Github user cloud-fan commented on a diff in the pull request: https://github.com/apache/spark/pull/19136#discussion_r138546295 --- Diff: sql/core/src/main/java/org/apache/spark/sql/sources/v2/reader/ReadTask.java --- @@ -0,0 +1,43 @@ +/* + * 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; + +import java.io.Serializable; + +/** + * A read task returned by a data source reader and is responsible to create the data reader. + * The relationship between `ReadTask` and `DataReader` is similar to `Iterable` and `Iterator`. + * + * Note that, the read task will be serialized and sent to executors, then the data reader will be + * created on executors and do the actual reading. + */ +public interface ReadTask<T> extends Serializable { + /** + * The preferred locations for this read task to run faster, but Spark can't guarantee that this + * task will always run on these locations. Implementations should make sure that it can + * be run on any location. + */ + default String[] preferredLocations() { --- End diff -- hmmm, do you mean create a `Host` class which only has a string field?
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