001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    package org.apache.hadoop.hdfs.server.datanode.fsdataset;
019    
020    import java.io.Closeable;
021    import java.io.IOException;
022    import java.util.Iterator;
023    
024    /**
025     * Rolling logs consist of a current log and a set of previous logs.
026     *
027     * The implementation should support a single appender and multiple readers.
028     */
029    public interface RollingLogs {
030      /**
031       * To iterate the lines of the logs.
032       */
033      public interface LineIterator extends Iterator<String>, Closeable {
034        /** Is the iterator iterating the previous? */
035        public boolean isPrevious();
036    
037        /**
038         * Is the last read entry from previous? This should be called after
039         * reading.
040         */
041        public boolean isLastReadFromPrevious();
042      }
043    
044      /**
045       * To append text to the logs.
046       */
047      public interface Appender extends Appendable, Closeable {
048      }
049    
050      /**
051       * Create an iterator to iterate the lines in the logs.
052       * 
053       * @param skipPrevious Should it skip reading the previous log? 
054       * @return a new iterator.
055       */
056      public LineIterator iterator(boolean skipPrevious) throws IOException;
057    
058      /**
059       * @return the only appender to append text to the logs.
060       *   The same object is returned if it is invoked multiple times.
061       */
062      public Appender appender();
063    
064      /**
065       * Roll current to previous.
066       *
067       * @return true if the rolling succeeded.
068       *   When it returns false, it is not equivalent to an error. 
069       *   It means that the rolling cannot be performed at the moment,
070       *   e.g. the logs are being read.
071       */
072      public boolean roll() throws IOException;
073    }