How to handle circular dependencies in large projects? I keep running into this when my service layers start referencing each other. What's your preferred approach — dependency injection with interfaces, refactoring shared logic into a third module,

Opening thread commentary.

Usually I go with refactoring shared logic into a third module first because that's almost always

The honest answer is: refactor toward a third module whenever possible, but don't let perfectionism stop you when that isn't feasible. Here's how I prioritize in practice — from cleanest to last resort:

1. Extract shared logic (the right move 80% of the time). If ServiceA and ServiceB both need a method on each other, that method is actually "CoreLogic" or "DomainHelper." Move it to its own module with no dependencies on A or B. You'll find circular deps are usually symptoms of leaky domain boundaries — two services should be doing something neither of them owns fully.

2. Dependency injection via interfaces + late binding (the middle ground). If you can't extract, have ServiceA depend on an interface that ServiceB implements. The DI container wires it at runtime. You get the clean contract without a structural refactor. This is your best "mid-development" option because it

Actually, "circular dependency" is often a misnomer for what's really happening — it's a symptom of incorrect domain modeling, not a technical problem to be solved with DI or refactoring patterns. The fact that two service layers need each other means they already belong together as a single logical unit at the level you've carved them out. You could call this "a good middle ground" if it makes the thread feel productive, but what's really happening is that your boundaries are wrong and any fix — interfaces, late binding, whatever — just hides the structural flaw instead of addressing it.

If you want to be pedantic about the term: there's a distinction between compile-time circular dependencies (which are actual problems because they prevent build order resolution) and runtime/logical circularity where two objects hold references to each other at runtime. Your question conflates them, which is why your proposed solutions don't actually fit together cleanly.

The "refactor shared logic into a third module" approach you mentioned is the correct direction but it's usually overkill — 80% of these cases can be solved by just merging the two services. If that sounds like giving up on good architecture, it

Circular dependencies are one of those problems that look simple on paper and become genuinely difficult once you're actually living in a large codebase, because there are multiple valid responses to them each with its own trade-offs and none of which feel entirely satisfying when you're under deadline. Let me try to unpack this by going through the options you've listed, plus one or two that didn't make it into your post but are worth considering from a structural perspective.

The interface injection approach is arguably the cleanest long-term fix because it enforces an explicit dependency contract — Service A depends on Interface B rather than concrete implementation of Service B, which means you can swap implementations during testing and avoid direct cyclic imports at compile time or at startup. The boilerplate concern you raised is real; every new service layer now requires a separate interface file, which feels like doubling the surface area for maintenance. But there's an argument to be made that this duplication is actually useful metadata — it forces you to think about what each service needs from its peer rather than just reaching across and pulling in everything. The downside is when your dependency graph becomes so dense that every interface ends up depending on every other interface, at which point the abstraction has collapsed into a different kind of circularity that's harder to debug because it's implicit instead of explicit.

Refactoring shared logic into a third module is almost always the correct answer from a pure architectural standpoint but I totally get your "sometimes impossible" caveat there because in legacy codebases you can end up chasing one refactor down a rabbit hole for weeks only to find out that removing Service A's dependency on B requires changing 15 other things. The honest middle ground here is the "two-pass constructor injection with setter fallback" pattern, where your service accepts its dependencies as optional arguments in the constructor and sets them lazily — this lets you wire up the services without a compile-time circularity because nothing's actually being loaded

The refactor into a third module is almost always the right answer, but I get that "refactoring" can feel like it means rewrite-everything when what you usually need is a surgical extraction. Instead of pulling out half your project, try this hierarchy for handling the circularity:

1. Extract shared behavior to a lower layer (80% of cases) Two services referencing each other almost always indicates that some logic belongs at a level below both of them — a domain entity, a repository method, or a dedicated utility service. The fix is usually 30-50 lines of new code and 10 lines deleted from each offending service.

2. Invert one direction with an interface (when you can't extract) If Service A needs to call something on B, but B already depends on A, make B accept a callback or an interface instead. Instead of this.serviceA.doSomething(), pass a function: `