I'm testing a flow that uses PutSQL to insert some simple data in a logging table. SQL is constructed first in a ReplaceText processor (to easily see SQL in queue before hitting PutSQL processor). SQL is correct and executes fine without using NiFI.

The flow often (not always!) stalls at the PutSQL processor. No errors shown for PutSQL, 1 thread running, but nothing getting through. The incoming queue from ReplaceText piles up, showing 10 in test screenshot. When trying to list the queue, it reports it's empty, despite just reporting 10 after refresh. It often starts flowing again after 10 mins or more, sometimes hours.

Neither Nifi nor database seem under load and otherwise feel responsive.

How can I debug what's causing the stalling?

Using HDF 2.1.1, connecting to a Sybase ASE database.