Github user jose-torres commented on a diff in the pull request:

    https://github.com/apache/spark/pull/20752#discussion_r172618681
  
    --- Diff: 
sql/core/src/main/java/org/apache/spark/sql/sources/v2/writer/streaming/StreamingDataWriterFactory.java
 ---
    @@ -0,0 +1,51 @@
    +/*
    + * 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.sources.v2.writer.streaming;
    +
    +import org.apache.spark.annotation.InterfaceStability;
    +import org.apache.spark.sql.sources.v2.writer.DataWriter;
    +import org.apache.spark.sql.sources.v2.writer.DataWriterFactory;
    +
    +@InterfaceStability.Evolving
    +public interface StreamingDataWriterFactory<T> extends 
DataWriterFactory<T> {
    +  /**
    +   * Returns a data writer to do the actual writing work.
    +   *
    +   * If this method fails (by throwing an exception), the action would 
fail and no Spark job was
    +   * submitted.
    +   *
    +   * @param partitionId A unique id of the RDD partition that the returned 
writer will process.
    +   *                    Usually Spark processes many RDD partitions at the 
same time,
    +   *                    implementations should use the partition id to 
distinguish writers for
    +   *                    different partitions.
    +   * @param attemptNumber Spark may launch multiple tasks with the same 
task id. For example, a task
    +   *                      failed, Spark launches a new task wth the same 
task id but different
    +   *                      attempt number. Or a task is too slow, Spark 
launches new tasks wth the
    +   *                      same task id but different attempt number, which 
means there are multiple
    +   *                      tasks with the same task id running at the same 
time. Implementations can
    +   *                      use this attempt number to distinguish writers 
of different task attempts.
    +   * @param epochId A monotonically increasing id for streaming queries 
that are split in to
    +   *                discrete periods of execution. For non-streaming 
queries,
    +   *                this ID will always be 0.
    +   */
    +  DataWriter<T> createDataWriter(int partitionId, int attemptNumber, long 
epochId);
    +
    +  @Override default DataWriter<T> createDataWriter(int partitionId, int 
attemptNumber) {
    +    throw new IllegalStateException("Streaming data writer factory cannot 
create data writers without epoch.");
    --- End diff --
    
    I suppose we could have it take a (partition, attempt number, epoch) => 
DataWriter lambda instead of Any if we really don't want to extend 
DataWriterFactory.


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