Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/11645#discussion_r56373034
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/HDFSBackedStateStoreProvider.scala
---
@@ -0,0 +1,465 @@
+/*
+ * 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.state
+
+import scala.collection.mutable
+import scala.util.Random
+import scala.util.control.NonFatal
+
+import org.apache.hadoop.conf.Configuration
+import org.apache.hadoop.fs.{FileStatus, Path}
+
+import org.apache.spark.{Logging, SparkConf}
+import org.apache.spark.serializer.{DeserializationStream, KryoSerializer,
SerializationStream}
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions.JoinedRow
+import org.apache.spark.util.{CompletionIterator, Utils}
+
+
+/**
+ * An implementation of [[StateStoreProvider]] and [[StateStore]] in which
all the data is backed
+ * by files in a HDFS-compatible file system. All updates to the store has
to be done in sets
+ * transactionally, and each set of updates increments the store's
version. These versions can
+ * be used to re-execute the updates (by retries in RDD operations) on the
correct version of
+ * the store, and regenerate the store version.
+ *
+ * Usage:
+ * To update the data in the state store, the following order of
operations are needed.
+ *
+ * - val store = StateStore.get(operatorId, partitionId, version) // to
get the right store
+ * - store.update(...)
+ * - store.remove(...)
+ * - store.commit() // commits all the updates to made with version
number
+ * - store.iterator() // key-value data after last commit as an iterator
+ * - store.updates() // updates made in the last as an iterator
+ *
+ * Fault-tolerance model:
+ * - Every set of updates is written to a delta file before committing.
+ * - The state store is responsible for managing, collapsing and cleaning
up of delta files.
+ * - Multiple attempts to commit the same version of updates must have the
same updates.
--- End diff --
Maybe even say that if they are different, which one wins is undefined? (It
is at least atomic right?)
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