Github user justinleet commented on the issue:
https://github.com/apache/metron/pull/970
Pulled in https://github.com/justinleet/metron/pull/18 from @merrimanr to
address the reported bugs when metaalerts become empty. Thanks for digging
into the bug you found, it's super helpful!
The short story for anyone here, is that the latest commit prevents the
removal of all child alerts from a metalert (which caused problems for Solr in
particular). Given that having an empty collection of alerts has no useful
meaning (and there's no score and such), it's just disallowed entirely if a
remove request will leave a metaalert empty.
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