Expose writable connection count

[sigerber]

This change would allow clients using the PushMonitor to ascertain whether there are currently any usable connections to APNS. This information is useful for upstream service monitoring as it is not possible to reliably figure this out only by listening for failed connections.

[jchambers]

This information is useful for upstream service monitoring as it is not possible to reliably figure this out only by listening for failed connections.

Out of curiosity, what decisions could you make based on this information? Following the "cycle connections more frequently" changes introduced in v0.4, this number could be extremely volatile.

[sigerber]

We are primarily interested in whether the count is > 0 or not. This would be useful to us as a basic health-check showing whether we have connectivity to APNS. Our production environment is complex and a misconfigured network is easily possible. We do expect Pushy's improved connection cycling in 0.4 to make this even more useful to us, as connection problems would surface sooner.

[jchambers]

Interesting.

Could you achieve the same result by using a bounded public queue (Pushy won't take notifications from the queue unless it has writable connections) and watching for timeouts on offer or exceptions on add?

[jchambers]

@sigerber It sounds like there's wider demand for a feature like this, but I think we're going to pass on this specific implementation for now in favor of doing something more robust (JMX, maybe?) as described in #175.

Thanks kindly for the contribution!