hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/common/blockaliasmap/BlockAliasMap.java

/**
 * 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.
 */
package org.apache.hadoop.hdfs.server.common.blockaliasmap;

import java.io.Closeable;
import java.io.IOException;
import java.util.Iterator;
import java.util.Optional;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.hdfs.protocol.Block;
import org.apache.hadoop.hdfs.server.common.BlockAlias;

/**
 * An abstract class used to read and write block maps for provided blocks.
 */
@InterfaceAudience.Public
@InterfaceStability.Unstable
public abstract class BlockAliasMap<T extends BlockAlias> {

  /**
   * ImmutableIterator is an Iterator that does not support the remove
   * operation. This could inherit {@link java.util.Enumeration} but Iterator
   * is supported by more APIs and Enumeration's javadoc even suggests using
   * Iterator instead.
   */
  public abstract class ImmutableIterator implements Iterator<T> {
    public void remove() {
      throw new UnsupportedOperationException(
          "Remove is not supported for provided storage");
    }
  }

  /**
   * An abstract class that is used to read {@link BlockAlias}es
   * for provided blocks.
   */
  public static abstract class Reader<U extends BlockAlias>
      implements Iterable<U>, Closeable {

    /**
     * reader options.
     */
    public interface Options { }

    /**
     * @param ident block to resolve
     * @return BlockAlias corresponding to the provided block.
     * @throws IOException
     */
    public abstract Optional<U> resolve(Block ident) throws IOException;
  }

  /**
   * Returns a reader to the alias map.
   * @param opts reader options
   * @param blockPoolID block pool id to use
   * @return {@link Reader} to the alias map. If a Reader for the blockPoolID
   * cannot be created, this will return null.
   * @throws IOException
   */
  public abstract Reader<T> getReader(Reader.Options opts, String blockPoolID)
      throws IOException;

  /**
   * An abstract class used as a writer for the provided block map.
   */
  public static abstract class Writer<U extends BlockAlias>
      implements Closeable {
    /**
     * writer options.
     */
    public interface Options { }

    /**
     * Store the BlockAlias; this may be a destructive store, replacing old
     * data.
     * @param blockAlias
     * @throws IOException
     */
    public abstract void store(U blockAlias) throws IOException;

    /**
     * Remove the block from the AliasMap.
     * @param block
     * @throws IOException
     */
    public abstract void remove(Block block) throws IOException;
  }

  /**
   * Returns the writer for the alias map.
   * @param opts writer options.
   * @param blockPoolID block pool id to use
   * @return {@link Writer} to the alias map.
   * @throws IOException
   */
  public abstract Writer<T> getWriter(Writer.Options opts, String blockPoolID)
      throws IOException;

  /**
   * Refresh the alias map.
   * @throws IOException
   */
  public abstract void refresh() throws IOException;

  public abstract void close() throws IOException;

}