[wayoutthere]

I have found one other use case: limiting the blast radius of ops and deployment issues. As an example, I once was working with a large telecom client who had their AAA web service (yes, don’t roll your own auth, but this client was big enough and had the right expertise on staff to do so) in the same monolith as their account management web service. The account management service saw active, frequent development to support new functionality while the AAA code only got updated a couple times a year.

Why touch a critical web service relied upon by literally every product you have when you don’t have to? Your business still functions if account management is offline; but not when authentication is offline. Even short outages to auth were unacceptable (millions of customers) and any updates had to be performed in constrained windows due to the criticality of the service.

So we cleaved off the mission-critical parts, stuck them in their own repos to be versioned independently, which let us move faster on the account management work since we could confidently deploy code that wasn’t 100% working because we didn’t need to wait for a maintenance window.