This is the same issue that was originally found in the arangodb-java-driver-async module. See: arangodb/arangodb-java-driver-async#21

After taking a closer look it occurred to me that the main problem is localized between ExtendedHostResolver and ArangoDBAsync.Builder, specifically in a way how the pools and connection factories are re-used for sync and async communications.

final HostResolver hostResolver = createHostResolver(createHostList(max, connectionFactory), max, connectionFactory);
final HostHandler hostHandler = createHostHandler(hostResolver);
return new ArangoDBAsyncImpl(asyncBuilder(hostHandler), util, syncBuilder(hostHandler), hostResolver, new ArangoContext());

As you can see both sync and async communication builders share the same HostHandler instance, and subsequently the same HostResolver that in turn points to a ConnectionFactory which has different implementations for sync and async variants. So according to the current architecture HostHandler and HostResolver classes even though don't directly depend on but de facto imply a concrete communication type (sync or async), and hence cannot be reused between them.

I'm not sure if I comprehend all the details and reasoning behind the current connection model and responsibility splitting between abstractions, but I have a feeling that some dependencies aren't justified, and that the issue could be fixed by applying some refactoring and rewire the classes a little bit.

Nevertheless I have prepared a quick and dirty fix that avoids refactoring and seemingly works (according to my tests and my understanding of how the things are organized in the driver). The way I approached it was creating an additional pair of HostHandler and HostResolver specifically for being used by the ArangoExecutorSync executor for the ExtendedHostResolver to connect to /_api/cluster/endpoints. This separates the sync executor used internally by the host resolver from the primary communication chain used to execute the client application requests.

@rashtao
I'll create a PR shortly. Could you please take a look at it, if it's something that makes sense?
Thank you!