Github user ilooner commented on a diff in the pull request:
https://github.com/apache/incubator-apex-malhar/pull/242#discussion_r59827085
--- Diff: library/src/main/java/org/apache/apex/malhar/lib/wal/WAL.java ---
@@ -0,0 +1,100 @@
+/**
+ * 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.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
+ * written to the WAL.
+ *
+ * @param <READER> Type of WAL Reader
+ * @param <WRITER> WAL Pointer Type.
+ */
+public interface WAL<READER extends WAL.WALReader, WRITER extends
WAL.WALWriter>
+{
+ void setup();
+
+ void teardown();
+
+ void beforeCheckpoint(long window);
+
+ void committed(long window);
+
+ READER getReader() throws IOException;
+
+ WRITER 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>
+ {
+ /**
+ * 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 pointer) 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();
+ }
+
+ /**
+ * Provide method to write entries to the WAL.
+ * @param <T>
+ * @param <P>
+ */
+ interface WALWriter<T, P>
+ {
+ /**
+ * Write an entry to the WAL
+ */
+ int append(T entry) throws IOException;
+
+ }
+
+ /**
+ * Serializer interface used while reading and writing entries to the
WAL.
+ * @param <T>
+ */
+ interface Serde<T>
--- End diff --
There is already a Serde interface here
https://github.com/apache/incubator-apex-malhar/blob/master/library/src/main/java/org/apache/apex/malhar/lib/spillable/Serde.java
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