catch (final Exception ex) {
+            callback.failed(ex);
+        }
+        return NOOP_CANCELLABLE;
+    }
+
+    public Cancellable updateEntry(
+            final String key, final HttpCacheCASOperation casOperation, final FutureCallback<Boolean> callback) {
+        Args.notEmpty(key, "Key");
+        Args.notNull(casOperation, "CAS operation");
+        Args.notNull(callback, "Callback");
+        try {
+            cacheStorage.updateEntry(key, casOperation);
+            callback.completed(Boolean.TRUE);
+        } catch (final Exception ex) {
+            callback.failed(ex);
+        }
+        return NOOP_CANCELLABLE;
+    }
+
+}

httpclient5-cache/src/main/java/org/apache/hc/client5/http/cache/HttpCacheStorage.java

@@ -27,9 +27,9 @@
 package org.apache.hc.client5.http.cache;
 
 /**
- * New storage backends should implement this {@link HttpCacheStorage}
- * interface. They can then be plugged into the existing caching
- * {@link org.apache.hc.client5.http.classic.HttpClient} implementation.
+ * {@literal HttpCacheStorage} represents an abstract HTTP cache
+ * storage backend that can then be plugged into the classic
+ * (blocking) request execution pipeline.
  *
  * @since 4.1
  */

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/AbstractBinaryAsyncCacheStorage.java

@@ -0,0 +1,46 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.impl.cache;
+
+import org.apache.hc.client5.http.cache.HttpCacheEntrySerializer;
+
+/**
+ * Abstract cache backend for serialized binary objects capable of CAS (compare-and-swap) updates.
+ *
+ * @since 5.0
+ */
+public abstract class AbstractBinaryAsyncCacheStorage<CAS> extends AbstractSerializingAsyncCacheStorage<byte[], CAS> {
+
+    public AbstractBinaryAsyncCacheStorage(final int maxUpdateRetries, final HttpCacheEntrySerializer<byte[]> serializer) {
+        super(maxUpdateRetries, serializer);
+    }
+
+    public AbstractBinaryAsyncCacheStorage(final int maxUpdateRetries) {
+        super(maxUpdateRetries, ByteArrayCacheEntrySerializer.INSTANCE);
+    }
+
+}

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/AbstractSerializingAsyncCacheStorage.java

