Tensions in Practice: Separate feature tests can miss their interaction¶
A physical prototype with two active modules
Two invented modules each draw 2 units from a shared supply that can provide 3. A prototype can test each module alone, or integrate both and exercise their joint state. The toy arithmetic is intentionally simple: either alone fits; together they exceed the limit. Testing a component and testing its interaction answer different questions.
Isolate a feature’s behavior
Keep the test narrow enough to interpret its result.
Expose joint constraints
Include states where features interact through a shared resource.
Why these aims pull against each other
A narrow test can be correct about everything it includes while missing a failure outside its scope. Integration includes the interaction but adds construction and diagnostic work.
Choose an arrangement to see what changes and what remains difficult.
Compare the same rows across alternatives. Cells state explicit toy quantities, membership or permissions; colors do not supply additional meaning.
What this choice protects
What it costs
When it fits
Compare the arrangements
Test each alone
Build and operate single-module prototypes. The joint active state is outside these tests.
| Load | Exercised? | Observation | |
|---|---|---|---|
| A only | 2 | Yes | Fits |
| B only | 2 | Yes | Fits |
| A+B | 4 | No | Untested |
- What it protects
- Attributes an observed single-module issue without interference from the other module.
- What it costs
- Does not exercise the both-on state; two successful separate tests do not establish joint feasibility.
- When it fits
- Plausible while learning basic module behavior or before an integrated prototype is affordable.
Illustration note: This is an editorial, deliberately bounded illustration. Its stated rules and any numbers are invented, not observations, recommended settings, or predictions.
Integrate and exercise
Build the two-module prototype and exercise each alone plus their joint active state.
| Load | Exercised? | Observation | |
|---|---|---|---|
| A only | 2 | Yes | Fits |
| B only | 2 | Yes | Fits |
| A+B | 4 | Yes | Exceeds 3 |
- What it protects
- Reveals the shared-limit violation in the both-on state.
- What it costs
- Requires integration work; in less transparent real systems, a joint failure can be harder to attribute.
- When it fits
- Plausible when simultaneous operation is intended and interaction risk justifies building the joint setup.
Illustration note: This is an editorial, deliberately bounded illustration. Its stated rules and any numbers are invented, not observations, recommended settings, or predictions.
What this illustration does—and does not—establish
The source establishes the structural tension; the concrete alternatives and their conditional costs are editorial synthesis. No arrangement is a universal recommendation.
- The modules, additive loads and limit are invented. This is not electrical sizing advice or an empirical prototype result.
- Integration does not guarantee realistic manufacturing, user behavior or every interaction. The toy’s known arithmetic is unusually easy to diagnose.
Source entries
Design Prototyping
Design prototyping T 5 supplies the local tension. The setting, alternative arrangements, and stipulated consequences are editorial applications.
Scope definition and feature creep
A narrowly scoped prototype (test one feature, one user scenario) learns precisely but may miss interactions with features not included in the prototype. A broadly scoped prototype (multiple features, multiple scenarios) risks becoming so complex that you cannot tell what failed or why.