A light per message stops meaning anything once there are enough messages

The background

A diagram of a running system usually draws the parts and not the traffic, so it looks identical whether the thing is working or has been dead for an hour. Lighting each connection as data goes down it is the obvious repair, and it holds until the system is asked to run faster than real time — at which point every line is flashing constantly, and the picture is back to saying nothing at all.

The requirement

A line has to show that something crossed it and go dark when nothing does. Two things it may not do: claim traffic that did not happen, and cost more to draw than the traffic costs to carry. Redrawing twenty component faces for every message would have the display competing for the machine with the system it is watching.

The options considered

Shortening the flash as the clock speeds up keeps the flicker and adds arithmetic. Sampling the traffic means throwing away messages that did happen. What won was changing what the light means: at real time, one fade per message; above it, a steady light, held while traffic keeps arriving and dropped a second after it stops. The panel says which of the two it is doing, because they are different claims.

The demo

Open it at the Operator view, press step 60 ticks, and watch the sensing path light and fade. Then set the clock to ×60 in the header: the same wires hold steady instead. The line under the chart names the topics whose lights are an approximation — the broker hands a subscriber a topic, never a sender.