Writing Effective Use Cases (Examples)
A great way for writing effective use cases is to walk through a simple
use case example and watch how it can be leveraged to something complex.
By absorbing the meaning of what use case diagrams, alternate flows and
basic flows, you will be able to apply use cases to your projects.
In this section, I'm going to walk you through writing effective use
cases for a mock Ebay site. Most of us know and love Ebay, so if you
don't know Ebay, go to Ebay and navigate through the site and come back
to our example. Imagine that only you know Ebay, but nobody has ever
seen Ebay and you have to write the requirements for Ebay using use
cases, and explain it to people who have never seen it.
Tip 1. When creating use cases, be productive without perfection
Tip 2. Define your use case actors
Tip 3. Define your "Sunny Day" Use Cases (Primary Use Cases)
Tip 4. Identify reuse opportunity for use cases
Tip 5. Create a use case index
Tip 6. Identify the key components of your use case
Tip 7. Name and briefly describe your use case
Tip 8. Create the use case basic flow
Tip 9. Create the use case alternate flows
Tip 10. Produce your use case document
Tip 11. Generate a Use Case Model Diagram
Tip 12. Do you need User Stories?
Tip 13. Agile Development with Use Cases
Tip 1. To write effective use cases, be productive without perfection
When it comes to writing effective use cases, you
don't need to be a perfectionist and concern yourself with getting it right the first time. Developing
use cases should be looked at as an iterative process where you work and refine. You can always refine it later, so again, don't
go for perfection from the get-go. Loosen up and have some fun while you're doing it.
Tip 2. Define your use case actors
There are possibly over a dozen actors that interact with Ebay, from buyers and sellers, down to
suppliers, wholesalers, auditors, and customer service. But we're going for grass-roots, so who are the
basic users of Ebay? BUYERS and SELLERS. So lets put them down as our first actors. (The visual notation in
the figures below is based on UML -- Unified Markup Language for Use Cases)
Do you notice how the actors aren't John and Sue which would be people? While John may be a seller
and Sue may be a buyer, an actor is a Role. And a role in this case would be that of a buyer and
that of a seller. Now that things are clicking, lets throw some more actors on your paper just so we can try and
identify more possible users.
Now we have a bunch of actors. Wait a minute? Paypal? That's not a
person. An actor can be a system, because a system plays another role in
the context of your
new system and has goals and interacts with other actors as you will see
later.
Tip 3. Define your Sunny Day Use Cases
For those of you who haven't heard the expression, "Sunny Day" use
cases, it is in reference to the use cases that are most likely going
to occur when all goes well. These are somtimes referred to as your
primary use cases. You always want to focus on the sunny day scenarios
first because
you can then pivot off these and figure out your "rainy day" scenarios
(or edge cases) later. |
Tip 4. Identify reuse opportunity for use cases
In this step, you are going to cross the bridge into object modeling. Don't get overly concerned
about terms like generalization, inheritance and extends. The goal of this Ebay use case example is to keep it
understandable so we will explain this concept in terms of the example.
What does the word general mean? Something is broad and not as detailed.
Generalization is when
you "inherit" from something general and then add more detail. A
"person" is very general. A "man" is still general, but not as general
as a "person". You can say that a "man" inherits behavior
and atributes of a "person".
Look at the requirements management use case diagram above and you will
see there is duplicate behavior in both the buyer and seller which
includes "create an account" and "search listings". Rather than have all
of this duplication, we will have a more general user that has this
behavior and then the actors will "inherit" this
behavior from the new user.
The above use case example diagram illustrates that a generic user
creates accounts and search listings and that a buyer and a seller have
their own behavior but also have the behavior of the generic user. The
benefits of generalization are that you eliminate duplicate behavior and
attributes that will ultimately make the system more understandable and
flexible. We will see in later steps that this inheritance applies both
to use cases and to the actors.
Tip 5. Create a use case index
After producing your initial visual list of use case actors and goals, we can take
this list and create an initial use case grid which provides the basis for the use case
index. Every use case will have various attributes relating both to the use case iteself
and to the project. At the project level, these attributes include scope, complexity, status
and priority.
This use case index should be used by the project team to define the use cases against. It will
serve as a master inventory to help writ effective use cases for the requirements phase of the project.
Tip 6. Identify the key components of your use case
The actual use case is a textual representation illustrating a sequence of events. In our
use case example, you will see that there are several components of a use case which we
will review. In the mean time, review the table below to get a basic understanding of what is in the
use case and then we will review each element as we progeress through our use case example.
Use Case Element | Description |
Use Case Number | ID to represent your use case |
Application | What system or application does this pertain to |
Use Case Name | The name of your use case, keep it short and sweet |
Use Case Description | Elaborate more on the name, in paragraph form. |
Primary Actor | Who is the main actor that this use case represents |
Precondition | What preconditions must be met before this use case can start |
Trigger | What event triggers this use case |
Basic Flow | The basic flow should be the events of the use case when everything is perfect; there are no errors, no exceptions. This is the "happy day scenario". The exceptions will be handled in the "Alternate Flows" section. |
Alternate Flows | The most significant alternatives and exceptions |
Tip 7. Name and briefly describe your use case
Now that you have a general understanding of what a use case consists
of, we are ready to start creating
our use case. Typically, while the name of your use case is being
discussed, people will start briefly describing
the use case. Use plain english and keep it simple. Getting back to our
use case example, I will begin with use case #1 from step number four.
