So I actually find that microservices should actually help tremendously here? Service A starts throwing 500s. Service A has a bunch of well defined API calls it makes with known, and logged, requests and responses. These responses should be validated on the way in and produce 400s if they aren't well formed. Most 500s IMHO result from the validations not catching all corner cases, or not handling the downstream errors properly. But in general it should be relatively easy to track down why one, or a series of calls failed.

I also find that by having separate distinct services, it puts up a lot of friction to scope creep in that service and also avoids side effect problems- IE you made this call, and little did you know this updated state somewhere you completely didn't expect and now touching this area is considered off limits, or at least scary because it has tentacles in so many different places. Eventually this will absolutely happen IME. No of course not on your team, you are better than that, but eventually teams change, this is now handled by the offshore or other B/C team, or a tyrant manager takes over for a year or two before that is obsessed with hitting the date, hacks or not, etc...

But I guess an absolutely critical key to that is having a logging/monitoring/testing/tracing workflow built in. Frameworks can help, Hapi.js makes a lot of this stuff a core concept for example. This is table stakes to be doing "micro" services though and any team that doesn't realize that has no business going near them. Based on the comments here though ignorance around this for teams embracing microservices might be more common than I had imagined.