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功能

hadoop_index任务中，IndexGeneratorJob负责启动一个MapReduce任务将原始数据形成Segment导入Druid的Storage中。因此，IndexGeneratorJob主要逻辑集中在IndexGeneratorMapper，IndexGeneratorReducer类中。

IndexGeneratorMapper

按行从hadoopShardSpecLookup条件中判断该行数据属于哪个Bucket，hadoopShardSpecLookup是determine_partitions过程计算结果，保存的是当前hadoop_index任务的数据分区规则。hadoopShardSpecLookup的产生逻辑在DeterminePartitionsJob或者DetermineHashedPartitionsJob中。
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Mapper输出格式中，格式<SortableBytes(groupKey, sortedKey), SerializedRow>。
groupKey包含了shardNum，time，partitionNum。MapReduce任务再Partition过程中用到shardNum作为分区的判断条件，DateTime time标识当前bucket所在的数据起始时间范围，partitionNum当前bucket下拆分的partition。
sortedKey包含truncated timestamp和hashed dimensions。reduce的spilling是什么过程，为什么有sortedKey就能减少reducer的spill？
SerializedRow，将一个inputRow二进制序列化

context.write(
    new SortableBytes(
        bucket.get().toGroupKey(),
        // sort rows by truncated timestamp and hashed dimensions to help reduce spilling on the reducer side
        ByteBuffer.allocate(Long.BYTES + hashedDimensions.length)
                  .putLong(truncatedTimestamp)
                  .put(hashedDimensions)
                  .array()
    ).toBytesWritable(),
    new BytesWritable(serializeResult.getSerializedRow())
);

IndexGeneratorReducer

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IndexGeneratorJob最重要逻辑都集中在IndexGeneratorReducer当中
1、PersistExecutor，用来持久化index文件
2、按行处理数据，累加总行数（maxRowCount）和内存数据量（maxBytesInMemory），并判断是否达到阈值。
如果达到阈值，会将该Index进行持久化生成文件。
persist(index, interval, mergedBase, progressIndicator);过程当中调用IndexMergerV9.persist()方法，形成version.bin、factory.json、0000.smoosh、meta.smoosh等文件。
疑问：version.bin、factory.json、0000.smoosh、meta.smoosh这些文件具体是什么作用？

[gh@20:47:40]~$ ll /var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/base-flush4512362737889488592/merged
total 32
-rw-r--r--  1 gh  staff     4  9  3 20:47 version.bin
-rw-r--r--  1 gh  staff    29  9  3 20:47 factory.json
-rw-r--r--  1 gh  staff  1306  9  3 20:47 00000.smoosh
-rw-r--r--  1 gh  staff   135  9  3 20:47 meta.smoosh

文件中内容：

[gh@20:51:24]/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/base-flush4512362737889488592/merged$ cat version.bin

 [gh@20:51:26]/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/base-flush4512362737889488592/merged$ cat factory.json
{"type":"mMapSegmentFactory"}

[gh@20:51:29]/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/base-flush4512362737889488592/merged$ cat 00000.smoosh
d{"valueType":"LONG","hasMultipleValues":false,"parts":[{"type":"long","byteOrder":"LITTLE_ENDIAN"}]} �,'5I,'5Ig{"valueType":"COMPLEX","hasMultipleValues":false,"parts":[{"type":"complex","typeName":"hyperUnique"}]}(�d{"valueType":"LONG","hasMultipleValues":false,"parts":[{"type":"long","byteOrder":"LITTLE_ENDIAN"}]} "d�2�{"valueType":"STRING","hasMultipleValues":false,"parts":[{"type":"stringDictionary","bitmapSerdeFactory":{"type":"roaring","compressRunOnSerialization":true},"byteOrder":"LITTLE_ENDIAN"}]}."a.example.comb.exmaple.com 8,:0:07'unique_hostsvisited_numhoshostI5',I:M�4{"type":"roaring","compressRunOnSerialization":true}{"container":{},"aggregators":[{"type":"hyperUnique","name":"unique_hosts","fieldName":"unique_hosts","isInputHyperUnique":false,"round":false},{"type":"longSum","name":"visited_num","fieldName":"visited_num","expression":null}],"timestampSpec":{"column":"timestamp","format":"yyyyMMddHH","missingValue":null},"queryGranularity":{"type":"none"},"rollup":true}

[gh@20:51:32]/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/base-flush4512362737889488592/merged$ cat meta.smoosh
v1,2147483647,1
__time,0,0,150
host,0,449,792
index.drd,0,792,947
metadata.drd,0,947,1306
unique_hosts,0,150,303
visited_num,0,303,449

3、是否采用extendable shardSpecs。这个是什么，没看出来具体作用呢？
4、生成DataSegment
5、将DataSegment打包成index.zip
6、生成SegmentDescriptor。SegmentDescriptor做什么用的？ SegmentDescriptor具体内容：

[gh@13:35:43]/private/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/junit2870916712364956185/junit7070039265224578711/website/2021-09-03T124308.104Z_4bd0315d3edf460daeb97264534a14a5/segmentDescriptorInfo$ ll
total 8
-rw-r--r--  1 gh  staff  612  9  4 13:34 website_2014-10-22T000000.000Z_2014-10-23T000000.000Z_2021-09-03T124308.104Z.json
[gh@13:35:43]/private/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/junit2870916712364956185/junit7070039265224578711/website/2021-09-03T124308.104Z_4bd0315d3edf460daeb97264534a14a5/segmentDescriptorInfo$ cat website_2014-10-22T000000.000Z_2014-10-23T000000.000Z_2021-09-03T124308.104Z.json
{
  "binaryVersion": 9,
  "dataSource": "website",
  "dimensions": "host",
  "identifier": "website_2014-10-22T00:00:00.000Z_2014-10-23T00:00:00.000Z_2021-09-03T12:43:08.104Z",
  "interval": "2014-10-22T00:00:00.000Z/2014-10-23T00:00:00.000Z",
  "loadSpec": {
    "path": "/private/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/junit2870916712364956185/junit7070039265224578711/website/2014-10-22T00:00:00.000Z_2014-10-23T00:00:00.000Z/2021-09-03T12:43:08.104Z/0/index.zip",
    "type": "local"
  },
  "metrics": "visited_num,unique_hosts",
  "shardSpec": {
    "partitionNum": 0,
    "partitions": 5,
    "type": "numbered"
  },
  "size": 1474,
  "version": "2021-09-03T12:43:08.104Z"
}
[gh@13:35:49]/private/var/folders/pv/dkqbk3wj51b7yb4r5wd9qgdw0000gn/T/junit2870916712364956185/junit7070039265224578711/website/2021-09-03T124308.104Z_4bd0315d3edf460daeb97264534a14a5/segmentDescriptorInfo$

numReducers处理逻辑：

公式： # of reducers = # of partitions by segmentGranularity x numShards
假如segmentGranularity设置为MONTH，"intervals": [ "’2020-01-01’/’2021-01-01’" ]，numShards：2
则reduceNum = 12 (Year / Month) * 2 = 24
注意：正因为Reducer的个数有计算公式和逻辑，所以在jobProperties中设置mapreduce.job.reduces：0是不起作用的

IndexGeneratorReducer类的处理逻辑中是将determine_partitions任务的结果，转换成Bucket类，从而得到reduceNum的值。getAllBuckets流程：
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