@@ -0,0 +1,228 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.impl.cache;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.apache.hc.client5.http.cache.HttpAsyncCacheStorage;
+import org.apache.hc.client5.http.cache.HttpCacheCASOperation;
+import org.apache.hc.client5.http.cache.HttpCacheEntry;
+import org.apache.hc.client5.http.cache.HttpCacheEntrySerializer;
+import org.apache.hc.client5.http.cache.HttpCacheStorageEntry;
+import org.apache.hc.client5.http.cache.HttpCacheUpdateException;
+import org.apache.hc.client5.http.cache.ResourceIOException;
+import org.apache.hc.core5.concurrent.Cancellable;
+import org.apache.hc.core5.concurrent.FutureCallback;
+import org.apache.hc.core5.util.Args;
+
+/**
+ * Abstract cache backend for serialized objects capable of CAS (compare-and-swap) updates.
+ *
+ * @since 5.0
+ */
+public abstract class AbstractSerializingAsyncCacheStorage<T, CAS> implements HttpAsyncCacheStorage {
+
+    private final int maxUpdateRetries;
+    private final HttpCacheEntrySerializer<T> serializer;
+
+    public AbstractSerializingAsyncCacheStorage(final int maxUpdateRetries, final HttpCacheEntrySerializer<T> serializer) {
+        this.maxUpdateRetries = Args.notNegative(maxUpdateRetries, "Max retries");
+        this.serializer = Args.notNull(serializer, "Cache entry serializer");
+    }
+
+    protected abstract String digestToStorageKey(String key);
+
+    protected abstract T getStorageObject(CAS cas) throws ResourceIOException;
+
+    protected abstract Cancellable store(String storageKey, T storageObject, FutureCallback<Boolean> callback);
+
+    protected abstract Cancellable restore(String storageKey, FutureCallback<T> callback);
+
+    protected abstract Cancellable getForUpdateCAS(String storageKey, FutureCallback<CAS> callback);
+
+    protected abstract Cancellable updateCAS(String storageKey, CAS cas, T storageObject, FutureCallback<Boolean> callback);
+
+    protected abstract Cancellable delete(String storageKey, FutureCallback<Boolean> callback);
+
+    @Override
+    public final Cancellable putEntry(
+            final String key, final HttpCacheEntry entry, final FutureCallback<Boolean> callback) {
+        Args.notNull(key, "Storage key");
+        Args.notNull(callback, "Callback");
+        try {
+            final String storageKey = digestToStorageKey(key);
+            final T storageObject = serializer.serialize(new HttpCacheStorageEntry(key, entry));
+            return store(storageKey, storageObject, callback);
+        } catch (final Exception ex) {
+            callback.failed(ex);
+            return NOOP_CANCELLABLE;
+        }
+    }
+
+    @Override
+    public final Cancellable getEntry(final String key, final FutureCallback<HttpCacheEntry> callback) {
+        Args.notNull(key, "Storage key");
+        Args.notNull(callback, "Callback");
+        try {
+            final String storageKey = digestToStorageKey(key);
+            return restore(storageKey, new FutureCallback<T>() {
+
+                @Override
+                public void completed(final T storageObject) {
+                    try {
+                        if (storageObject != null) {
+                            final HttpCacheStorageEntry entry = serializer.deserialize(storageObject);
+                            if (key.equals(entry.getKey())) {
+                                callback.completed(entry.getContent());
+                            } else {
+                                callback.completed(null);
+                            }
+                        } else {
+                            callback.completed(null);
+                        }
+                    } catch (final Exception ex) {
+                        callback.failed(ex);
+                    }
+                }
+
+                @Override
+                public void failed(final Exception ex) {
+                    callback.failed(ex);
+                }
+
+                @Override
+                public void cancelled() {
+                    callback.cancelled();
+                }
+
+            });
+        } catch (final Exception ex) {
+            callback.failed(ex);
+            return NOOP_CANCELLABLE;
+        }
+    }
+
+    @Override
+    public final Cancellable removeEntry(final String key, final FutureCallback<Boolean> callback) {
+        Args.notNull(key, "Storage key");
+        Args.notNull(callback, "Callback");
+        try {
+            final String storageKey = digestToStorageKey(key);
+            return delete(storageKey, callback);
+        } catch (final Exception ex) {
+            callback.failed(ex);
+            return NOOP_CANCELLABLE;
+        }
+    }
+
+    @Override
+    public final Cancellable updateEntry(
+            final String key, final HttpCacheCASOperation casOperation, final FutureCallback<Boolean> callback) {
+        Args.notNull(key, "Storage key");
+        Args.notNull(casOperation, "CAS operation");
+        Args.notNull(callback, "Callback");
+        final ComplexCancellable complexCancellable = new ComplexCancellable();
+        final AtomicInteger count = new AtomicInteger(0);
+        atemmptUpdateEntry(key, casOperation, complexCancellable, count, callback);
+        return complexCancellable;
+    }
+
+    private void atemmptUpdateEntry(
+            final String key,
+            final HttpCacheCASOperation casOperation,
+            final ComplexCancellable complexCancellable,
+            final AtomicInteger count,
+            final FutureCallback<Boolean> callback) {
+        try {
+            final String storageKey = digestToStorageKey(key);
+            complexCancellable.setDependency(getForUpdateCAS(storageKey, new FutureCallback<CAS>() {
+
+                @Override
+                public void completed(final CAS cas) {
+                    try {
+                        HttpCacheStorageEntry storageEntry = cas != null ? serializer.deserialize(getStorageObject(cas)) : null;
+                        if (storageEntry != null && !key.equals(storageEntry.getKey())) {
+                            storageEntry = null;
+                        }
+                        final HttpCacheEntry existingEntry = storageEntry != null ? storageEntry.getContent() : null;
+                        final HttpCacheEntry updatedEntry = casOperation.execute(existingEntry);
+                        if (existingEntry == null) {
+                            putEntry(key, updatedEntry, callback);
+                        } else {
+                            final T storageObject = serializer.serialize(new HttpCacheStorageEntry(key, updatedEntry));
+                            complexCancellable.setDependency(updateCAS(storageKey, cas, storageObject, new FutureCallback<Boolean>() {
+
+                                @Override
+                                public void completed(final Boolean result) {
+                                    if (result) {
+                                        callback.completed(result);
+                                    } else {
+                                        if (!complexCancellable.isCancelled()) {
+                                            final int numRetries = count.incrementAndGet();
+                                            if (numRetries >= maxUpdateRetries) {
+                                                callback.failed(new HttpCacheUpdateException("Cache update failed after " + numRetries + " retries"));
+                                            } else {
+                                                atemmptUpdateEntry(key, casOperation, complexCancellable, count, callback);
+                                            }
+                                        }
+                                    }
+                                }
+
+                                @Override
+                                public void failed(final Exception ex) {
+                                    callback.failed(ex);
+                                }
+
+                                @Override
+                                public void cancelled() {
+                                    callback.cancelled();
+                                }
+
+                            }));
+                        }
+                    } catch (final Exception ex) {
+                        callback.failed(ex);
+                    }
+                }
+
+                @Override
+                public void failed(final Exception ex) {
+                    callback.failed(ex);
+                }
+
+                @Override
+                public void cancelled() {
+                    callback.cancelled();
+                }
+
+            }));
+        } catch (final Exception ex) {
+            callback.failed(ex);
+        }
+    }
+
+}

