Github user lw-lin commented on a diff in the pull request:
https://github.com/apache/spark/pull/17219#discussion_r106800936
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/BatchCommitLog.scala
---
@@ -0,0 +1,80 @@
+/*
+ * 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.execution.streaming
+
+import java.io.{InputStream, OutputStream}
+import java.nio.charset.StandardCharsets._
+
+import scala.io.{Source => IOSource}
+
+import org.apache.spark.sql.SparkSession
+
+/**
+ * Used to write log files that represent batch commit points in
structured streaming.
+ * A commit log file will be written immediately after the successful
completion of a
+ * batch, and before processing the next batch. Here is an execution
summary:
+ * - trigger batch 1
+ * - obtain batch 1 offsets and write to offset log
+ * - process batch 1
+ * - write batch 1 to completion log
+ * - trigger batch 2
+ * - obtain bactch 2 offsets and write to offset log
+ * - process batch 2
+ * - write batch 2 to completion log
+ * ....
+ *
+ * The current format of the batch completion log is:
+ * line 1: version
+ * line 2: metadata (optional json string)
+ */
+class BatchCommitLog(sparkSession: SparkSession, path: String)
+ extends HDFSMetadataLog[String](sparkSession, path) {
+
+ override protected def deserialize(in: InputStream): String = {
+ // called inside a try-finally where the underlying stream is closed
in the caller
+ val lines = IOSource.fromInputStream(in, UTF_8.name()).getLines()
+ if (!lines.hasNext) {
+ throw new IllegalStateException("Incomplete log file in the offset
commit log")
+ }
+ val version = lines.next().trim.toInt
+ if (BatchCommitLog.VERSION < version) {
+ throw new IllegalStateException(s"Incompatible log file version
${version}")
+ }
--- End diff --
nit: now that https://github.com/apache/spark/pull/17070 has been merged,
maybe let's use `parseVersion(lines.next(), BatchCommitLog.VERSION)`
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