we are using Flink's

AsyncIO

function with Futures to make external gRPC calls. Currently, we have set the async capacity to 1, and we are using a blocking stub to make those calls. For each event, we trigger 4 Futures (i.e., 4 gRPC calls per event). Does this mean that the

Executors.newFixedThreadPool()

needs to have at least 4 threads to avoid queue starvation? Also, if we increase the async capacity to 2, should we increase the thread pool size to 8 to keep up with the parallel calls?

Generally speaking -- yes. You'll need to ensure that your available thread pool has at least the number of potential concurrent blocking all that you might be making. At a high-level, you'd probably want to target a formula like:

(capacity) * (calls per event)

The above formula would scale based on the capacity. So in your example case, you'd likely want a minimum of 4 threads and if you increased the capacity to 2, then you'd scale that up further to a minimum of 8 threads.

Thanks for the response @Rion Williams 👍