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/ComplexCancellable.java

@@ -0,0 +1,74 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.impl.cache;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+
+import org.apache.hc.core5.concurrent.Cancellable;
+import org.apache.hc.core5.util.Args;
+
+/**
+ * TODO: replace with ComplexCancellable from HttpCore 5.0b2
+ */
+final class ComplexCancellable implements Cancellable {
+
+    private final AtomicReference<Cancellable> dependencyRef;
+    private final AtomicBoolean cancelled;
+
+    public ComplexCancellable() {
+        this.dependencyRef = new AtomicReference<>(null);
+        this.cancelled = new AtomicBoolean(false);
+    }
+
+    public boolean isCancelled() {
+        return cancelled.get();
+    }
+
+    public void setDependency(final Cancellable dependency) {
+        Args.notNull(dependency, "dependency");
+        if (!cancelled.get()) {
+            dependencyRef.set(dependency);
+        } else {
+            dependency.cancel();
+        }
+    }
+
+    @Override
+    public boolean cancel() {
+        if (cancelled.compareAndSet(false, true)) {
+            final Cancellable dependency = dependencyRef.getAndSet(null);
+            if (dependency != null) {
+                dependency.cancel();
+            }
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+}

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/memcached/MemcachedHttpAsyncCacheStorage.java

@@ -0,0 +1,250 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.impl.cache.memcached;
+
+import java.io.IOException;
+import java.net.InetSocketAddress;
+import java.util.concurrent.ExecutionException;
+
+import org.apache.hc.client5.http.cache.HttpCacheEntrySerializer;
+import org.apache.hc.client5.http.cache.ResourceIOException;
+import org.apache.hc.client5.http.impl.cache.AbstractBinaryAsyncCacheStorage;
+import org.apache.hc.client5.http.impl.cache.ByteArrayCacheEntrySerializer;
+import org.apache.hc.client5.http.impl.cache.CacheConfig;
+import org.apache.hc.core5.concurrent.Cancellable;
+import org.apache.hc.core5.concurrent.FutureCallback;
+import org.apache.hc.core5.util.Args;
+
+import net.spy.memcached.CASResponse;
+import net.spy.memcached.CASValue;
+import net.spy.memcached.MemcachedClient;
+import net.spy.memcached.internal.GetCompletionListener;
+import net.spy.memcached.internal.GetFuture;
+import net.spy.memcached.internal.OperationCompletionListener;
+import net.spy.memcached.internal.OperationFuture;
+
+/**
+ * <p>
+ * This class is a storage backend that uses an external <i>memcached</i>
+ * for storing cached origin responses. This storage option provides a
+ * couple of interesting advantages over the default in-memory storage
+ * backend:
+ * </p>
+ * <ol>
+ * <li>in-memory cached objects can survive an application restart since
+ * they are held in a separate process</li>
+ * <li>it becomes possible for several cooperating applications to share
+ * a large <i>memcached</i> farm together</li>
+ * </ol>
+ * <p>
+ * Note that in a shared memcached pool setting you may wish to make use
+ * of the Ketama consistent hashing algorithm to reduce the number of
+ * cache misses that might result if one of the memcached cluster members
+ * fails (see the <a href="http://dustin.github.com/java-memcached-client/apidocs/net/spy/memcached/KetamaConnectionFactory.html">
+ * KetamaConnectionFactory</a>).
+ * </p>
+ * <p>
+ * Because memcached places limits on the size of its keys, we need to
+ * introduce a key hashing scheme to map the annotated URLs the higher-level
+ * caching HTTP client wants to use as keys onto ones that are suitable
+ * for use with memcached. Please see {@link KeyHashingScheme} if you would
+ * like to use something other than the provided {@link SHA256KeyHashingScheme}.
+ * </p>
+ *
+ * <p>
+ * Please refer to the <a href="http://code.google.com/p/memcached/wiki/NewStart">
+ * memcached documentation</a> and in particular to the documentation for
+ * the <a href="http://code.google.com/p/spymemcached/">spymemcached
+ * documentation</a> for details about how to set up and configure memcached
+ * and the Java client used here, respectively.
+ * </p>
+ *
+ * @since 5.0
+ */
+public class MemcachedHttpAsyncCacheStorage extends AbstractBinaryAsyncCacheStorage<CASValue<Object>> {
+
+    private final MemcachedClient client;
+    private final KeyHashingScheme keyHashingScheme;
+
+    /**
+     * Create a storage backend talking to a <i>memcached</i> instance
+     * listening on the specified host and port. This is useful if you
+     * just have a single local memcached instance running on the same
+     * machine as your application, for example.
+     * @param address where the <i>memcached</i> daemon is running
+     * @throws IOException in case of an error
+     */
+    public MemcachedHttpAsyncCacheStorage(final InetSocketAddress address) throws IOException {
+        this(new MemcachedClient(address));
+    }
+
+    /**
+     * Create a storage backend using the pre-configured given
+     * <i>memcached</i> client.
+     * @param cache client to use for communicating with <i>memcached</i>
+     */
