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A module that is everything required to understands Druid Segments
There is a newer version: 30.0.1
/*
 * 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.druid.timeline.partition;

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.databind.ObjectMapper;

/**
 * This interface is used in the segment allocation protocol when it is coordinated by the Overlord; when appending
 * segments to an existing datasource (either streaming ingestion or batch append) or any case when segment
 * lock is used. The implementations of this interface contain all information of the corresponding {@link ShardSpec}
 * except the partition ID.
 * The ingestion tasks send all information required for allocating a new segment using this interface and the Overlord
 * determines the partition ID to create a new segment.
 */
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
@JsonSubTypes({
    @Type(name = "numbered", value = NumberedPartialShardSpec.class),
    @Type(name = HashBasedNumberedPartialShardSpec.TYPE, value = HashBasedNumberedPartialShardSpec.class),
    @Type(name = "single_dim", value = SingleDimensionPartialShardSpec.class),
    @Type(name = "numbered_overwrite", value = NumberedOverwritePartialShardSpec.class),
})
public interface PartialShardSpec
{
  /**
   * Creates a new ShardSpec with given partitionId and numCorePartitions.
   *
   * @param objectMapper      jsonMapper used only for {@link HashBasedNumberedShardSpec}
   * @param partitionId       partitionId of the shardSpec. must be carefully chosen to be unique in a time chunk
   * @param numCorePartitions the core partition set size. Should be set properly to determine if this segment belongs
   *                          to the core partitions.
   */
  ShardSpec complete(ObjectMapper objectMapper, int partitionId, int numCorePartitions);

  /**
   * Returns the class of the shardSpec created by this factory.
   */
  @JsonIgnore
  Class getShardSpecClass();

  /**
   * Returns true if this partialShardSpec needs a partitionId of a non-root generation.
   * Any partialShardSpec to overwrite a subset of segments in a time chunk such as
   * {@link NumberedOverwritePartialShardSpec} should return true.
   *
   *
   * @see PartitionIds
   */
  default boolean useNonRootGenerationPartitionSpace()
  {
    return false;
  }
}