A system for processing application programming interface (API) calls and event messages may include a gateway subsystem and an events hub. The gateway subsystem can route API calls submitted by user agents connected to the gateway subsystem to appropriate service subsystems. The events hub can receive event messages from at least some of the service subsystems and publish the received event messages for consumption by one or more consuming subsystems including the gateway subsystem. A first published event message may specify a first user and the gateway subsystem can (i) determine every user agent that is associated with the first user and that is currently connected to the gateway subsystem and (ii) transmit information that represents the first published event message to every user agent that is determined to be associated with the first user and currently connected to the gateway subsystem.