+    public MemcachedHttpAsyncCacheStorage(final MemcachedClient cache) {
+        this(cache, CacheConfig.DEFAULT, ByteArrayCacheEntrySerializer.INSTANCE, SHA256KeyHashingScheme.INSTANCE);
+    }
+
+    /**
+     * Create a storage backend using the given <i>memcached</i> client and
+     * applying the given cache configuration, serialization, and hashing
+     * mechanisms.
+     * @param client how to talk to <i>memcached</i>
+     * @param config apply HTTP cache-related options
+     * @param serializer alternative serialization mechanism
+     * @param keyHashingScheme how to map higher-level logical "storage keys"
+     *   onto "cache keys" suitable for use with memcached
+     */
+    public MemcachedHttpAsyncCacheStorage(
+            final MemcachedClient client,
+            final CacheConfig config,
+            final HttpCacheEntrySerializer<byte[]> serializer,
+            final KeyHashingScheme keyHashingScheme) {
+        super((config != null ? config : CacheConfig.DEFAULT).getMaxUpdateRetries(),
+                serializer != null ? serializer : ByteArrayCacheEntrySerializer.INSTANCE);
+        this.client = Args.notNull(client, "Memcached client");
+        this.keyHashingScheme = keyHashingScheme;
+    }
+
+    @Override
+    protected String digestToStorageKey(final String key) {
+        return keyHashingScheme.hash(key);
+    }
+
+    private byte[] castAsByteArray(final Object storageObject) throws ResourceIOException {
+        if (storageObject == null) {
+            return null;
+        }
+        if (storageObject instanceof byte[]) {
+            return (byte[]) storageObject;
+        } else {
+            throw new ResourceIOException("Unexpected cache content: " + storageObject.getClass());
+        }
+    }
+
+    @Override
+    protected byte[] getStorageObject(final CASValue<Object> casValue) throws ResourceIOException {
+        return castAsByteArray(casValue.getValue());
+    }
+
+    private <T> Cancellable operation(final OperationFuture<T> operationFuture, final FutureCallback<T> callback) {
+        operationFuture.addListener(new OperationCompletionListener() {
+
+            @Override
+            public void onComplete(final OperationFuture<?> future) throws Exception {
+                try {
+                    callback.completed(operationFuture.get());
+                } catch (final ExecutionException ex) {
+                    if (ex.getCause() instanceof Exception) {
+                        callback.failed((Exception) ex.getCause());
+                    } else {
+                        callback.failed(ex);
+                    }
+                }
+            }
+
+        });
+        return new Cancellable() {
+
+            @Override
+            public boolean cancel() {
+                return operationFuture.cancel();
+            }
+
+        };
+    }
+
+    @Override
+    protected Cancellable store(final String storageKey, final byte[] storageObject, final FutureCallback<Boolean> callback) {
+        return operation(client.set(storageKey, 0, storageObject), callback);
+    }
+
+    @Override
+    protected Cancellable restore(final String storageKey, final FutureCallback<byte[]> callback) {
+        final GetFuture<Object> getFuture = client.asyncGet(storageKey);
+        getFuture.addListener(new GetCompletionListener() {
+
+            @Override
+            public void onComplete(final GetFuture<?> future) throws Exception {
+                try {
+                    callback.completed(castAsByteArray(getFuture.get()));
+                } catch (final ExecutionException ex) {
+                    if (ex.getCause() instanceof Exception) {
+                        callback.failed((Exception) ex.getCause());
+                    } else {
+                        callback.failed(ex);
+                    }
+                }
+            }
+
+        });
+        return new Cancellable() {
+
+            @Override
+            public boolean cancel() {
+                return getFuture.cancel(true);
+            }
+
+        };
+    }
+
+    @Override
+    protected Cancellable getForUpdateCAS(final String storageKey, final FutureCallback<CASValue<Object>> callback) {
+        return operation(client.asyncGets(storageKey), callback);
+    }
+
+    @Override
+    protected Cancellable updateCAS(
+            final String storageKey, final CASValue<Object> casValue, final byte[] storageObject, final FutureCallback<Boolean> callback) {
+        return operation(client.asyncCAS(storageKey, casValue.getCas(), storageObject), new FutureCallback<CASResponse>() {
+
+            @Override
+            public void completed(final CASResponse result) {
+                callback.completed(result == CASResponse.OK);
+            }
+
+            @Override
+            public void failed(final Exception ex) {
+                callback.failed(ex);
+            }
+
+            @Override
+            public void cancelled() {
+                callback.cancelled();
+            }
+
+        });
+    }
+
+    @Override
+    protected Cancellable delete(final String storageKey, final FutureCallback<Boolean> callback) {
+        return operation(client.delete(storageKey), callback);
+    }
+
+}

