[GitHub] [druid] gianm edited a comment on pull request #12137: Batch ingestion replace

[GitBox <gi...@apache.org>]

gianm edited a comment on pull request #12137:
URL: https://github.com/apache/druid/pull/12137#issuecomment-1030344922


   Very interesting!
   
   Some comments on the design first. Afterwards, a musing 🙂
   
   - Compatibility scenario 1: what happens if someone using dropExisting today does a rolling cluster update? MMs are typically updated first. When an ingest task gets launched on a new MM, it will generate tombstones. The Overlords, Coordinators, and Brokers will still be running old code. Will they be able to handle the tombstones properly? If not, then we should avoid repurposing the `dropExisting` flag.
   - Compatibility scenario 2: what happens if runs a job that generates tombstones and then rolls back to an earlier version of Druid? This is sort of a more extreme version of scenario 1. The Overlords, Coordinators, Brokers, and Historicals will all be running the older code. Will they be able to work in some reasonable way or will they have trouble handling the tombstones? If there will be trouble, we'll need to make it clear that people should avoid using this feature until they're sure they won't need to roll back.
   - Zero-sized segments make me wonder if some BalancerStrategy implementations will have problems handling them. So I'd suggest having tests with all current BalancerStrategy impls to make sure this works, or alternatively, using size: 1.
   - Not all query types are expected to return zero rows when the input segment has zero rows. This may be an issue for the NoOpQueryRunner based approach. For example, segmentMetadata queries on segments with zero rows should still return the segment id, etc.
   - In the case where tombstones + new segments fully overshadow an existing segment, there isn't a reason to keep the tombstones loaded up forever. They could be dropped from the cluster after the segment they're meant to shadow has been unloaded. I think doing this would involve some logic adjustments to the Coordinator, and would be valuable so people don't run into proliferations of tombstones. Would you be up for adding this logic, either in this patch or in a follow-up?
   - Special case of the above: what happens if a datasource is 100% composed of tombstones? Ideally, those tombstones would all get dropped and the datasource would be completely unloaded.
   
   ---
   
   The musing I promised: IMO this is going to be a very useful feature beyond the `dropExisting` use case. It'll give us a "delete" operation that is distinct from the "drop" operation. That's important because "drop" should be reversible by "re-load" but "delete" shouldn't be. For example, consider this case:
   
   1. You delete some time ranges, either using one of the Coordinator DELETE APIs or by using `dropExisting`. Today, these are implemented by marking the relevant segments unused.
   2. You don't ever add more data for these time ranges.
   2. Later on, you drop the table to save space. This marks all the rest of the segments in the table unused.
   3. Later still, you reload the table so you can query it again. This goes through and marks all the topmost segments for each time chunk as used.
   4. Surprise! The time ranges you thought you deleted in (1) are now back!
   
   This is confusing, and it's fixed by creating tombstones in (1) rather than marking the segments unused. That way, in step (3), the tombstones would block the unwanted segments from getting reloaded.
   
   So, his patch updates `dropExisting` to create tombstones, but IMO in the future we should also be creating tombstones for any operation that the user would logically think is more of a "delete" than a "drop".


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