connect/api/src/main/java/org/apache/kafka/connect/sink/SinkTask.java

/*
 * 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.
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package org.apache.kafka.connect.sink;

import org.apache.kafka.clients.consumer.OffsetAndMetadata;
import org.apache.kafka.common.TopicPartition;
import org.apache.kafka.connect.connector.Task;
import org.apache.kafka.connect.transforms.Transformation;

import java.util.Collection;
import java.util.Map;

/**
 * SinkTask is a Task that takes records loaded from Kafka and sends them to another system. Each task
 * instance is assigned a set of partitions by the Connect framework and will handle all records received
 * from those partitions. As records are fetched from Kafka, they will be passed to the sink task using the
 * {@link #put(Collection)} API, which should either write them to the downstream system or batch them for
 * later writing. Periodically, Connect will call {@link #flush(Map)} to ensure that batched records are
 * actually pushed to the downstream system.
 * <p>
 * Below we describe the lifecycle of a SinkTask.
 *
 * <ol>
 *     <li><b>Initialization:</b> SinkTasks are first initialized using {@link #initialize(SinkTaskContext)}
 *     to prepare the task's context and {@link #start(Map)} to accept configuration and start any services
 *     needed for processing.</li>
 *     <li><b>Partition Assignment:</b> After initialization, Connect will assign the task a set of partitions
 *     using {@link #open(Collection)}. These partitions are owned exclusively by this task until they
 *     have been closed with {@link #close(Collection)}.</li>
 *     <li><b>Record Processing:</b> Once partitions have been opened for writing, Connect will begin forwarding
 *     records from Kafka using the {@link #put(Collection)} API. Periodically, Connect will ask the task
 *     to flush records using {@link #flush(Map)} as described above.</li>
 *     <li><b>Partition Rebalancing:</b> Occasionally, Connect will need to change the assignment of this task.
 *     When this happens, the currently assigned partitions will be closed with {@link #close(Collection)} and
 *     the new assignment will be opened using {@link #open(Collection)}.</li>
 *     <li><b>Shutdown:</b> When the task needs to be shutdown, Connect will close active partitions (if there
 *     are any) and stop the task using {@link #stop()}</li>
 * </ol>
 *
 */
public abstract class SinkTask implements Task {

    /**
     * <p>
     * The configuration key that provides the list of topics that are inputs for this
     * SinkTask.
     * </p>
     */
    public static final String TOPICS_CONFIG = "topics";

    /**
     * <p>
     * The configuration key that provides a regex specifying which topics to include as inputs
     * for this SinkTask.
     * </p>
     */
    public static final String TOPICS_REGEX_CONFIG = "topics.regex";

    protected SinkTaskContext context;

    /**
     * Initialize the context of this task. Note that the partition assignment will be empty until
     * Connect has opened the partitions for writing with {@link #open(Collection)}.
     * @param context The sink task's context
     */
    public void initialize(SinkTaskContext context) {
        this.context = context;
    }

    /**
     * Start the Task. This should handle any configuration parsing and one-time setup of the task.
     * @param props initial configuration
     */
    @Override
    public abstract void start(Map<String, String> props);

    /**
     * Put the records in the sink. This should either write them to the downstream system or batch them for
     * later writing. If this method returns before the records are written to the downstream system, the task must
     * implement {@link #flush(Map)} or {@link #preCommit(Map)} to ensure that offsets are only committed for records
     * that have been written to the downstream system (hence avoiding data loss during failures).
     * <p>
     * If this operation fails, the SinkTask may throw a {@link org.apache.kafka.connect.errors.RetriableException} to
     * indicate that the framework should attempt to retry the same call again. Other exceptions will cause the task to
     * be stopped immediately. {@link SinkTaskContext#timeout(long)} can be used to set the maximum time before the
     * batch will be retried.
     *
     * @param records the collection of records to send
     */
    public abstract void put(Collection<SinkRecord> records);

    /**
     * Flush all records that have been {@link #put(Collection)} for the specified topic-partitions.
     *
     * @param currentOffsets the current offset state as of the last call to {@link #put(Collection)}, provided for
     *                       convenience but could also be determined by tracking all offsets included in the
     *                       {@link SinkRecord}s passed to {@link #put}. Note that the topic, partition and offset
     *                       here correspond to the original Kafka topic partition and offset, before any
     *                       {@link Transformation transformations} have been applied. These can be tracked by the task
     *                       through the {@link SinkRecord#originalTopic()}, {@link SinkRecord#originalKafkaPartition()}
     *                       and {@link SinkRecord#originalKafkaOffset()} methods.
     */
    public void flush(Map<TopicPartition, OffsetAndMetadata> currentOffsets) {
    }

    /**
     * Pre-commit hook invoked prior to an offset commit.
     * <p>
     * The default implementation simply invokes {@link #flush(Map)} and is thus able to assume all {@code currentOffsets}
     * are safe to commit.
     *
     * @param currentOffsets the current offset state as of the last call to {@link #put(Collection)}, provided for
     *                       convenience but could also be determined by tracking all offsets included in the
     *                       {@link SinkRecord}s passed to {@link #put}. Note that the topic, partition and offset
     *                       here correspond to the original Kafka topic partition and offset, before any
     *                       {@link Transformation transformations} have been applied. These can be tracked by the task
     *                       through the {@link SinkRecord#originalTopic()}, {@link SinkRecord#originalKafkaPartition()}
     *                       and {@link SinkRecord#originalKafkaOffset()} methods.
     *
     * @return an empty map if Connect-managed offset commit is not desired, otherwise a map of offsets by topic-partition that are
     *         safe to commit. Note that the returned topic-partition to offsets map should use the original Kafka
     *         topic partitions and offsets instead of the transformed values.
     */
    public Map<TopicPartition, OffsetAndMetadata> preCommit(Map<TopicPartition, OffsetAndMetadata> currentOffsets) {
        flush(currentOffsets);
        return currentOffsets;
    }

    /**
     * The SinkTask uses this method to create writers for newly assigned partitions in case of partition
     * rebalance. This method will be called after partition re-assignment completes and before the SinkTask starts
     * fetching data. Any errors raised from this method will cause the task to stop.
     * <p>
     * Note that the topic partitions here correspond to the original Kafka topic partitions, before any
     * {@link Transformation transformations} have been applied.
     *
     * @param partitions The list of partitions that are now assigned to the task (may include
     *                   partitions previously assigned to the task)
     */
    public void open(Collection<TopicPartition> partitions) {
    }

    /**
     * The SinkTask uses this method to close writers for partitions that are no
     * longer assigned to the SinkTask. This method will be called before a rebalance operation starts
     * and after the SinkTask stops fetching data. After being closed, Connect will not write
     * any records to the task until a new set of partitions has been opened. Any errors raised
     * from this method will cause the task to stop.
     * <p>
     * Note that the topic partitions here correspond to the original Kafka topic partitions, before any
     * {@link Transformation transformations} have been applied.
     *
     * @param partitions The list of partitions that should be closed
     */
    public void close(Collection<TopicPartition> partitions) {
    }

    /**
     * Perform any cleanup to stop this task. In SinkTasks, this method is invoked only once outstanding calls to other
     * methods have completed (e.g., {@link #put(Collection)} has returned) and a final {@link #flush(Map)} and offset
     * commit has completed. Implementations of this method should only need to perform final cleanup operations, such
     * as closing network connections to the sink system.
     */
    @Override
    public abstract void stop();
}