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insert-segment-to-db tool

In older versions of Apache Druid, insert-segment-to-db was a tool that could scan deep storage and insert data from there into Druid metadata storage. It was intended to be used to update the segment table in the metadata storage after manually migrating segments from one place to another, or even to recover lost metadata storage by telling it where the segments are stored.

In Druid 0.14.x and earlier, Druid wrote segment metadata to two places: the metadata store's druid_segments table, and descriptor.json files in deep storage. This practice was stopped in Druid 0.15.0 as part of consolidated metadata management, for the following reasons:

1. If any segments are manually dropped or re-enabled by cluster operators, this information is not reflected in deep storage. Restoring metadata from deep storage would undo any such drops or re-enables.
2. Ingestion methods that allocate segments optimistically (such as native Kafka or Kinesis stream ingestion, or native batch ingestion in 'append' mode) can write segments to deep storage that are not meant to actually be used by the Druid cluster. There is no way, while purely looking at deep storage, to differentiate the segments that made it into the metadata store originally (and therefore should be used) from the segments that did not (and therefore should not be used).
3. Nothing in Druid other than the insert-segment-to-db tool read the descriptor.json files.

After this change, Druid stopped writing descriptor.json files to deep storage, and now only writes segment metadata to the metadata store. This meant the insert-segment-to-db tool is no longer useful, so it was removed in Druid 0.15.0.

It is highly recommended that you take regular backups of your metadata store, since it is difficult to recover Druid clusters properly without it.