Balrog Rules have always been a bit difficult to understand. Much of this is rooted in the fact that they work together as set instead of individually - this means you when you alter one Rule, you can affect things you didn't mean to. For example, given two Rules: priority: 80, product: Firefox, channel: release, mapping: Firefox-53.0.1-build1 priority: 50, product: Firefox, mapping: No-Update In the above, All Firefox release channel update requests are returned Firefox-53.0.1-build1. Implicitly, this means that any other Firefox update channel would receive No-Update. If we were to modify _just_ the first rule, and remove its channel, _all_ Firefox channels would begin to receive Firefox-53.0.1-build1. Because unspecified parts of the Rule are treated as wildcards, it's extremely difficult to answer questions like "If I remove the channel from this Rule, which platforms/locales/products/channels/etc. will it affect?". bug 1241766 has an idea about how to do that, but I think the most important thing we can do is make it easier for humans to understand the Rules, and validate them before they become live. We've got numerous other bugs on file with concrete ways to improve this, so we'll treat this as a tracking bug.

This was a tracking bug for a bunch of work that we wanted to do in Q3/Q4. We've managed to improve things to the point where we're able to make changes more easily than before - I think this bug has outlived its usefulness.