Look for the responsibility that stays together.
Consider a hypothetical reference register. One part owns the records, while another applies a review decision. It may be tempting to divide the application by screen: a list module, an editing module, and a review page module. That division can leave the same record rules scattered across several places.
An alternative starts with responsibility. Which part defines what a valid record is? Which part decides whether a review may be approved? Which information must the two parts exchange? These questions reveal a boundary that can be discussed independently of the eventual layout.
The objective is not to identify a single perfect decomposition. It is to make the ownership clear enough that a future change has a sensible home.
Explain the connection in terms of meaning.
A module interface can state the inputs, results, and conditions another part can rely on. The contract should explain what the values mean, not only their technical types. A record identifier, a display label, and an approval state have different roles even when they are all represented as strings.
Include the ordinary unsuccessful outcome. If a record does not exist, a request is incomplete, or an action is not permitted, the caller needs an agreed response. Leaving those conditions implicit makes the connection harder to review and maintain.
A short interface note with representative examples can be more useful than a large diagram that does not explain the behavior at the boundary.
Follow the direction of dependency.
When one module relies on another’s internal storage or undocumented behavior, the apparent boundary may be weaker than the folder structure suggests. A change inside one part can unexpectedly require a coordinated change elsewhere.
Identify which information should cross through the agreed interface and which implementation choices should remain internal. This does not require hiding everything behind a universal abstraction. It requires a reason for the dependency and an understanding of its consequences.
Circular dependencies deserve attention because they can blur the ownership of a decision. Sometimes the right response is a clearer interface; sometimes the proposed parts actually belong together.
Do not confuse modules with separate services.
A modular application can run as one deployable unit. Separating responsibilities in code is different from introducing network communication, independent deployment, and additional operating responsibilities. A small system may benefit from clear internal modules without needing a distributed architecture.
Choose deployment boundaries for a concrete need, such as a genuinely independent operational requirement, rather than treating separation as proof of sophistication. The environment and maintenance capacity should influence the decision.
Record why the chosen structure is proportionate. That explanation can help the next team reconsider it when the conditions change.
Use a real change to examine the boundary.
Take a plausible revision: adding a field, changing a review rule, or replacing a data source. Ask which module should change and which contract should remain stable. If every revision affects every part, the boundaries may not reflect the actual responsibilities.
A small map, a clear contract, and a documented reason for ownership can provide a useful first structure. The value lies in making change understandable, not in maximizing the number of modules.