Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/22009#discussion_r208438912
  
    --- Diff: 
sql/core/src/main/java/org/apache/spark/sql/sources/v2/reader/streaming/MicroBatchReadSupport.java
 ---
    @@ -0,0 +1,49 @@
    +/*
    + * 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.reader.streaming;
    +
    +import org.apache.spark.annotation.InterfaceStability;
    +import org.apache.spark.sql.execution.streaming.BaseStreamingSource;
    +import org.apache.spark.sql.sources.v2.reader.ScanConfig;
    +import org.apache.spark.sql.sources.v2.reader.ScanConfigBuilder;
    +
    +/**
    + * An interface which defines how to scan the data from data source for 
streaming processing with
    + * micro-batch mode.
    + */
    +@InterfaceStability.Evolving
    +public interface MicroBatchReadSupport extends StreamingReadSupport, 
BaseStreamingSource {
    +
    +  /**
    +   * Returns a builder of {@link ScanConfig}. The builder can take some 
query specific information
    +   * like which operators to pushdown, streaming offsets, etc., and keep 
these information in the
    +   * created {@link ScanConfig}.
    +   *
    +   * This is the first step of the data scan. All other methods in {@link 
MicroBatchReadSupport}
    +   * needs to take {@link ScanConfig} as an input.
    +   *
    +   * If this method fails (by throwing an exception), the action will fail 
and no Spark job will be
    +   * submitted.
    +   */
    +  ScanConfigBuilder newScanConfigBuilder(Offset start, Offset end);
    +
    +  /**
    +   * Returns the most recent offset available.
    +   */
    +  Offset latestOffset(Offset start);
    --- End diff --
    
    a concrete example is in kafka data source, it needs to know the start 
offset to decide the latest offset, for purposes like rate control.


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