Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/11625#discussion_r55738627
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/HDFSMetadataLog.scala
---
@@ -0,0 +1,168 @@
+/*
+* 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.{DataOutput, EOFException, IOException}
+import java.nio.ByteBuffer
+
+import scala.reflect.ClassTag
+
+import org.apache.hadoop.fs.{Path, PathFilter}
+
+import org.apache.spark.SparkException
+import org.apache.spark.serializer.JavaSerializer
+import org.apache.spark.sql.SQLContext
+import org.apache.spark.util.Utils
+
+/**
+ * A [[MetadataLog]] implementation based on HDFS. [[HDFSMetadataLog]]
uses the specified `path`
+ * as the metadata storage.
+ *
+ * There should only be one [[HDFSMetadataLog]] using `path` at the same
time. [[HDFSMetadataLog]]
+ * uses a file ".lock" in the directory to make sure there is always only
one user using `path`.
+ * When [[HDFSMetadataLog]] is created, it will create a ".lock" file. If
this step fails,
+ * [[HDFSMetadataLog]] will throw an exception saying there is someone
using the same directory.
+ * When [[HDFSMetadataLog]] is stopped, it will delete the ".lock" file.
+ *
+ * However, in extreme case, e.g., power outage, [[HDFSMetadataLog]] won't
be able to delete ".lock"
+ * file. Then [[HDFSMetadataLog]] cannot be created even if no one is
using the `path`. In such
+ * case, the user has to delete the lock file in the exception message to
restart their application.
+ *
+ * Note: [[HDFSMetadataLog]] doesn't support S3-like file systems as they
don't guarantee listing
+ * files in a directory always shows the latest files.
+ */
+class HDFSMetadataLog[T: ClassTag](sqlContext: SQLContext, path: String)
extends MetadataLog[T] {
+
+ private val metadataPath = new Path(path)
+ private val fs =
metadataPath.getFileSystem(sqlContext.sparkContext.hadoopConfiguration)
+ if (!fs.exists(metadataPath)) {
+ fs.mkdirs(metadataPath)
+ }
+
+ tryAcquireLock()
+
+ /**
+ * A `PathFilter` to filter only batch files
+ */
+ private val batchFilesFilter = new PathFilter {
+ override def accept(path: Path): Boolean = try {
+ path.getName.toLong
+ true
+ } catch {
+ case _: NumberFormatException => false
+ }
+ }
+
+ private val serializer = new
JavaSerializer(sqlContext.sparkContext.conf).newInstance()
+
+ private def tryAcquireLock(): Unit = {
--- End diff --
What happens if you die while you are holding the lock? It seems like your
streaming job will be unable to restart without human intervention. Is there a
reason that we can't detect problems when writing a new log entry?
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