The easiest solution would be to keep a record of all hosts which checked in at some point in time, even if a host is currently no longer utliized. And the Dash could just display this information as a [hostname, num_procs] table.

We could then add API to the dash to purge stale, non-participating hosts.

The cool thing about the way the monitor currently tracks instances, is that we can come up with alerts such as "X worker instances failed to check-in on Host Y, in the last Z minutes", or "Host Y disappeared Z minutes ago". However, some API will be needed to mute alerts when operational changes are being made to Vumi systems.