[jira] [Commented] (APEXMALHAR-1965) Create a WAL in Malhar

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/APEXMALHAR-1965?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15239975#comment-15239975 ] 

ASF GitHub Bot commented on APEXMALHAR-1965:
--------------------------------------------

Github user chandnisingh commented on a diff in the pull request:

    https://github.com/apache/incubator-apex-malhar/pull/204#discussion_r59621181
  
    --- Diff: library/src/main/java/org/apache/apex/malhar/lib/wal/WAL.java ---
    @@ -0,0 +1,130 @@
    +/**
    + * 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.apex.malhar.lib.wal;
    +
    +import java.io.Closeable;
    +import java.io.IOException;
    +
    +/**
    + * This interface represents a write ahead log that can be used by operator.
    + * the WAL is split into two interfaces, a WALWriter which allows writing
    + * data, and WALReader which provides iterator like interface to read entries
    + * writen to the WAL.
    + *
    + * @param <T> Tuple type
    + * @param <P> WAL Pointer Type.
    + */
    +public interface WAL<T, P>
    +{
    +  WALReader<T, P> getReader() throws IOException;
    +
    +  WALWriter<T, P> getWriter() throws IOException;
    +
    +  /**
    +   * Provides iterator like interface to read entries from the WAL.
    +   * @param <T> type of WAL entries
    +   * @param <P> type of Pointer in the WAL
    +   */
    +  interface WALReader<T, P> extends Closeable
    +  {
    +    /**
    +     * Close WAL after read.
    +     *
    +     * @param offset seek offset.
    +     * @throws IOException
    +     */
    +    @Override
    +    void close() throws IOException;
    +
    +    /**
    +     * Seek to middle of the WAL. This is used primarily during recovery,
    +     * when we need to start recovering data from middle of WAL file.
    +     */
    +    void seek(P offset) throws IOException;
    +
    +    /**
    +     * Advance WAL by one entry, returns true if it can advance, else false
    +     * in case of any other error throws an Exception.
    +     *
    +     * @return true if next data item is read successfully, false if data can not be read.
    +     * @throws IOException
    +     */
    +    boolean advance() throws IOException;
    +
    +    /**
    +     * Return current entry from WAL, returns null if end of file has reached.
    +     *
    +     * @return MutableKeyValue
    +     */
    +    T get();
    +
    +    /**
    +     * Return the offset corresponding to the last read entry.
    +     * @return
    +     */
    +    P getOffset();
    +  }
    +
    +  /**
    +   * Provide method to write entries to the WAL.
    +   * @param <T>
    +   * @param <P>
    +   */
    +  interface WALWriter<T, P>
    +  {
    +    /**
    +     * flush pending data to disk and close file.
    +     *
    +     * @throws IOException
    +     */
    +    void close() throws IOException;
    +
    +    /**
    +     * Write an entry to the WAL, this operation need not flush the data.
    +     */
    +    int append(T entry) throws IOException;
    +
    +    /**
    +     * Flush data to persistent storage.
    +     *
    +     * @throws IOException
    +     */
    +    void flush() throws IOException;
    +
    +    /**
    +     * Returns size of the WAL, last part of the log may not be persisted on disk.
    +     * In case of file backed WAL this will be the size of file, in case of kafka
    +     * like log, this will be similar to the message offset.
    +     *
    +     * @return The log size
    +     */
    +    P getOffset();
    --- End diff --
    
    @tushargosavi the javadoc is not clear here. Can you please tell me the intention of this method? I have changed the implementation of FsWALWriter and I am unable to understand  the significance of this method.


> Create a WAL in Malhar
> ----------------------
>
>                 Key: APEXMALHAR-1965
>                 URL: https://issues.apache.org/jira/browse/APEXMALHAR-1965
>             Project: Apache Apex Malhar
>          Issue Type: Task
>            Reporter: Chandni Singh
>            Assignee: Tushar Gosavi
>
> In Malhar we have an IdempotentStorageManager which we use like a Write Ahead Logger. There have been some other places where we have created a different flavor of Write Ahead Logger. 
> We need to find overlap between all these flavors and create a common Write Ahead Logger for use in Apex core and Apex malhar.



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