Conceptual Design vs Structural Design: Where They Meet
Every robust system begins with a spark of idea—often sketched on a sticky note or scribbled on a whiteboard. That’s where CRC conceptual modeling thrives. It’s about exploring behavior, responsibilities, and interactions in a way that feels natural to teams. But as systems grow, so must their structure. That’s where UML structural modeling takes over—offering precision, consistency, and a shared language for developers, architects, and testers.
What I’ve learned over two decades of working with teams is this: conceptual design isn’t the enemy of structural design. They’re partners. One fuels the other. The key is knowing when to let each phase lead, and how to smoothly transition from brainstorming to documentation without losing intent.
By the end of this chapter, you’ll understand how CRC cards evolve into full-fledged UML class diagrams. You’ll see how a team’s collaborative insights become structured, persistent, and testable design artifacts. You’ll learn the practical workflows that make this shift not just possible—but reliable, repeatable, and team-friendly.
Understanding the Two Design Phases
Conceptual Design: The Power of CRC Modeling
CRC cards are not just a diagramming tool—they’re a conversation engine. When a team sits around a table, each card represents a class, its responsibilities, and collaborations. It’s lightweight, tactile, and encourages shared ownership.
But it’s not about perfect syntax. It’s about asking: What does this object do? Who does it work with? How does it behave under different conditions?
This stage thrives on discovery. It’s where you explore, challenge, and refine ideas before any code is written. CRC conceptual modeling is where design begins—not as a static blueprint, but as a dynamic, evolving discussion.
Structural Design: The Discipline of UML Modeling
UML structural modeling is the next evolution. Here, the goal shifts from “what might happen?” to “what must be?”. The focus is on stability, consistency, and precision.
Classes become nodes with attributes, operations, and visibility. Relationships are defined by multiplicity, direction, and dependency. Constraints, interfaces, and inheritance hierarchies now carry formal meaning.
This phase isn’t about replacing the CRC cards. It’s about documenting and validating the insights they helped uncover. It’s where the team’s collaborative insight becomes a shared, formal contract.
Where They Meet: The Transition Point
There’s no single moment when conceptual design ends and structural design begins. The transition happens gradually—through iteration, review, and refinement. But there are clear signals that it’s time to shift gears.
When the team agrees on key classes, their core responsibilities, and how they interact—without ambiguity—it’s a sign the conceptual phase is mature enough for formalization.
Here’s a practical checklist to guide your transition:
- All major domain objects are identified and named consistently.
- Responsibilities are clearly defined and not overlapping.
- Collaborations are traceable and stable across team discussions.
- No major contradictions or missing roles are reported in the latest review.
- At least two team members can independently explain the core flow.
When these conditions are met, it’s time to move from CRC transition workflows into UML structural modeling.
From Cards to Code: The Practical Workflow
Let’s walk through a real-world CRC transition workflow.
- Collect CRC cards from the brainstorming session. Organize by domain category—e.g., “Customer,” “Order,” “Payment.”
- Extract classes from card names. Ensure single responsibility per class.
- Map responsibilities to methods and attributes. Ask: Is this behavior about state or interaction?
- Model collaborations as associations with proper multiplicity and direction.
- Apply naming conventions—PascalCase for classes, lowercase for attributes, camelCase for methods.
- Review for inheritance or interfaces where behavior is shared across multiple classes.
- Validate with a simple walkthrough: “Can we simulate a checkout process using only these classes?”
Each step reinforces the shift from idea to artifact. The diagram isn’t a translation—it’s a refinement.
Comparing CRC and UML: A Side-by-Side View
| Element | CRC Conceptual Modeling | UML Structural Modeling |
|---|---|---|
| Class | Card with class name, responsibilities, collaborators | Rectangle with name, attributes, operations |
| Responsibility | Verb phrase: “calculate total,” “notify customer” | Method: calculateTotal(): double |
| Collaboration | Text: “Customer,” “PaymentProcessor” | Association: Order — PaymentProcessor with multiplicity |
| Visibility | Not specified | + (public), - (private), # (protected) |
| Abstraction | Implied through shared behavior | Explicit: abstract class, interface |
This table isn’t just comparison—it’s a guide to translation. Every CRC insight must be mapped with intent, not just syntax.
Why This Matters: Intent Over Notation
One of the most common pitfalls I’ve seen is teams treating UML as a checklist: “Did we add the attributes? Did we draw the associations?” But that’s not the point.
The real value lies in preserving the intent behind the design. A class named OrderProcessor might seem fine, but if its only responsibility is “send confirmation email,” you’re missing the deeper abstraction: NotificationService.
So ask: Does this structure reflect the behavior we discovered in the CRC session? If not, revise the model—don’t just fix the diagram.
Frequently Asked Questions
What’s the main difference between CRC and UML?
CRC models are informal, exploratory, and team-driven. They focus on behavior and collaboration. UML is formal, standardized, and documentation-oriented. It defines structure, visibility, and constraints. CRC is for thinking; UML is for sharing and building.
Can I skip CRC and go straight to UML?
Technically yes—but it’s risky. Without CRC’s collaborative discovery, you might miss key roles, responsibilities, or edge cases. CRC helps expose assumptions early. Skipping it increases the chance of rework later.
How do I handle conflicting responsibilities in UML?
Go back to the CRC session. Re-examine how the team described the interaction. Often, a merged responsibility indicates a need for a new class. For example, if both “calculate total” and “apply discount” are assigned to the same class, consider refactoring into a PricingEngine.
Do I need to model every CRC card in UML?
Not necessarily. Some cards reflect temporary ideas or edge cases. Prioritize cards that appear in core workflows. Use CRC transition workflows to filter for relevance. If a class isn’t involved in the main interaction flow, defer it or remove it.
Can UML models be updated after development starts?
Yes. But don’t treat them as immutable. UML is not a one-way contract. As implementation reveals new needs, revisit the model with the team. Use CRC insights to guide refinement—especially in agile environments where design evolves with feedback.
What tools support CRC-to-UML workflows?
Tools like Visual Paradigm offer support for both CRC-style whiteboarding and full UML modeling. Some even allow importing CRC notes and auto-generating class diagrams. But the real power comes from team alignment, not automation.
Remember: the tool doesn’t replace the conversation. It just makes it easier to preserve.
