1. Use case diagrams
A use case diagram is UML’s notation for showing the relationships among a set of use cases and actors
A use case diagram can help the software engineer to convey a high-level picture of the functionality of the system

2. Library system use case diagram
Example: Use-case diagram for a course registration system

3. Create Use Cases: Walk Through
Identify actors
Define use cases
Discover reuseable use cases
Put uses into tables with id, name, primary actors etc.
Write use case description for each use case
brief description
Basic flow
Alternative flow
Reference:

4. Example

5. Identify Reuse In Use Case

6. Create use case index

8. Use case Description 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.

9. Basic Flow
Next 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.

10. Alternative flow A few examples of alternate flows are:
credit card failed when a customer places an order,
user session times out when placing an order

11. Extensions
Used to make optional interactions explicit or to handle exceptional cases
By creating separate use case extensions, the description of the basic use case remains simple
A use case extension must list all the steps from the beginning of the use case to the end
Including the handling of the unusual situation

12. Generalizations Much like superclasses in a class diagram
A generalized use case represents several similar use cases
One or more specializations provides details of the similar use cases

13. Inclusions
Allow one to express commonality between several different use cases
Are included in other use cases
Even very different use cases can share sequence of actions
Enable you to avoid repeating details in multiple use cases

14. Example of generalization, extension and inclusion
Example: Use-case diagram for a home security system