srowen commented on a change in pull request #25583: [MINOR][SS] Reuse 
KafkaSourceInitialOffsetWriter to deduplicate
URL: https://github.com/apache/spark/pull/25583#discussion_r317621165
 
 

 ##########
 File path: 
external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaSourceInitialOffsetWriter.scala
 ##########
 @@ -0,0 +1,63 @@
+/*
+ * 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.spark.sql.kafka010
+
+import java.io._
+import java.nio.charset.StandardCharsets
+
+import org.apache.commons.io.IOUtils
+
+import org.apache.spark.sql.SparkSession
+import org.apache.spark.sql.execution.streaming.{HDFSMetadataLog, 
SerializedOffset}
+
+/** A version of [[HDFSMetadataLog]] specialized for saving the initial 
offsets. */
+private[kafka010] class KafkaSourceInitialOffsetWriter(
+    sparkSession: SparkSession,
+    metadataPath: String)
+  extends HDFSMetadataLog[KafkaSourceOffset](sparkSession, metadataPath) {
+
+  val VERSION = 1
+
+  override def serialize(metadata: KafkaSourceOffset, out: OutputStream): Unit 
= {
+    out.write(0) // A zero byte is written to support Spark 2.1.0 (SPARK-19517)
+    val writer = new BufferedWriter(new OutputStreamWriter(out, 
StandardCharsets.UTF_8))
+    writer.write("v" + VERSION + "\n")
+    writer.write(metadata.json)
+    writer.flush
+  }
+
+  override def deserialize(in: InputStream): KafkaSourceOffset = {
+    in.read() // A zero byte is read to support Spark 2.1.0 (SPARK-19517)
+    val content = IOUtils.toString(new InputStreamReader(in, 
StandardCharsets.UTF_8))
+    // HDFSMetadataLog guarantees that it never creates a partial file.
+    assert(content.length != 0)
+    if (content(0) == 'v') {
+      val indexOfNewLine = content.indexOf("\n")
+      if (indexOfNewLine > 0) {
+        validateVersion(content.substring(0, indexOfNewLine), VERSION)
+        KafkaSourceOffset(SerializedOffset(content.substring(indexOfNewLine + 
1)))
+      } else {
+        throw new IllegalStateException(
+          s"Log file was malformed: failed to detect the log file version 
line.")
 
 Review comment:
   Nit: no need for interpolation

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