Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/13342#discussion_r66534481
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/ForeachWriter.scala
---
@@ -0,0 +1,63 @@
+/*
+ * 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
+import org.apache.spark.sql.streaming.ContinuousQuery
+
+/**
+ * :: Experimental ::
+ * A writer to consume data generated by a [[ContinuousQuery]]. Each
partition will use a new
+ * deserialized instance, so you usually should do the initialization work
in the `open` method.
+ *
+ * @since 2.0.0
+ */
+@Experimental
+abstract class ForeachWriter[T] extends Serializable {
+
+ /**
+ * Called when starting to process one partition of new data in the
executor side. `version` is
+ * for data deduplication. When recovering from a failure, some data may
be processed twice. But
+ * it's guarantee that they will be opened with the same "version".
+ *
+ * If this method finds this is a partition from a duplicated data set,
it can return `false` to
+ * skip the further data processing. However, `close` still will be
called for cleaning up
+ * resources.
+ *
+ * @param partitionId the partition id.
+ * @param version a unique id for data deduplication.
+ * @return a flat that indicates if the data should be processed.
--- End diff --
return true if the corresponding partition and version id should be
processed.
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