content: /*
   * The Alluxio Open Foundation licenses this work under the Apache License, version 2.0
   * (the "License"). You may not use this work except in compliance with the License, which is
   * available at www.apache.org/licenses/LICENSE-2.0
   *
   * This software is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
   * either express or implied, as more fully set forth in the License.
   *
   * See the NOTICE file distributed with this work for information regarding copyright ownership.
   */
  
  package alluxio.master.journal;
  
  import alluxio.master.journal.checkpoint.CheckpointInputStream;
  import alluxio.proto.journal.Journal.JournalEntry;
  
  import java.io.Closeable;
  import java.io.IOException;
  import javax.annotation.concurrent.NotThreadSafe;
  
  /**
   * This class manages reading from the journal.
   */
  @NotThreadSafe
  public interface JournalReader extends Closeable {
    /**
     * Returns the next entry in the journal.
     *
     * This will be null unless the last call to {@link #advance()} returned {@link State#LOG}.
     * Multiple calls to getEntry without calling advance will return the same entry.
     *
     * @return the journal entry, or null if the next element isn't a journal entry
     */
    JournalEntry getEntry();
  
    /**
     * Returns the input stream for reading a checkpoint.
     *
     * This will be null unless the last call to {@link #advance()} returned {@link State#CHECKPOINT}.
     * Multiple calls to getCheckpoint without calling advance will return the same stream.
     *
     * @return the input stream for reading the checkpoint, or null if the next element isn't a
     *         checkpoint
     */
    CheckpointInputStream getCheckpoint();
  
    /**
     * Gets the the sequence number of the next journal log entry to read. This method is valid
     * no matter whether this JournalReader is closed or not.
     *
     * @return the next sequence number
     */
    long getNextSequenceNumber();
  
    /**
     * Advances the reader to the next element.
     *
     * @return the next element, see {@link State}
     */
    State advance() throws IOException;
  
    /**
     * States that the reader can be after calling {@link #advance()}.
     */
    enum State {
      /**
       * Indicates that the next item to process is a checkpoint. The caller should call
       * {@link #getCheckpoint()}.
       */
      CHECKPOINT,
      /**
       * Indicates that the next item to process is an edit log. The caller should call
       * {@link #getEntry()}.
       */
      LOG,
      /**
       * Indicates that there is nothing left to read.
       */
      DONE
    }
  }
  