Finding out which agents are active

The ‘active_hosts’ topic in the SAM namespace can be subscribed to by nodes that wish to know which agents are connected to the SAM network at a given time. This topic is latched, and the list is updated whenever a host appears on / disappears from the network. This can be used, for instance, to know which robots are running at any given time. Note, however, that non-communicative hosts will only be removed from the list after a pre-defined amount of time (by default, 20 seconds, but configurable as the ‘~keepalive_period’ parameter), so this list is not guaranteed to be delay-free.