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    * Licensed to the Apache Software Foundation (ASF) under one
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    * distributed with this work for additional information
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    * 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/>.
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  package org.apache.http.impl.client.cache.memcached;
  
  import java.io.IOException;
  import java.net.InetSocketAddress;
  
  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;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.http.client.cache.HttpCacheEntry;
  import org.apache.http.client.cache.HttpCacheEntrySerializer;
  import org.apache.http.client.cache.HttpCacheStorage;
  import org.apache.http.client.cache.HttpCacheUpdateCallback;
  import org.apache.http.client.cache.HttpCacheUpdateException;
  import org.apache.http.impl.client.cache.CacheConfig;
  
  /**
   * <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>
   * Because this hashing scheme can potentially result in key collisions (though
   * highly unlikely), we need to store the higher-level logical storage key along
   * with the {@link HttpCacheEntry} so that we can re-check it on retrieval. There
   * is a default serialization scheme provided for this, although you can provide
   * your own implementations of {@link MemcachedCacheEntry} and
   * {@link MemcachedCacheEntryFactory} to customize this serialization.
   * </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 4.1
   */
  public class MemcachedHttpCacheStorage implements HttpCacheStorage {
  
      private static final Log log = LogFactory.getLog(MemcachedHttpCacheStorage.class);
  
      private final MemcachedClientIF client;
      private final KeyHashingScheme keyHashingScheme;
     private final MemcachedCacheEntryFactory memcachedCacheEntryFactory;
     private final int maxUpdateRetries;
 
     /**
      * 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 MemcachedHttpCacheStorage(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 MemcachedHttpCacheStorage(final MemcachedClientIF cache) {
         this(cache, CacheConfig.DEFAULT, new MemcachedCacheEntryFactoryImpl(),
                 new SHA256KeyHashingScheme());
     }
 
     /**
      * Create a storage backend using the given <i>memcached</i> client and
      * applying the given cache configuration and cache entry serialization
      * mechanism. <b>Deprecation note:</b> In the process of fixing a bug
      * based on the need to hash logical storage keys onto memcached cache
      * keys, the serialization process was revamped. This constructor still
      * works, but the serializer argument will be ignored and default
      * implementations of the new framework will be used. You can still
      * provide custom serialization by using the
      * {@link #MemcachedHttpCacheStorage(MemcachedClientIF, CacheConfig,
      * MemcachedCacheEntryFactory, KeyHashingScheme)} constructor.
      * @param client how to talk to <i>memcached</i>
      * @param config apply HTTP cache-related options
      * @param serializer <b>ignored</b>
      *
      * @deprecated (4.2) do not use
      */
     @Deprecated
     public MemcachedHttpCacheStorage(final MemcachedClientIF client, final CacheConfig config,
             final HttpCacheEntrySerializer serializer) {
         this(client, config, new MemcachedCacheEntryFactoryImpl(),
                 new SHA256KeyHashingScheme());
     }
 
     /**
      * 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 memcachedCacheEntryFactory Factory pattern used for obtaining
      *   instances of alternative cache entry serialization mechanisms
      * @param keyHashingScheme how to map higher-level logical "storage keys"
      *   onto "cache keys" suitable for use with memcached
      */
     public MemcachedHttpCacheStorage(final MemcachedClientIF client, final CacheConfig config,
             final MemcachedCacheEntryFactory memcachedCacheEntryFactory,
             final KeyHashingScheme keyHashingScheme) {
         this.client = client;
         this.maxUpdateRetries = config.getMaxUpdateRetries();
         this.memcachedCacheEntryFactory = memcachedCacheEntryFactory;
         this.keyHashingScheme = keyHashingScheme;
     }
 
     @Override
     public void putEntry(final String url, final HttpCacheEntry entry) throws IOException  {
         final byte[] bytes = serializeEntry(url, entry);
         final String key = getCacheKey(url);
         if (key == null) {
             return;
         }
         try {
             client.set(key, 0, bytes);
         } catch (final OperationTimeoutException ex) {
             throw new MemcachedOperationTimeoutException(ex);
         }
     }
 
     private String getCacheKey(final String url) {
         try {
             return keyHashingScheme.hash(url);
         } catch (final MemcachedKeyHashingException mkhe) {
             return null;
         }
     }
 
     private byte[] serializeEntry(final String url, final HttpCacheEntry hce) throws IOException {
         final MemcachedCacheEntry mce = memcachedCacheEntryFactory.getMemcachedCacheEntry(url, hce);
         try {
             return mce.toByteArray();
         } catch (final MemcachedSerializationException mse) {
             final IOException ioe = new IOException();
             ioe.initCause(mse);
             throw ioe;
         }
     }
 
     private byte[] convertToByteArray(final Object o) {
         if (o == null) {
             return null;
         }
         if (!(o instanceof byte[])) {
             log.warn("got a non-bytearray back from memcached: " + o);
             return null;
         }
         return (byte[])o;
     }
 
     private MemcachedCacheEntry reconstituteEntry(final Object o) {
         final byte[] bytes = convertToByteArray(o);
         if (bytes == null) {
             return null;
         }
         final MemcachedCacheEntry mce = memcachedCacheEntryFactory.getUnsetCacheEntry();
         try {
             mce.set(bytes);
         } catch (final MemcachedSerializationException mse) {
             return null;
         }
         return mce;
     }
 
     @Override
     public HttpCacheEntry getEntry(final String url) throws IOException {
         final String key = getCacheKey(url);
         if (key == null) {
             return null;
         }
         try {
             final MemcachedCacheEntry mce = reconstituteEntry(client.get(key));
             if (mce == null || !url.equals(mce.getStorageKey())) {
                 return null;
             }
             return mce.getHttpCacheEntry();
         } catch (final OperationTimeoutException ex) {
             throw new MemcachedOperationTimeoutException(ex);
         }
     }
 
     @Override
     public void removeEntry(final String url) throws IOException {
         final String key = getCacheKey(url);
         if (key == null) {
             return;
         }
         try {
             client.delete(key);
         } catch (final OperationTimeoutException ex) {
             throw new MemcachedOperationTimeoutException(ex);
         }
     }
 
     @Override
     public void updateEntry(final String url, final HttpCacheUpdateCallback callback)
             throws HttpCacheUpdateException, IOException {
         int numRetries = 0;
         final String key = getCacheKey(url);
         if (key == null) {
             throw new HttpCacheUpdateException("couldn't generate cache key");
         }
         do {
             try {
                 final CASValue<Object> v = client.gets(key);
                 MemcachedCacheEntry mce = (v == null) ? null
                         : reconstituteEntry(v.getValue());
                 if (mce != null && (!url.equals(mce.getStorageKey()))) {
                     mce = null;
                 }
                 final HttpCacheEntry existingEntry = (mce == null) ? null
                         : mce.getHttpCacheEntry();
                 final HttpCacheEntry updatedEntry = callback.update(existingEntry);
 
                 if (existingEntry == null) {
                     putEntry(url, updatedEntry);
                     return;
 
                 } else {
                     final byte[] updatedBytes = serializeEntry(url, updatedEntry);
                     final CASResponse casResult = client.cas(key, v.getCas(),
                             updatedBytes);
                     if (casResult != CASResponse.OK) {
                         numRetries++;
                     } else {
                         return;
                     }
                 }
             } catch (final OperationTimeoutException ex) {
                 throw new MemcachedOperationTimeoutException(ex);
             }
         } while (numRetries <= maxUpdateRetries);
 
         throw new HttpCacheUpdateException("Failed to update");
     }
 }