My first thought was "why don't they use Akka"?

> Akka has no true lightweight threads (the actors are actually callbacks)

Would you care to elaborate? I'm not too familiar with the internals of Akka, but they definitely don't use "heavyweight" threads (which I assume are threads that are 1:1 mapped to OS threads).

Also, I didn't get "the actors are actually callbacks". Yes, there may be callbacks involved internally (why not?), but there is a big difference whether I am sending a message to an actor (which may be processed at any time) vs. calling a callback (which is immediately executed on the very same thread that I'm running on).

Sorry if this sounds dismissive, but I'd really like to learn why you choose to implement your own solution, because you've obviously put some time into evaluating what is out there.