Too Long; Didn't Read
The Realtime API Family [A Deep Dive] is an umbrella for event-driven, streaming, pub/sub, push, and other APIs. Ably defines realtime API as anything that happens in under 100ms as this is typically the threshold for perceiving something as happening immediately. Any API designed such that data flowing from producers to consumers happens in the shortest amount of time possible can therefore be described as realtime. The company behind Ably's Pub/Sub Channels API acts as a serverless message bus, and subscribers can listen for events based on those topics.