Github user StephanEwen commented on the issue:
https://github.com/apache/flink/pull/5996
I had a quick look at the code example, and it looks like it might not
actually do asynchronous I/O.
It dispatches a synchronous HTTP request on a direct executor
(`onComplete`s in a direct executor as well), which should result us purely
synchronous operations.
Have you verified that this does in fact send off multiple requests
concurrently (beyond the parallelism)? ---
