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The SegmentRefreshTask reads the latest table config and refreshes the segments if they are not consistent with the table config. When an inconsistency between segments and table config is detected, it will download the segments from the deep store, process and regenerate the segments, and then push them back to replace the old segments atomically.

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Supported Operations

The following operations can be applied to the segments to match the table config:
  • Time partitioning: Re-partition the segments to be time partitioned (all the records within a segment are in the same time bucket).
  • Value partitioning: Re-partition the segments according to the partitioning config.
  • Merge/Split: Merge small segments or split large segments (with rollup support) to ensure segments are properly sized.
  • Other table config changes that cannot be applied on the server side with segment reload:
    • Change time column
    • Change sorted column
    • Change column data type
    • Change column encoding
complexTypeConfig transforms are not supported with the Segment Refresh Task. The complexTypeConfig configuration is used as a part of the ingestionConfig to flatten and process complex-type data. Do not use the Segment Refresh Task when the complexTypeConfig configuration is enabled. This may result in records being duplicated and unintended changes to tables.

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How to configure the SegmentRefreshTask

Configure the SegmentRefreshTask under the taskConfig section in the table configuration.

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Segment Index Check

When segment index check is enabled, the task generator will pull the segment metadata for each segment from the servers, and compare it with the table config. If the segment metadata is not consistent with the table config, the segment will be refreshed. The following properties are compared between the segment metadata and the table config:
  • Whether the time column is the same.
  • Whether the partitioning info matches:
    • Same partition column
    • Same partition function
    • Same partition count
    • Segment belong to a single partition
  • Sorted column in the table config is sorted in the segment.
  • Checks for all columns:
    • Column is added (while this can be handled through a server reload, the task has been extended, so that it can be performed in one go and through Minions.)
    • Column is deleted.
    • Column field type change.
    • Column data type change.
    • Column SV/MV change.
    • Column encoding change.

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Example Configuration

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Real-time Tables

For real-time tables, the SegmentRefreshTask considers segments which are in COMPLETED state. The consuming segments are left untouched.

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Upsert Table

This task can work with real-time table enabled Upserts as well. The task can compact segments, i.e. removing invalid docs; and then merge segments into bigger one. There are some limitations when this task is enabled for upsert tables, mainly due to the complexity from managing the upsert metadata consistently for upsert to work.
  1. Records can’t be rolled up because the primary keys must be kept intact before and after refreshing.
  2. The task performs M:1 merging, i.e. combining multiple input segments into one new segment, to simplify failure handling for tracking upsert metadata consistently. With M:1 mapping, data repartitioning and bucketizing are not supported.
Most of the task configs described above are ignored, except "maxNumRecordsPerSegment": "2000000" . This setting configures how large the output segment should be, and the input segments are automatically decided for each task, based on how many valid records they have. For the in-depth documentation on running Segment Refresh Task on upsert tables, see Compaction after Upserts.