A ServiceKey is an Akka Typed identifier for registering and discovering actors that provide a particular service protocol. Its name and protocol type together distinguish one key from another. section 6.1
Hotel concierge discovery
HotelConcierge defines ServiceKey[VIPGuest.Command]("concierge-key") and subscribes to it. When VIP guests enter, they register themselves; the concierge receives the listing and logs each guest. A test verifies that the concierge reports both the existing and newly registered guests.
A ServiceKey can also define the membership of an Akka group router. Independently instantiated routees register under the key, while the router discovers them and distributes messages; no routees means messages cannot yet reach a processor. section 6.3.3
