TL;DR: I played a fairly large role in writing this package, and I wouldn’t use it outside Uber’s environment at that time (thousands of engineers, thousands of microservices, tens of thousands of Go repositories).

At the time we wrote Fx, Uber had ~1500 engineers writing Go. The company had 25 million lines of Go, spread across more than a thousand microservices and an unknown number of shared libraries (likely hundreds, perhaps as many as a thousand). Nearly every project was in a separate git repository, with effectively no tools to make large cross-repository refactorings. As an engineering organization, we struggled to make relatively simple cross-cutting changes.

For example, we spent years rolling out distributed tracing. The actual change required was simple: upgrade all your dependencies to something recent-ish, add the tracing library, construct a tracer in main (or something main-adjacent), use the tracer to construct an RPC interceptor, and add the interceptor to your API server. All in, we're talking about ~20 lines of code and a dependency upgrade. It took multiple TPMs, spreadsheets, quarterly planning, and several high-level edicts to get this mostly done.

Why were changes so painful? At root, because nobody cared much about most of the Go repositories. From the perspective of the teams who nominally owned the code (often after several reorganizations over the years), the code worked fine and solved the business purpose - why invest time in changing anything? On the ground, changes were painful. Go's simplicity makes semantic versioning _very_ restrictive, so trying to pull in a year's worth of dependency updates often produced a variety of breakages. (Keep in mind that many of these libraries were used only in a handful of projects and weren't particularly carefully designed or maintained.) Taking on all this pain to change 20 lines of code in main was a difficult sell.

Fx codified some basic back-compat best practices (if you're paranoid) - mostly param and result structs, so constructors have more flexibility to add inputs and outputs. Fx also made most of these problems a negotiation directly between library authors, leaving the microservice team out of the picture: the "standard Uber stuff" package provides a distributed tracer, and the "RPC stuff" package takes an _optional_ tracer and installs the appropriate interceptor. No changes to main or application logic necessary, just a dependency update (which is hopefully safer, since more libraries are forced to follow better semver practices). The reflection-based wiring came with lots of magic and downsides, but the tradeoff was worth it across the engineering organization - it made us _overall_ more able to change our own systems.

Bluntly, IMO Fx made individual codebases less understandable (especially codebases carefully maintained by engineers who like Go). It made the whole company's code more maintainable. The bulk of the Go engineers at the company agreed (the developer experience org tracked NPS, which went from double-digit negative to +40ish).

In the years since Fx, I left Uber and the company has moved most of their Go to a monorepo. I'm not sure what the current cost/benefit tradeoff of this approach is.