We use akka-persistence with a home made Couchbase plugin.
Every session is persisted as a sequence of events. Those events are used for session recovery when needed. They can also be streamed to long term storage/dashboards. There are some additional advantages of event sourcing, like retroactive report generation, auditing, troubleshooting, etc.
Additional possible usage is that several services will subscribe for those events and react in real time/near real time to those events. For this use case it would be useful to be able to subscribe for session’s events, at session granularity.
I doubt there is a single data store today capable of meeting the defined requirements (1M live subscriptions, latency of ~20msec, average).
I do think it would be very useful for applications using event sourcing.