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/memcached/MemcachedHttpCacheStorage.java

@@ -32,14 +32,13 @@ import java.net.InetSocketAddress;
 import org.apache.hc.client5.http.cache.HttpCacheEntrySerializer;
 import org.apache.hc.client5.http.cache.ResourceIOException;
 import org.apache.hc.client5.http.impl.cache.AbstractBinaryCacheStorage;
-import org.apache.hc.client5.http.impl.cache.CacheConfig;
 import org.apache.hc.client5.http.impl.cache.ByteArrayCacheEntrySerializer;
+import org.apache.hc.client5.http.impl.cache.CacheConfig;
 import org.apache.hc.core5.util.Args;
 
 import net.spy.memcached.CASResponse;
 import net.spy.memcached.CASValue;
 import net.spy.memcached.MemcachedClient;
-import net.spy.memcached.MemcachedClientIF;
 import net.spy.memcached.OperationTimeoutException;
 
 /**
@@ -82,7 +81,7 @@ import net.spy.memcached.OperationTimeoutException;
  */
 public class MemcachedHttpCacheStorage extends AbstractBinaryCacheStorage<CASValue<Object>> {
 
-    private final MemcachedClientIF client;
+    private final MemcachedClient client;
     private final KeyHashingScheme keyHashingScheme;
 
     /**
@@ -102,7 +101,7 @@ public class MemcachedHttpCacheStorage extends AbstractBinaryCacheStorage<CASVal
      * <i>memcached</i> client.
      * @param cache client to use for communicating with <i>memcached</i>
      */
-    public MemcachedHttpCacheStorage(final MemcachedClientIF cache) {
+    public MemcachedHttpCacheStorage(final MemcachedClient cache) {
         this(cache, CacheConfig.DEFAULT, ByteArrayCacheEntrySerializer.INSTANCE, SHA256KeyHashingScheme.INSTANCE);
     }
 
@@ -117,7 +116,7 @@ public class MemcachedHttpCacheStorage extends AbstractBinaryCacheStorage<CASVal
      *   onto "cache keys" suitable for use with memcached
      */
     public MemcachedHttpCacheStorage(
-            final MemcachedClientIF client,
+            final MemcachedClient client,
             final CacheConfig config,
             final HttpCacheEntrySerializer<byte[]> serializer,
             final KeyHashingScheme keyHashingScheme) {
@@ -134,11 +133,7 @@ public class MemcachedHttpCacheStorage extends AbstractBinaryCacheStorage<CASVal
 
     @Override
     protected void store(final String storageKey, final byte[] storageObject) throws ResourceIOException {
-        try {
         client.set(storageKey, 0, storageObject);
-        } catch (final OperationTimeoutException ex) {
-            throw new MemcachedOperationTimeoutException(ex);
-        }
     }
 
     private byte[] castAsByteArray(final Object storageObject) throws ResourceIOException {
@@ -184,11 +179,7 @@ public class MemcachedHttpCacheStorage extends AbstractBinaryCacheStorage<CASVal
 
     @Override
     protected void delete(final String storageKey) throws ResourceIOException {
-        try {
         client.delete(storageKey);
-        } catch (final OperationTimeoutException ex) {
-            throw new MemcachedOperationTimeoutException(ex);
-        }
     }
 
 }

httpclient5-cache/src/test/java/org/apache/hc/client5/http/impl/cache/TestAbstractSerializingAsyncCacheStorage.java

