You can get the number of dropped messages for a specific call to “propagate” by looking at the value of the resolved future from “propagate”. It will be the number of messages actually sent through to Druid, so if it’s less than the number of messages you passed to “propagate”, then you can figure out how many messages were dropped by subtracting.
If you want to see the messages that were dropped, the simplest way is just to log out the messages from calls to “propagate” where its return value indicates that it dropped some messages. This will also potentially log out some messages that were not dropped, however, since it’s possible for tranquility to drop part of a batch while sending the rest. If you want to only see the dropped messages, you can provide a beamDecorateFn to the DruidBeams builder. This wraps the innermost beam in the beam stack (see the ASCII art on ClusteredBeam.scala) and so it should be all-or-nothing in terms of whether the batch of messages it sees made it through or not.