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    * 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.Collection;
  import java.util.HashMap;
  import java.util.Map;
  
  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.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;
  
  /**
   * <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 4.1
   */
  public class MemcachedHttpCacheStorage extends AbstractBinaryCacheStorage<CASValue<Object>> {
  
      private final MemcachedClientIF 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 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 MemcachedClient cache) {
         this(cache, CacheConfig.DEFAULT, ByteArrayCacheEntrySerializer.INSTANCE, SHA256KeyHashingScheme.INSTANCE);
     }
 
     /**
      * Create a storage backend using the pre-configured given
      * <i>memcached</i> client.
      *
      * @param cache client to use for communicating with <i>memcached</i>
      *
      * @since 5.2
      */
     public MemcachedHttpCacheStorage(final MemcachedClientIF 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 MemcachedHttpCacheStorage(
             final MemcachedClient client,
             final CacheConfig config,
             final HttpCacheEntrySerializer<byte[]> serializer,
             final KeyHashingScheme keyHashingScheme) {
         this((MemcachedClientIF) client, config, serializer, keyHashingScheme);
     }
 
     /**
      * 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
      * @since 5.2
      */
     public MemcachedHttpCacheStorage(
             final MemcachedClientIF 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);
     }
 
     @Override
     protected void store(final String storageKey, final byte[] storageObject) throws ResourceIOException {
         client.set(storageKey, 0, storageObject);
     }
 
     private byte[] castAsByteArray(final Object storageObject) throws ResourceIOException {
         if (storageObject == null) {
             return null;
         }
         if (storageObject instanceof byte[]) {
             return (byte[]) storageObject;
         }
         throw new ResourceIOException("Unexpected cache content: " + storageObject.getClass());
     }
 
     @Override
     protected byte[] restore(final String storageKey) throws ResourceIOException {
         try {
             return castAsByteArray(client.get(storageKey));
         } catch (final OperationTimeoutException ex) {
             throw new MemcachedOperationTimeoutException(ex);
         }
     }
 
     @Override
     protected CASValue<Object> getForUpdateCAS(final String storageKey) throws ResourceIOException {
         try {
             return client.gets(storageKey);
         } catch (final OperationTimeoutException ex) {
             throw new MemcachedOperationTimeoutException(ex);
         }
     }
 
     @Override
     protected byte[] getStorageObject(final CASValue<Object> casValue) throws ResourceIOException {
         return castAsByteArray(casValue.getValue());
     }
 
     @Override
     protected boolean updateCAS(
             final String storageKey, final CASValue<Object> casValue, final byte[] storageObject) throws ResourceIOException {
         final CASResponse casResult = client.cas(storageKey, casValue.getCas(), storageObject);
         return casResult == CASResponse.OK;
     }
 
     @Override
     protected void delete(final String storageKey) throws ResourceIOException {
         client.delete(storageKey);
     }
 
     @Override
     protected Map<String, byte[]> bulkRestore(final Collection<String> storageKeys) throws ResourceIOException {
         final Map<String, ?> storageObjectMap = client.getBulk(storageKeys);
         final Map<String, byte[]> resultMap = new HashMap<>(storageObjectMap.size());
         for (final Map.Entry<String, ?> resultEntry: storageObjectMap.entrySet()) {
             resultMap.put(resultEntry.getKey(), castAsByteArray(resultEntry.getValue()));
         }
         return resultMap;
     }
 
 }