@@ -0,0 +1,450 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.impl.cache;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.apache.hc.client5.http.cache.HttpCacheCASOperation;
+import org.apache.hc.client5.http.cache.HttpCacheEntry;
+import org.apache.hc.client5.http.cache.HttpCacheStorageEntry;
+import org.apache.hc.client5.http.cache.HttpCacheUpdateException;
+import org.apache.hc.client5.http.cache.ResourceIOException;
+import org.apache.hc.core5.concurrent.Cancellable;
+import org.apache.hc.core5.concurrent.FutureCallback;
+import org.hamcrest.CoreMatchers;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Answers;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+import org.mockito.invocation.InvocationOnMock;
+import org.mockito.junit.MockitoJUnitRunner;
+import org.mockito.stubbing.Answer;
+
+@RunWith(MockitoJUnitRunner.class)
+public class TestAbstractSerializingAsyncCacheStorage {
+
+    @Mock
+    private Cancellable cancellable;
+    @Mock
+    private FutureCallback<Boolean> operationCallback;
+    @Mock
+    private FutureCallback<HttpCacheEntry> cacheEntryCallback;
+
+    private AbstractBinaryAsyncCacheStorage<String> impl;
+
+    public static byte[] serialize(final String key, final HttpCacheEntry value) throws ResourceIOException {
+        return ByteArrayCacheEntrySerializer.INSTANCE.serialize(new HttpCacheStorageEntry(key, value));
+    }
+
+    @Before
+    @SuppressWarnings("unchecked")
+    public void setUp() {
+        impl = Mockito.mock(AbstractBinaryAsyncCacheStorage.class,
+                Mockito.withSettings().defaultAnswer(Answers.CALLS_REAL_METHODS).useConstructor(3));
+    }
+
+    @Test
+    public void testCachePut() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry value = HttpTestUtils.makeCacheEntry();
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.store(
+                Mockito.eq("bar"),
+                Mockito.<byte[]>any(),
+                Mockito.<FutureCallback<Boolean>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<Boolean> callback = invocation.getArgument(2);
+                callback.completed(true);
+                return cancellable;
+            }
+
+        });
+
+        impl.putEntry(key, value, operationCallback);
+
+        final ArgumentCaptor<byte[]> argumentCaptor = ArgumentCaptor.forClass(byte[].class);
+        Mockito.verify(impl).store(Mockito.eq("bar"), argumentCaptor.capture(), Mockito.<FutureCallback<Boolean>>any());
+        Assert.assertArrayEquals(serialize(key, value), argumentCaptor.getValue());
+        Mockito.verify(operationCallback).completed(Boolean.TRUE);
+    }
+
+    @Test
+    public void testCacheGetNullEntry() throws Exception {
+        final String key = "foo";
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.restore(Mockito.eq("bar"), Mockito.<FutureCallback<byte[]>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<byte[]> callback = invocation.getArgument(1);
+                callback.completed(null);
+                return cancellable;
+            }
+
+        });
+
+        impl.getEntry(key, cacheEntryCallback);
+        final ArgumentCaptor<HttpCacheEntry> argumentCaptor = ArgumentCaptor.forClass(HttpCacheEntry.class);
+        Mockito.verify(cacheEntryCallback).completed(argumentCaptor.capture());
+        Assert.assertThat(argumentCaptor.getValue(), CoreMatchers.nullValue());
+        Mockito.verify(impl).restore(Mockito.eq("bar"), Mockito.<FutureCallback<byte[]>>any());
+    }
+
+    @Test
+    public void testCacheGet() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry value = HttpTestUtils.makeCacheEntry();
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.restore(Mockito.eq("bar"), Mockito.<FutureCallback<byte[]>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<byte[]> callback = invocation.getArgument(1);
+                callback.completed(serialize(key, value));
+                return cancellable;
+            }
+
+        });
+
+        impl.getEntry(key, cacheEntryCallback);
+        final ArgumentCaptor<HttpCacheEntry> argumentCaptor = ArgumentCaptor.forClass(HttpCacheEntry.class);
+        Mockito.verify(cacheEntryCallback).completed(argumentCaptor.capture());
+        final HttpCacheEntry resultingEntry = argumentCaptor.getValue();
+        Assert.assertThat(resultingEntry, HttpCacheEntryMatcher.equivalent(value));
+        Mockito.verify(impl).restore(Mockito.eq("bar"), Mockito.<FutureCallback<byte[]>>any());
+    }
+
+    @Test
+    public void testCacheGetKeyMismatch() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry value = HttpTestUtils.makeCacheEntry();
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.restore(Mockito.eq("bar"), Mockito.<FutureCallback<byte[]>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<byte[]> callback = invocation.getArgument(1);
+                callback.completed(serialize("not-foo", value));
+                return cancellable;
+            }
+
+        });
+
+        impl.getEntry(key, cacheEntryCallback);
+        final ArgumentCaptor<HttpCacheEntry> argumentCaptor = ArgumentCaptor.forClass(HttpCacheEntry.class);
+        Mockito.verify(cacheEntryCallback).completed(argumentCaptor.capture());
