LESSON 1 / 8 · Module 1 · Separation of Concerns & Layers

1. Layers and Boundaries

[THEORY]

[COMPREHENSION CHECK]

1. The shop switches its order input from semicolon-separated lines to JSON. In the layered version of this lesson, which code has to change?

All three parts, because they are connected by calls Only handle, because it owns the input format OrderService, because it receives the product and the quantity StockStore, because it holds the data the input refers to

2. A teammate adds a println with the remaining stock to OrderService.place. What is wrong with that?

println is slow, so placing an order would take noticeably longer Nothing, as long as the method also returns a value to its caller A class may not print in Kotlin; only top-level functions are allowed to do that The domain now decides wording and output, so a UI change reaches the rules

3. Marketing wants orders of more than 50 units to need approval. Where does that rule belong?

In a new layer of its own, because every rule needs one In StockStore, because it already knows the stock In OrderService, because it is a business rule In handle, because it is the first code that sees the quantity

4. To save a call, someone changes handle to use StockStore directly: it checks store.available(product) and calls store.remove(...) itself. What is the problem?

None. Skipping a layer is faster, and fewer calls are always better StockStore may only be used by other data access classes, never by functions The stock rule now lives in handle, so every new caller has to repeat it Kotlin does not allow a function to use a class from a lower layer

5. A colleague wants to split a 15-line script into presentation, domain and data access classes. The script converts one CSV file into a report and runs once a year. What is the best answer?

Do not: no part has its own reason to change, so each boundary only adds cost Do it: three layers are the minimum for any program, even a short script Do not: layers are only worth it in programs that talk to a database Do it, because a script that grows later will need the layers anyway, so start now

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