5. Event Flows

One of the requirements of the UML language is the self-sufficiency of diagrams for presenting information about the models of designed systems. However, as noted above, use-case diagrams describe what the system does, without specifying how it does it.

For a real description of the system will require more specific data that are reflected in the flow of events. Event streams specify or detail the sequence of actions performed by the system when executing its use cases, and also describe the logic of transitions through use cases.

The flow of events is a specific sequence of actions that describes the actions of actors and the behavior of the modeled system in the form of plain text.

Event streams are textual descriptions of step-by-step execution of precedents; they are understandable not only to the developer, but also to the outside reader. Their task is to further elaborate the description of the functionality of the system before the developers start to write the program code, and to eliminate possible misunderstanding of the required functionality, to bring together the developer’s ideas about the system and the customer as much as possible.

Event streams are of three types: main, alternative, and error stream.

The main (main) thread describes the best scenario or the most used way of precedent execution.

An alternate stream specifies deviations from the main stream that are not considered erroneous.

The error stream is considered as a deviation from the alternative or main one, which gives rise to the conditions for the formation of an error.

Example. We describe the flow of event precedent Order goods.

The main stream of events.

1.President starts with the choice by the buyer of the catalog display mode.

2. The system opens the directory.

3. The buyer chooses the basket display mode.

A1. The buyer views the catalog and launches the “add product to cart” stream

4. The system opens the basket.

5. The buyer clicks the button "checkout".

A2. The buyer views the basket and launches the “basket change” stream.

A3. The buyer decides to return to the catalog.

6. The system proceeds to the first step of placing an order: it asks the buyer for personal data and telephone.

7. The buyer enters personal data and phone.

8. The system proceeds to the second step of ordering: shows the contents of the order and asks to confirm the order.

9. The buyer confirms the order.

A4. The buyer returns to the basket.

10. The system proceeds to the third step of order placement: it requests the type of credit card, its number, secret code, the name of the owner and the expiration date.

11. The buyer enters the type of credit card, its number, secret code, the name of the holder and the expiration date.

12. The system proceeds to the fourth step of the order: confirms the payment.

A5. User account not found. A6. Not enough money in the account. E1. Payment system is not available.

13.System assigns order number and sends it together.

confirming the order to the email address of the buyer.

14. The use case is completed.

Alternative streams.

A1. add product to cart

1.Buyer selects size.

2. The buyer chooses the quantity.

3. The buyer clicks the Add to cart button.

4. The system puts the selected product in the basket.

A7. Buyer did not choose the size.

A8. The buyer did not choose the quantity.

5. The system displays a message stating that the item has been added to the cart.

6. The flow returns to the second stage of the main flow.

A2. Change cart

1. The buyer clicks the "Delete" button in front of one selected item.

A9. The buyer changes the number of positions of one selected item.

2. The system removes the goods from the basket.

3. The flow returns to step 4 of the main flow.

A3. Buyer decides to return to catalog

1. The flow returns to stage 2 of the main flow.

A4. Buyer returns to cart

1. The flow returns to step 4 of the main flow.

A5. User account not found

1. The system displays a message stating that a user account was not found.

2. The flow returns to step 11 of the main flow.

A6. Not enough money in the account

1. The system displays a message stating that there is not enough money in the user’s account to complete the transaction.

2. The flow returns to step 11 of the main flow.

A7. Buyer did not choose the size

1. The system displays a message about the need to select the size.

2. The flow returns to stage 1 of flow A1.

A8. Buyer did not select quantity

1. The system displays a message about the need to select the quantity of goods.

2. The stream returns to stage 2 of stream A1.

A9. The buyer changes the number of positions of one selected product

1. The buyer increases the quantity of goods.

2. The system updates the cart.

A10. Not enough item in stock.

3. The flow returns to step 4 of the main flow.

A10. Not enough item in stock

1. The system displays a message about how many items the customer can order.

2. The flow returns to stage 4 of the main flow.

Error streams.

E1. Payment system is not available

1. The system displays a message about unavailability of the payment

system.

2. The flow returns to step 11 of the main flow.