+        Assert.assertThat(argumentCaptor.getValue(), CoreMatchers.nullValue());
+        Mockito.verify(impl).restore(Mockito.eq("bar"), Mockito.<FutureCallback<byte[]>>any());
+    }
+
+    @Test
+    public void testCacheRemove()  throws Exception{
+        final String key = "foo";
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.delete(
+                Mockito.eq("bar"),
+                Mockito.<FutureCallback<Boolean>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<Boolean> callback = invocation.getArgument(1);
+                callback.completed(true);
+                return cancellable;
+            }
+
+        });
+        impl.removeEntry(key, operationCallback);
+
+        Mockito.verify(impl).delete("bar", operationCallback);
+        Mockito.verify(operationCallback).completed(Boolean.TRUE);
+    }
+
+    @Test
+    public void testCacheUpdateNullEntry() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry updatedValue = HttpTestUtils.makeCacheEntry();
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<byte[]> callback = invocation.getArgument(1);
+                callback.completed(null);
+                return cancellable;
+            }
+
+        });
+        Mockito.when(impl.store(
+                Mockito.eq("bar"),
+                Mockito.<byte[]>any(),
+                Mockito.<FutureCallback<Boolean>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<Boolean> callback = invocation.getArgument(2);
+                callback.completed(true);
+                return cancellable;
+            }
+
+        });
+
+        impl.updateEntry(key, new HttpCacheCASOperation() {
+
+            @Override
+            public HttpCacheEntry execute(final HttpCacheEntry existing) throws ResourceIOException {
+                Assert.assertThat(existing, CoreMatchers.nullValue());
+                return updatedValue;
+            }
+
+        }, operationCallback);
+
+        Mockito.verify(impl).getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any());
+        Mockito.verify(impl).store(Mockito.eq("bar"), Mockito.<byte[]>any(), Mockito.<FutureCallback<Boolean>>any());
+        Mockito.verify(operationCallback).completed(Boolean.TRUE);
+    }
+
+    @Test
+    public void testCacheCASUpdate() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry existingValue = HttpTestUtils.makeCacheEntry();
+        final HttpCacheEntry updatedValue = HttpTestUtils.makeCacheEntry();
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<String> callback = invocation.getArgument(1);
+                callback.completed("stuff");
+                return cancellable;
+            }
+
+        });
+        Mockito.when(impl.getStorageObject("stuff")).thenReturn(serialize(key, existingValue));
+        Mockito.when(impl.updateCAS(
+                Mockito.eq("bar"),
+                Mockito.eq("stuff"),
+                Mockito.<byte[]>any(),
+                Mockito.<FutureCallback<Boolean>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<Boolean> callback = invocation.getArgument(3);
+                callback.completed(true);
+                return cancellable;
+            }
+
+        });
+
+        impl.updateEntry(key, new HttpCacheCASOperation() {
+
+            @Override
+            public HttpCacheEntry execute(final HttpCacheEntry existing) throws ResourceIOException {
+                return updatedValue;
+            }
+
+        }, operationCallback);
+
+        Mockito.verify(impl).getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any());
+        Mockito.verify(impl).getStorageObject("stuff");
+        Mockito.verify(impl).updateCAS(Mockito.eq("bar"), Mockito.eq("stuff"), Mockito.<byte[]>any(), Mockito.<FutureCallback<Boolean>>any());
+        Mockito.verify(operationCallback).completed(Boolean.TRUE);
+    }
+
+    @Test
+    public void testCacheCASUpdateKeyMismatch() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry existingValue = HttpTestUtils.makeCacheEntry();
+        final HttpCacheEntry updatedValue = HttpTestUtils.makeCacheEntry();
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any())).thenAnswer(
+                new Answer<Cancellable>() {
+
+                    @Override
+                    public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                        final FutureCallback<String> callback = invocation.getArgument(1);
+                        callback.completed("stuff");
+                        return cancellable;
+                    }
+
+                });
+        Mockito.when(impl.getStorageObject("stuff")).thenReturn(serialize("not-foo", existingValue));
+        Mockito.when(impl.store(
+                Mockito.eq("bar"),
+                Mockito.<byte[]>any(),
+                Mockito.<FutureCallback<Boolean>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<Boolean> callback = invocation.getArgument(2);
+                callback.completed(true);
+                return cancellable;
+            }
+
+        });
+
+        impl.updateEntry(key, new HttpCacheCASOperation() {
+
+            @Override
+            public HttpCacheEntry execute(final HttpCacheEntry existing) throws ResourceIOException {
+                Assert.assertThat(existing, CoreMatchers.nullValue());
