…It seems obvious until you deal with integration tests and products with no automation.  I got tripped up by this example:

ProductA calls ProductB’s service, ServiceB.  Both products are owned by the same dev shop.  ServiceB keeps breaking in production, disrupting ProductA. ProductA has automated checks.  ProductB does NOT have automated checks.  Automated checks for ServiceB might help. Where would the automated checks for ServiceB live?

It’s tempting to say ProductA because ProductA has an automation framework with its automated checks running in a Continuous Integration on merge-to-dev.  It would be much quicker to add said automated checks to ProductA than ProductB.  However, said checks wouldn’t help b/c they would run in ProductA’s CI.  ProductB could still deploy to production with a broken ServiceB.

My lesson learned: Despite the ease of adding a check to ProductA’s CI, the check needs to be coupled with ProductB. 

In my case, until we invest in test automation for ProductB, said check(s) for ServiceB will be checks performed by humans.