> In real life, using a hermetic build system for tests does not actually scale. The dependency graph of your repo becomes entwined with how much of your repo gets tested. Change a deeply nested component often? Even though you only want to deploy one out of all your services, you’ll get full rebuilds of your repo.I still think a hermetic build system is the way to go; but I wouldn’t gate merges on testing the full repo. Instead, let’s give some flexibility to our engineers.

Scaling a monorepo also involves scaling with the number of engineers at the company. The strategy mentioned here, of putting the trust in your engineers to determine what tests need to be run after platform-wide changes does not scale. Eventually there will be enough product teams who care more about speed than quality. Likely resulting in increased outages, because changes won't be guaranteed to be fully tested in CI.

A more reliable way to solve this problem would be to scale out testing in CI. For example with Bazel's remote execution or any type of CI sharding strategy. Optimizing CI build and test times for changes impacting many targets is a scaling problem any monorepo will eventually need to tackle.