druid/docs/content/Tasks.md

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Tasks are run on middle managers and always operate on a single data source.

There are several different types of tasks.

Segment Creation Tasks

Index Task

The Index Task is a simpler variation of the Index Hadoop task that is designed to be used for smaller data sets. The task executes within the indexing service and does not require an external Hadoop setup to use. The grammar of the index task is as follows:

{
  "type" : "index",
  "dataSource" : "example",
  "granularitySpec" : {
    "type" : "uniform",
    "gran" : "DAY",
    "intervals" : [ "2010/2020" ]
  },
  "aggregators" : [ {
     "type" : "count",
     "name" : "count"
   }, {
     "type" : "doubleSum",
     "name" : "value",
     "fieldName" : "value"
  } ],
  "firehose" : {
    "type" : "local",
    "baseDir" : "/tmp/data/json",
    "filter" : "sample_data.json",
    "parser" : {
      "timestampSpec" : {
        "column" : "timestamp"
      },
      "data" : {
        "format" : "json",
        "dimensions" : [ "dim1", "dim2", "dim3" ]
      }
    }
  }
}

property	description	required?
type	The task type, this should always be "index".	yes
id	The task ID.	no
granularitySpec	See granularitySpec	yes
spatialDimensions	Dimensions to build spatial indexes over. See Spatial-Indexing	no
aggregators	The metrics to aggregate in the data set. For more info, see Aggregations	yes
indexGranularity	The rollup granularity for timestamps.	no
targetPartitionSize	Used in sharding. Determines how many rows are in each segment.	no
firehose	The input source of data. For more info, see Firehose	yes
rowFlushBoundary	Used in determining when intermediate persist should occur to disk.	no

Index Hadoop Task

The Hadoop Index Task is used to index larger data sets that require the parallelization and processing power of a Hadoop cluster.

{
  "type" : "index_hadoop",
  "config": <Hadoop index config>
}

property	description	required?
type	The task type, this should always be "index_hadoop".	yes
config	See Batch Ingestion	yes

Realtime Index Task

The indexing service can also run real-time tasks. These tasks effectively transform a middle manager into a real-time node. We introduced real-time tasks as a way to programmatically add new real-time data sources without needing to manually add nodes. The grammar for the real-time task is as follows:

{
  "type" : "index_realtime",
  "id": "example",
  "resource": {
    "availabilityGroup" : "someGroup",
    "requiredCapacity" : 1
  },
  "schema": {
    "dataSource": "dataSourceName",
    "aggregators": [
      {
        "type": "count",
        "name": "events"
      },
      {
        "type": "doubleSum",
        "name": "outColumn",
        "fieldName": "inColumn"
      }
    ],
    "indexGranularity": "minute",
    "shardSpec": {
      "type": "none"
    }
  },
  "firehose": {
    "type": "kafka-0.7.2",
    "consumerProps": {
      "zk.connect": "zk_connect_string",
      "zk.connectiontimeout.ms": "15000",
      "zk.sessiontimeout.ms": "15000",
      "zk.synctime.ms": "5000",
      "groupid": "consumer-group",
      "fetch.size": "1048586",
      "autooffset.reset": "largest",
      "autocommit.enable": "false"
    },
    "feed": "your_kafka_topic",
    "parser": {
      "timestampSpec": {
        "column": "timestamp",
        "format": "iso"
      },
      "data": {
        "format": "json"
      },
      "dimensionExclusions": [
        "value"
      ]
    }
  },
  "fireDepartmentConfig": {
    "maxRowsInMemory": 500000,
    "intermediatePersistPeriod": "PT10m"
  },
  "windowPeriod": "PT10m",
  "segmentGranularity": "hour",
  "rejectionPolicy": {
    "type": "messageTime"
  }
}

Id: The ID of the task. Not required.

Resource: A JSON object used for high availability purposes. Not required.

Field	Type	Description	Required
availabilityGroup	String	An uniqueness identifier for the task. Tasks with the same availability group will always run on different middle managers. Used mainly for replication.	yes
requiredCapacity	Integer	How much middle manager capacity this task will take.	yes

Schema: See Schema.

Fire Department Config: See Config.

Firehose: See Firehose.

Window Period: See Realtime.

Segment Granularity: See Realtime.

Rejection Policy: See Realtime.

Segment Merging Tasks

Append Task

Append tasks append a list of segments together into a single segment (one after the other). The grammar is:

{
    "id": <task_id>,
    "dataSource": <task_datasource>,
    "segments": <JSON list of DataSegment objects to append>
}

Merge Task

Merge tasks merge a list of segments together. Any common timestamps are merged. The grammar is:

{
    "id": <task_id>,
    "dataSource": <task_datasource>,
    "segments": <JSON list of DataSegment objects to append>
}

Segment Destroying Tasks

Delete Task

Delete tasks create empty segments with no data. The grammar is:

{
    "id": <task_id>,
    "dataSource": <task_datasource>,
    "segments": <JSON list of DataSegment objects to append>
}

Kill Task

Kill tasks delete all information about a segment and removes it from deep storage. Killable segments must be disabled (used==0) in the Druid segment table. The available grammar is:

{
    "id": <task_id>,
    "dataSource": <task_datasource>,
    "segments": <JSON list of DataSegment objects to append>
}

Misc. Tasks

Version Converter Task

These tasks convert segments from an existing older index version to the latest index version. The available grammar is:

{
    "id": <task_id>,
    "groupId" : <task_group_id>,
    "dataSource": <task_datasource>,
    "interval" : <segment_interval>,
    "segment": <JSON DataSegment object to convert>
}

Noop Task

These tasks start, sleep for a time and are used only for testing. The available grammar is:

{
    "id": <optional_task_id>,
    "interval" : <optional_segment_interval>,
    "runTime" : <optional_millis_to_sleep>,
    "firehose": <optional_firehose_to_test_connect>
}

Locking

Once an overlord node accepts a task, a lock is created for the data source and interval specified in the task. Tasks do not need to explicitly release locks, they are released upon task completion. Tasks may potentially release locks early if they desire. Tasks ids are unique by naming them using UUIDs or the timestamp in which the task was created. Tasks are also part of a "task group", which is a set of tasks that can share interval locks.