Designing with CRC Cards

Programmers tend to use software to solve absolutely all problems. Need to take notes? Here is an application for taking notes. Brainstorming? All sorts of mind mappers are available for you. Even though in most cases it makes perfect sense, sometimes I find that a piece of paper and a pen are better tools for the job.

This is the first blog post in a series about using such low-tech tools as paper cards for brainstorming, planning, and designing. I’d like to start by introducing one of the most interesting and, unfortunately, underused tools for brainstorming: CRC cards. CRC stands for Candidate, Responsibility, and Collaboration.

What are CRC cards?

CRC cards are index cards on which there are written:

• A candidate’s name.
• A candidate’s information description. Just one or two sentences describing what each candidate is, and what it does.
• A candidate’s characterization. It’s role stereotype, a pattern, etc.
• A candidate’s responsibilities.
• A candidate’s collaborators.

Front Side

The front side of a card is used to record a candidate’s name, purpose, and stereotype.

Back Side

The back side of a card is used for recording a candidate’s responsibilities and collaborators.

How to use CRC cards?

Start by writing down each candidate’s name, and maybe their stereotypes. Next, move the cards around, and think about each candidate for a little bit. Then, describe each candidate — write down its purpose.

Often at this point I have a pretty good idea of what my design looks like. If that’s the case, I just stop. After all, CRC cards are just a tool aiming to flesh out the design of a new feature, or system. The goal is not to formally describe collaboration patterns, but to come up with a few good ones. In some cases, however, I pick three or four the most important candidates and write down their responsibilities and collaborators.

Candidate or Class?

Originally, CRC stood for Class, Responsibility, and Collaboration. But soon most practitioners realized that it’s too early to start thinking in terms of classes, or you may not use classes at all! It’s better to think about them as abstract candidates: they can be classes, objects, functions, actors, etc.

Why Cards?

Why not to draw all the same stuff on a whiteboard? Why cards? Because you can move them around, discard them, group them by an abstraction layer or a subdomain. You can do it without redrawing the whole picture. I find that the process of moving stuff around is what usually sparkles creative ideas.

Read More

Kent Beck and Ward Cunningham described CRC cards in their classic paper “A Laboratory For Teaching Object-Oriented Thinking”.

The way I use CRC cards is described in the book “Object Design: Roles, Responsibilities, and Collaborations” by Rebecca Wirfs-Brock, Alan McKean.

Follow @victorsavkin to learn more about software design.

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