[beebs93]

> synthetic in the sense of synthetic traffic, since it isn't traffic from genuine users.

Yup - I think that lines up.

> what is the master aggregate "switch" ? what does it do?

We have a hierarchy of aggregrate monitors (or "switches") that watch n amount of either specific metrics or other sub-aggregate monitors.

In the case of production deployments, we watch a specific rollback aggregrate monitor for either a fixed amount of time or customer traffic that will auto-trigger a rollback if it goes into alarm (aka switches on).

We also have a master aggregrate monitor that will switch on if any sub-monitors get swtiched on for any reason. We typically watch this master aggregate alarm to auto-disable any promotions in our code pipeline.