Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/16758#discussion_r99748948
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/KeyedState.scala ---
@@ -0,0 +1,134 @@
+/*
+ * 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
+
+import org.apache.spark.annotation.{Experimental, InterfaceStability}
+import org.apache.spark.sql.catalyst.plans.logical.LogicalKeyedState
+
+/**
+ * :: Experimental ::
+ *
+ * Wrapper class for interacting with keyed state data in
`mapGroupsWithState` and
+ * `flatMapGroupsWithState` operations on
+ * [[KeyValueGroupedDataset]].
+ *
+ * Detail description on `[map/flatMap]GroupsWithState` operation
+ * ------------------------------------------------------------
+ * Both, `mapGroupsWithState` and `flatMapGroupsWithState` in
[[KeyValueGroupedDataset]]
+ * will invoke the user-given function on each group (defined by the
grouping function in
+ * `Dataset.groupByKey()`) while maintaining user-defined per-group state
between invocations.
+ * For a static batch Dataset, the function will be invoked once per
group. For a streaming
+ * Dataset, the function will be invoked for each group repeatedly in
every trigger.
+ * That is, in every batch of the [[streaming.StreamingQuery
StreamingQuery]],
+ * the function will be invoked once for each group that has data in the
batch.
+ *
+ * The function is invoked with following parameters.
+ * - The key of the group.
+ * - An iterator containing all the values for this key.
+ * - A user-defined state object set by previous invocations of the given
function.
+ * In case of a batch Dataset, there is only invocation and state object
will be empty as
+ * there is no prior state. Essentially, for batch Datasets,
`[map/flatMap]GroupsWithState`
+ * is equivalent to `[map/flatMap]Groups`.
+ *
+ * Important points to note about the function.
+ * - In a trigger, the function will be called only the groups present in
the batch. So do not
+ * assume that the function will be called in every trigger for every
group that has state.
+ * - There is no guaranteed ordering of values in the iterator in the
function, neither with
+ * batch, nor with streaming Datasets.
+ * - All the data will be shuffled before applying the function.
+ *
+ * Important points to note about using KeyedState.
+ * - The value of the state cannot be null. So updating state with null
is same as removing it.
+ * - Operations on `KeyedState` are not thread-safe. This is to avoid
memory barriers.
+ * - If the `remove()` is called, then `exists()` will return `false`, and
--- End diff --
done.
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