There’s a moment of despair when extracting functionality from a larger library, framework, or program. The idea grows, a seed at first and then a full-blown tree, that the coupling in this functionality isn’t all bad. A lot of people talk only about coupling and leave out cohesion. They aren’t mutually exclusive! When the two are balanced, it’s hard to come up with a reason to start extracting.
On the other hand, sometimes that moment of despair strikes when you start really digging into the domain and realize this chunk of functionality isn’t what you thought it was. Maybe it’s not coherent (see above!) or perhaps the model of the domain isn’t deep enough. This is a pretty good signal to hit the brakes on the refactoring, figure the domain out, and reconsider the course of action.
Feature envy rears its head in extractions too. Patterns of crosstalk between the existing thing and the new thing are a sure sign of feature envy. It’s tempting to say, hey maybe you really need a third thing in the middle. That’s probably making matters worse though.
That said, changing bidirectional communication to unidirectional is usually a positive thing. Same for replacing any kind of asynchronous communication with synchronous. Or replacing lockstep coordination with asynchronous messaging. Envy is tricky!
(I) often encourage starting a new service or application within your existing “mothership”. The trendy way to say this right now is “monorepo all the things” or build a “modular monolith”. I find this compelling because you can leverage a lot of existing effort into operationalizing, tooling, and infrastructure. Once you know the domain and technical concerns specific to the new thing, you can easily extract into its own thing if you need to. The other edge of a monorepolith is that path dependence is a hell of a thing. Today is almost certainly an easier day to split stuff out than tomorrow.
A thing to consider pursuing is a backend-for-frontend service in pursuit of a specific frontend. It doesn’t even have to serve an application. You may have services that are specific to mobile, desktop, apps, APIs, integrations, etc. Each of these may need drastically different rates of change, technical features, and team sets.
Probably don’t split out a service so that a bunch of specialized people can build a “center of excellence” for the rest of the organization to rely upon. This is a very fancy way to say “we are too cool for everyone else and we just can’t stand the work everyone else is doing”. On their best day, the Excellence team will be overwhelmed by the volume of work they have put in front of themselves to make Everything Good. On their worst day, they will straight give up.
If you split something out, realize you’re going to have to maintain it until you replace it. And you’re going to rebuild the airplane while it’s flying. If you’re not really into that, stop now. Just because you can’t stand Rails, relational databases, or whatever doesn’t mean you should jump into an extraction.