vogievetsky opened a new issue #8120: Confusing error for when Kafka sampler 
can not resolve DNS
URL: https://github.com/apache/incubator-druid/issues/8120
 
 
   ### Affected Version
   
   Druid 0.16.0-snapshot
   
   ### Description
   
   For Kafka ingestion to work the hostnames of the bootstrap servers ip 
addresses need to be resolvable. This is a common issue for people where they 
set up networking in such a way that the hostname is not resolvable. I will 
admit that I banged my head against the wall on this one for like an hour at 
one point.
   
   Right now if the Druid nodes can not resolve the Kafka hostnames this error 
is returned from the sampler (used for the streaming data loader):
   
   ![Screen Shot 2019-07-21 at 12 37 39 
PM](https://user-images.githubusercontent.com/177816/61598160-4f168780-abce-11e9-9473-338e1b5f90a7.png)
   
   The raw response from the sampler is `{"error":"Failed to sample data: No 
entry found for connection 2"}`
   
   (the connection number changes)
   
   It would be great to provide a more detailed error message in this case.
   
   Note: sometimes in this case when the hostname is not resolvable the sampler 
returns with 200 and `[]` as the sample which make the data loader show:
   
   ![Screen Shot 2019-07-21 at 12 38 58 
PM](https://user-images.githubusercontent.com/177816/61598203-c2b89480-abce-11e9-9a8f-5852cd0805e4.png)
   
   This is not easily reproducible and I believe is somehow to do with some 
timing.
   
   Steps to repro:
   - Spin up a Druid with the streaming data loader (master)
   - Make your network not able to resolve hostnames
   - Try to sample a Kafka broker via IP address
   

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