Use case extensions

Extension to a step of the main success scenario

To define an extension to a step of the main success scenario, first define a handle on the step. The handle identifies the step, and can then be used to reference the step from the extension.

Then define a condition element within extensions sub-element of the use case. The condition refers to the step of the main success scenario via its handle and defines the extension to the step.

<use-case goal="Transfer funds" ...>
        <success>
                <step>User identifies debit account.</step>
                <step handle="credit">User identifies credit account.</step>
                ...
        </success>
        <extensions>
                <condition step-ref="credit" when="Debit and credit accounts are the same">
                        <step>System displays error message and lets the user identify other accounts.</step>
                </condition>
        </extensions>
</use-case>

Attributes of condition:

Name Description
step-ref The value of the handle attribute of the step of main success scenario that this extension extends.
when The condition under which the extension applies. May alternatively be expressed using when sub-element.
inline-step Action of the extension. May alternatively be expressed using step sub-element(s).

Alternative notation of extensions

As you can see from the table above, there are alternative ways to specify the extension.

  1. Attribute when and attribute inline-step: 
    <condition step-ref="handle" when="Some condition" inline-step="Some action." />
  2. Attribute when and element(s) step: 
    <condition step-ref="handle" when="Some condition">
        <step>Some action.</step>
        <step>Some other action.</step>
</condition>
  3. Element when and element(s) step: 
    <condition step-ref="handle">
        <when>Some condition</when>
        <step>Some action.</step>
        <step>Some other action.</step>
</condition>

Extending range of steps of main success scenario

To extend a range of steps of the main success scenario, express the range using handle1-handle2 notation in the step-ref attribute of the condition element:

<use-case goal="Transfer funds" ...>
        <success>
                <step handle="debit">User identifies debit account.</step>
                <step>User identifies credit account.</step>
                <step handle="amount">User enters amount.</step>
                <step>User submits the transaction.</step>
        </success>
        <extensions>
                <condition step-ref="debit-amount" when="User cancels the transaction">
                        <step>System logs transaction failure and its reason to audit log.</step>
                        <step>The use case processing ends.</step>
                </condition>
        </extensions>
</use-case>

Extending all steps of main success scenario

To extend all steps of the main success scenario, use asterisk (*) in the step-ref attribute of the condition element:

<use-case goal="Transfer funds" ...>
        <success>
                ...
        </success>
        <extensions>
                <condition step-ref="*" when="Client walks away at any time">
                        <step>User takes appropriate action...</step>
                </condition>
        </extensions>
</use-case>

Extending extensions

Each step within an extension can itself be extended, just use a step-extensions element after the step:

<condition step-ref="debit" when="User decides to verify signature">
        <step>User verifies client's signature for the debit account.</step>
        <step-extensions>
                <step-condition when="Signature verification fails">
                        <step>The use case ends with error.</step>
                </step-condition>
        </step-extensions>
</condition>
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