Segment refresh

The SegmentRefreshTask reads the latest table config, refreshes the segments if they are not consistent with the table config. When 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.

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 per 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

How to config SegmentRefreshTask

Configure the SegmentRefreshTask under the taskConfig in the table config.

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 via server reload, we have extended support to handle addition to this task, so that we can do it in one shot and via Minions)
  • Column is deleted
  • Column field type change
  • Column data type change
  • Column SV/MV change
  • Column encoding change

Example Configuration

    "task": {
      "taskTypeConfigsMap": {
        "SegmentRefreshTask": {
          "bucketTimePeriod": "1d",
          "maxNumRecordsPerSegment": "2000000",
          "maxNumRecordsPerTask": "10000000"
          "schedule": "0 */30 * * * ?"
        }
      }
    }

Real-time tables

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

Upsert table

This task is made to work with real-time table enabled Upsert 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.

Firstly, the records can't be rolled up because we need to keep the primary keys intact before/after refreshing.

Secondly, the task does M:1 merging, i.e. combining multiple input segments into one new segment, to simplify the failure handling for tracking the upsert metadata consistently. With M:1 mapping, data repartitioning and bucketizing are not supported.

Most task configs said above are ignored, but this one "maxNumRecordsPerSegment": "2000000", to configure 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.

Limitation

It's to be validated if this task can work with real-time table enabled Dedup.