+                return updatedValue;
+            }
+
+        }, operationCallback);
+
+        Mockito.verify(impl).getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any());
+        Mockito.verify(impl).getStorageObject("stuff");
+        Mockito.verify(impl, Mockito.never()).updateCAS(
+                Mockito.eq("bar"), Mockito.eq("stuff"), Mockito.<byte[]>any(), Mockito.<FutureCallback<Boolean>>any());
+        Mockito.verify(impl).store(Mockito.eq("bar"), Mockito.<byte[]>any(), Mockito.<FutureCallback<Boolean>>any());
+        Mockito.verify(operationCallback).completed(Boolean.TRUE);
+    }
+
+    @Test
+    public void testSingleCacheUpdateRetry() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry existingValue = HttpTestUtils.makeCacheEntry();
+        final HttpCacheEntry updatedValue = HttpTestUtils.makeCacheEntry();
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any())).thenAnswer(
+                new Answer<Cancellable>() {
+
+                    @Override
+                    public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                        final FutureCallback<String> callback = invocation.getArgument(1);
+                        callback.completed("stuff");
+                        return cancellable;
+                    }
+
+                });
+        Mockito.when(impl.getStorageObject("stuff")).thenReturn(serialize(key, existingValue));
+        final AtomicInteger count = new AtomicInteger(0);
+        Mockito.when(impl.updateCAS(
+                Mockito.eq("bar"),
+                Mockito.eq("stuff"),
+                Mockito.<byte[]>any(),
+                Mockito.<FutureCallback<Boolean>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<Boolean> callback = invocation.getArgument(3);
+                if (count.incrementAndGet() == 1) {
+                    callback.completed(false);
+                } else {
+                    callback.completed(true);
+                }
+                return cancellable;
+            }
+
+        });
+
+        impl.updateEntry(key, new HttpCacheCASOperation() {
+
+            @Override
+            public HttpCacheEntry execute(final HttpCacheEntry existing) throws ResourceIOException {
+                return updatedValue;
+            }
+
+        }, operationCallback);
+
+        Mockito.verify(impl, Mockito.times(2)).getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any());
+        Mockito.verify(impl, Mockito.times(2)).getStorageObject("stuff");
+        Mockito.verify(impl, Mockito.times(2)).updateCAS(
+                Mockito.eq("bar"), Mockito.eq("stuff"), Mockito.<byte[]>any(), Mockito.<FutureCallback<Boolean>>any());
+        Mockito.verify(operationCallback).completed(Boolean.TRUE);
+    }
+
+    @Test
+    public void testCacheUpdateFail() throws Exception {
+        final String key = "foo";
+        final HttpCacheEntry existingValue = HttpTestUtils.makeCacheEntry();
+        final HttpCacheEntry updatedValue = HttpTestUtils.makeCacheEntry();
+
+        Mockito.when(impl.digestToStorageKey(key)).thenReturn("bar");
+        Mockito.when(impl.getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any())).thenAnswer(
+                new Answer<Cancellable>() {
+
+                    @Override
+                    public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                        final FutureCallback<String> callback = invocation.getArgument(1);
+                        callback.completed("stuff");
+                        return cancellable;
+                    }
+
+                });
+        Mockito.when(impl.getStorageObject("stuff")).thenReturn(serialize(key, existingValue));
+        final AtomicInteger count = new AtomicInteger(0);
+        Mockito.when(impl.updateCAS(
+                Mockito.eq("bar"),
+                Mockito.eq("stuff"),
+                Mockito.<byte[]>any(),
+                Mockito.<FutureCallback<Boolean>>any())).thenAnswer(new Answer<Cancellable>() {
+
+            @Override
+            public Cancellable answer(final InvocationOnMock invocation) throws Throwable {
+                final FutureCallback<Boolean> callback = invocation.getArgument(3);
+                if (count.incrementAndGet() <= 3) {
+                    callback.completed(false);
+                } else {
+                    callback.completed(true);
+                }
+                return cancellable;
+            }
+
+        });
+
+        impl.updateEntry(key, new HttpCacheCASOperation() {
+
+            @Override
+            public HttpCacheEntry execute(final HttpCacheEntry existing) throws ResourceIOException {
+                return updatedValue;
+            }
+
+        }, operationCallback);
+
+        Mockito.verify(impl, Mockito.times(3)).getForUpdateCAS(Mockito.eq("bar"), Mockito.<FutureCallback<String>>any());
+        Mockito.verify(impl, Mockito.times(3)).getStorageObject("stuff");
+        Mockito.verify(impl, Mockito.times(3)).updateCAS(
+                Mockito.eq("bar"), Mockito.eq("stuff"), Mockito.<byte[]>any(), Mockito.<FutureCallback<Boolean>>any());
+        Mockito.verify(operationCallback).failed(Mockito.<HttpCacheUpdateException>any());
+    }
+
+}