Writing effective Use Case 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.\

STEP 1. When creating use cases, be productive without perfection

STEP 2. Define your use case actors

STEP 3. Define your use case Actor Goals

STEP 4. Identify reuse opportunity for use cases

STEP 5. Create a use case index

STEP 6. Identify the key components of your use case

STEP 7. Name and briefly describe your use case

STEP 8. Create the use case basic flow

STEP 9. Create the use case alternate flows

STEP 10. Produce your use case document

STEP 11. Generate a Use Case Model Diagram

STEP 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. STEP 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.

STEP 3. Define your use case Actor Goals Now that you have established your actors (system users), or most of them, the next logical step is to outline what the goals are of each actor. One you have established your actors and the goals of each actor, you have now created your initial list of high level use cases. In the example below you will see the representation of actor goals. Again, don't try and capture 100% of this step, in requirements management, refinement is always part of the entire process. Effective use cases should have understandable actors and goals.

STEP 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.

STEP 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.

STEP 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

STEP 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.

STEP 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.

STEP 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

STEP 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.

STEP 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.