seastar

The observable class keeps the list of std::function<void(Args...)> subscribers onboard. When set (called) the value notifies all observers about the change. Being void the callback that needs to start an asynchronous operation can do nothing but send it into the background by ignoring the top-most future. It would be nice if observable::operator() returned back a future. This would requir

Prometheus can add latency to the data path.

Use a low priority scheduling group for the metrics.

ref: https://groups.google.com/forum/#!topic/seastar-dev/adKyzRgqc5E

> with_scheduling_group(maintenance_scheduling_group, [&] { 
>                   return 
> prometheus_server.listen(ipv4_addr{prom_addr.addr_list.front(), 
> pport}).handle_exception([pport, &cfg] (auto ep) { 
>