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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