Everyone tells you that you should strive for:

High cohesion, low coupling.

A definition:

Cohesion is about how well elements within a module belong together and serve a common purpose. Coupling is about how much one module depends or interacts with other modules. Thus, cohesion is an intra-module concern whereas coupling cuts across modules [aka inter-module].

(Devopedia)

Cohesion is a property of code that can be measured. It’s like clarity/obscurity, obviousness, or beauty – you need to analyze the code, then you can tell if it’s cohesive, and how. In boxes-and-arrows graphical analysis, it boils down to how many arrows stay inside the group instead of crossing boundaries.

• Example of High Cohesion and Low Coupling with Presenter, View, and View Model: These components can form a logical unit, a module; or you can group them ‘by name’ and destroy cohesion.

Yourdon’s 6 Levels of Cohesion

Levels of cohesion according to Yourdon, pp. 108-132.¹

1. accidental (coincidental)
   • Reducing code duplication for the sake of reducing code duplication results in “coincidental cohesive modules”. See Don't Repeat Yourself.
   • The code doesn’t have a shared meaning/intent, only superficial similarity.
2. logical
3. temporal
4. communicational
5. sequential
   • Reactive code is sequentially cohesive: a sequence of operators, from network request to UI update, can encapsulate the steps even more clearly than a regular, imperative function with multiple lines.
6. functional

I call 1–3 “code-level cohesion”; 4–6 is cohesion reflecting the problem space.

When I write “code-level”, this is meant to invoke something similar to “code smell”: it’s purely derived from looking at the code as code, without interpreting its meaning and how it creates higher-level structures or expresses intent.

SMC Cohesion

Yourdon’s 6 levels are built upon the principles behind “SMC Cohesion”. “SMC” stands for Stevens, Myers, and Constantine.² Constantine is co-author of Yourdon’s book, so it checks out that the following 7 levels found their way from the SMC paper² into the Yourdon/Constantine book.

1. coincidental association
2. logical association
3. temporal association
4. procedural association
5. communicational association
6. sequential association
7. functional association