druid/docs/content/ingestion/compaction.md

layout	title
doc_page	Compaction Task

Compaction Task

Compaction tasks merge all segments of the given interval. The syntax is:

{
    "type": "compact",
    "id": <task_id>,
    "dataSource": <task_datasource>,
    "interval": <interval to specify segments to be merged>,
    "dimensions" <custom dimensionsSpec>,
    "keepSegmentGranularity": <true or false>,
    "segmentGranularity": <segment granularity after compaction>,
    "targetCompactionSizeBytes": <target size of compacted segments>
    "tuningConfig" <index task tuningConfig>,
    "context": <task context>
}

Field	Description	Required
type	Task type. Should be compact	Yes
id	Task id	No
dataSource	DataSource name to be compacted	Yes
interval	Interval of segments to be compacted	Yes
dimensionsSpec	Custom dimensionsSpec. Compaction task will use this dimensionsSpec if exist instead of generating one. See below for more details.	No
metricsSpec	Custom metricsSpec. Compaction task will use this metricsSpec if specified rather than generating one.	No
segmentGranularity	If this is set, compactionTask will change the segment granularity for the given interval. See segmentGranularity of Uniform Granularity Spec for more details. See the below table for the behavior.	No
keepSegmentGranularity	Deprecated. Please use segmentGranularity instead. See the below table for its behavior.	No
targetCompactionSizeBytes	Target segment size after comapction. Cannot be used with maxRowsPerSegment, maxTotalRows, and numShards in tuningConfig.	No
tuningConfig	Index task tuningConfig	No
context	Task context	No

Used segmentGranularity based on segmentGranularity and keepSegmentGranularity

SegmentGranularity	keepSegmentGranularity	Used SegmentGranularity
Non-null	True	Error
Non-null	False	Given segmentGranularity
Non-null	Null	Given segmentGranularity
Null	True	Original segmentGranularity
Null	False	ALL segmentGranularity. All events will fall into the single time chunk.
Null	Null	Original segmentGranularity

An example of compaction task is

{
  "type" : "compact",
  "dataSource" : "wikipedia",
  "interval" : "2017-01-01/2018-01-01"
}

This compaction task reads all segments of the interval 2017-01-01/2018-01-01 and results in new segments. Since both segmentGranularity and keepSegmentGranularity are null, the original segment granularity will be remained and not changed after compaction. To control the number of result segments per time chunk, you can set maxRowsPerSegment or numShards. Please note that you can run multiple compactionTasks at the same time. For example, you can run 12 compactionTasks per month instead of running a single task for the entire year.

A compaction task internally generates an index task spec for performing compaction work with some fixed parameters. For example, its firehose is always the ingestSegmentSpec, and dimensionsSpec and metricsSpec include all dimensions and metrics of the input segments by default.

Compaction tasks will exit with a failure status code, without doing anything, if the interval you specify has no data segments loaded in it (or if the interval you specify is empty).

The output segment can have different metadata from the input segments unless all input segments have the same metadata.

• Dimensions: since Druid supports schema change, the dimensions can be different across segments even if they are a part of the same dataSource. If the input segments have different dimensions, the output segment basically includes all dimensions of the input segments. However, even if the input segments have the same set of dimensions, the dimension order or the data type of dimensions can be different. For example, the data type of some dimensions can be changed from string to primitive types, or the order of dimensions can be changed for better locality. In this case, the dimensions of recent segments precede that of old segments in terms of data types and the ordering. This is because more recent segments are more likely to have the new desired order and data types. If you want to use your own ordering and types, you can specify a custom dimensionsSpec in the compaction task spec.
• Roll-up: the output segment is rolled up only when rollup is set for all input segments. See Roll-up for more details. You can check that your segments are rolled up or not by using Segment Metadata Queries.