Atomic Vertical Slicing vs Horizontal Layered Architecture— Beyond the ‘Service’ Suffix — e.g. CarService — Toward Functional Single Responsibility with Atomicity

In many codebases, the use of UserService, CarService, or any other [Noun]Service is a common convention. I’ve been using that convention for many years. But now let’s look at it again to question its real intent. At first glance, it may seem like a valid separation of concerns. After all, we’re just grouping domain-related operations under a domain-named class. But is that truly single responsibility? Or is it merely noun-based technical separation of concerns?

I’ve been reflecting on this distinction, and I’ve come to believe that [Noun]Service often masks a deeper problem: lack of clear functional separation. While there’s no universal rule in software architecture—context always matters—there are principles that can guide us toward cleaner, more maintainable structures.

Let’s take a closer look.

The Illusion of Single Responsibility

Say we have a CarService class in our business logic layer. Over time, it accumulates methods like:

• addEngine(), addTransmission(), addMirror(), addBumper(), addWheel(), addSeats(), addDoors()