Use Case Number: | 1 |
Use Case Name: | Buyer Places a Bid |
Description: | An EBAY buyer has identified an item they wish to buy, so they will place a bid for an item with the intent of winning the auction and paying for the item. |
Tip 8. Create the use case basic flow
The basic flow of a use case represents the most important course of
events or what happens most of the time, sometimes referred to as the
'Happy
Day Scenario' because it is what occurs when everything goes well -- no
errors or exceptions. Another reason why the basic flow is so critical
is because it's much easier to fully comprehend the exceptions once the
norm is understood and if the basic flow represents 70% of
the system, the development staff is much more prone to implementing
the correct code in the first pass.
For our use case example, the basic flow should be to describe the happy
day scenario for your use cases such as "placing a bid". For a consumer
to play a successful bid, what is the primary flow when everything goes
as planned. An effective use cases needs to have the basic flow before
moving forward with writing the alternate flows.
Tip 9. Create the use case alternate flows
The basic flow is the key ingredient to your use case and some can argue that you can stop once you're done with the
basic flow. It really depends on the level of detail you wish to achieve. However, providing more detail to the
consumers of your use case is always a good thing.
The alternate flows providing the following:
An exception or error flow to any line item in your basic flow
- An additional flow, not necessarily error based, but a flow that COULD happen
A few examples of alternate flows are:
While a customer places an order, their credit card failed
While a customer places an order, their user session times out
While a customer uses an ATM machine, the machine runs out of receipts and needs to warn the customer
Tip 10. Produce your effective use case document
Recently at a new project assignment, I introduced a mid level developer to the concept of use cases which was totally foreign
to him. Once walking him through the basic concepts and showing him the use case example, the lightbulb went off in his head
on how convenient and simple it was to grasp the project.
A few reasons why it's that much easier to learn a system through use
cases then a traditional requirements document
is probably because with use cases, you are introduced to concepts at a
high level, walk through a living scenario and then presented
with specifications last.
In several places in this document, I have stated "effective use cases"
rather than just "use cases". The purpose of the use cases is for
effective knowledge transfer from the domain expert to the software
developer -- these use cases will serve as software requirements. If
they don't make sense to the person building the software, they are not
effective. There are several sources on the web for writing effective
use cases including the book by Alistair Cockburn.
Tip 11. Generate a Use Case Model Diagram
You can use the Gatherspace.com use case modeling tool to produce a use
case model within a few clicks. Once you define your use cases and
actors, just go into the reporting section and click on the 'Use Case
Model' report and that's it. From the main use case model, you can
continue to drill down into the use cases.
To see what this looks like, click the use case model sample now.
In several places in this document, I have stated "effective use cases"
rather than just "use cases". The purpose of the use cases is for
effective knowledge transfer from the domain expert to the software
developer -- these use cases will serve as software requirements. If
they don't make sense to the person building the software, they are not
effective. There are several sources on the web for writing effective
use cases including the book by Alistair Cockburn.
Tip 12. What's the difference between a User Story and a Use Case?
With so many engineering teams making the paradigm shift from waterfall
to Agile, people often get caught up in having a pure Agile process
which would include the use of User Stories. So what's all of the hoopla
with User Stories?
What are they, how are they different from use cases, do I need them,
and where do they fit in the process?
What is a User Story? Simply put, written from the context of the user as a simple statement about their feature need. They should generally have this format. "As a -role-, I want -goal/desire- so that -benefit-"
How is a User Story different than a Use Case? While a use case is highly structured and tells a story, the User Story sets the stage by stating the need. A User Story is the prelude to the use case by stating the need before the use case tells the story.
How does the User Story fit into the process? User Stories are great as an activity in collecting and prioritizing the high level features. Getting this initial feedback from the customer is a simple way of trying to get all of their needs identified and prioritized. The User Stories will then morph themselves into the business requirements and use cases.
Tip 13. In Agile Development, Keep Use Cases Agile, Mean and Lean
A common myth with Agile Development is that you *must* use user
stories, and not use cases. Like anything else in life, nothing is black
and white -- being Agile is really about smaller iterations, learning
and adapting to the market.
If you are using Agile, Scrum and moving away from waterfall, what you
want to do is make sure to iterate with your use cases. All that means
is that your flows will be smaller and less feature rich. While the
theme of the use case may appear the same from iteration to iteration,
what is changing is the level of detail and the features inside the
particular sprint.
Creating a use case to long winded with too many features can
potentially put a product at risk. What happens is that you can extend
your release to market from two weeks to several months without the
ability to learn from the iteration and adapt to the market. Keep those
use cases leaner!
Comments
flag
like
great article and explains it plain and simple :), i just cant understand why would i want someone to hold my steak :P
This is fantastic! I was in search of an easy way to explain this to my workgroups - this will certaintly help them to understand better!
Awesome job. I wish you would provide a similar argument for software architecture design.
Great job.
great article!,explains use cases in simple language, easy to understand.
thanks!
![rss](use_case_example_files/feed.png)