Tier 2: named processes, read on the tick.

Never :erlang.processes(). On a node with half a million processes that is ruinous at 1 Hz, and it is the single easiest way to turn a lightweight observer into the thing you are debugging. Kepler reads a declared watchlist and nothing else.

Watches on the same process are read together — one Process.info/2 call per target per tick, regardless of how many measurements you declared on it.

Measurements

:message_queue_len, :memory, :heap_size, :total_heap_size, :stack_size, :reductions, and :alive.

:alive is 1 or 0, so "the exporter died" is a condition you can write:

watch :exporter_down do
  source process: MyApp.Exporter
  measure :alive
  fire when: value == 0, sustained: :timer.seconds(10)
end

Every other measurement reports no data when the process is not running, so a watch on a restarting process holds its state rather than firing on the gap.