Github user tdas commented on a diff in the pull request:

    https://github.com/apache/spark/pull/11645#discussion_r56227397
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/HDFSBackedStateStoreProvider.scala
 ---
    @@ -0,0 +1,454 @@
    +/*
    + * 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.
    + * - Background management of files ensures that last versions of the 
store is always recoverable
    + * to ensure re-executed RDD operations re-apply updates on the correct 
past version of the
    + * store.
    + */
    +class HDFSBackedStateStoreProvider(
    +    val id: StateStoreId,
    +    val directory: String,
    +    numBatchesToRetain: Int = 2,
    +    maxDeltaChainForSnapshots: Int = 10
    +  ) extends StateStoreProvider with Logging {
    +  type MapType = mutable.HashMap[InternalRow, InternalRow]
    +
    +  import StateStore._
    +
    +
    +  class HDFSBackedStateStore( val version: Long, mapToUpdate: MapType)
    +    extends StateStore {
    +
    +    trait STATE
    +    case object UPDATING extends STATE
    +    case object COMMITTED extends STATE
    +    case object CANCELLED extends STATE
    +
    +    private val newVersion = version + 1
    +    private val tempDeltaFile = new Path(baseDir, 
s"temp-${Random.nextLong}")
    +    private val tempDeltaFileStream =
    +      serializer.newInstance().serializeStream(fs.create(tempDeltaFile, 
true))
    +    private val allUpdates = new mutable.HashMap[InternalRow, StoreUpdate]
    +
    +    @volatile private var state: STATE = UPDATING
    +    @volatile private var finalDeltaFile: Path = null
    +
    +    override def id: StateStoreId = HDFSBackedStateStoreProvider.this.id
    +
    +    /** Update the value of a key using the value generated by the update 
function */
    +    override def update(key: InternalRow, updateFunc: Option[InternalRow] 
=> InternalRow): Unit = {
    +      verify(state == UPDATING, "Cannot update after already committed or 
cancelled")
    +      val oldValueOption = mapToUpdate.get(key)
    +      val value = updateFunc(oldValueOption)
    +      mapToUpdate.put(key, value)
    +      allUpdates.get(key) match {
    +        case Some(ValueAdded(_, _)) =>
    +          allUpdates.put(key, ValueAdded(key, value))
    +        case Some(ValueUpdated(_, _)) | Some(KeyRemoved(_)) =>
    +          allUpdates.put(key, ValueUpdated(key, value))
    +        case None =>
    +          val update =
    +            if (oldValueOption.nonEmpty) ValueUpdated(key, value) else 
ValueAdded(key, value)
    +          allUpdates.put(key, update)
    +      }
    +      tempDeltaFileStream.writeObject(ValueUpdated(key, value))
    +    }
    +
    +    /** Remove keys that match the following condition */
    +    override def remove(condition: InternalRow => Boolean): Unit = {
    +      verify(state == UPDATING, "Cannot remove after already committed or 
cancelled")
    +      val keyIter = mapToUpdate.keysIterator
    +      while (keyIter.hasNext) {
    +        val key = keyIter.next
    +        if (condition(key)) {
    +          mapToUpdate.remove(key)
    +          allUpdates.get(key) match {
    +            case Some(ValueUpdated(_, _)) | None =>
    +              allUpdates.put(key, KeyRemoved(key))
    +            case Some(ValueAdded(_, _)) =>
    +              allUpdates.remove(key)
    +            case Some(KeyRemoved(_)) =>
    --- End diff --
    
    add more inline comments to explain the cases.


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