Lecture 9

Descriptors, Events & Event Tables

INFO1409 Systems Analysis & Design ModuleHND Year 1

2008/9

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Lecture 9Learning Objectives

At the end of this lecture you should be able to:

- Explain what is meant by an event.

- Identify events which trigger the system.

- Draw an events table and a Use Case model

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Modelling systems recap

We develop a model after completing our fact finding.

• One way to do this is to identify Users and Processes.

• Once identified, we can produce Role Descriptors and Business Process descriptors.

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Accountant

E.G : Staff Administration system

Add Staff

Amend staff

Calculate pay

We have previously described the work of the Accountant and the processes (Use Cases) in the form of descriptors.

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Using Events Tables:

Another method is to use ‘Events’ which trigger business processes

Recap: In lecture 2 we examined how an Event

could trigger 3 business processes. The event was called “Receive Sales Order”

Question: What type of event is this? (Answer given on the next slide).

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A simple business model might consist of an Event 3 business processes and a result

This event is a response to an external trigger

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Types of event

• Events trigger all the processing a system does.

• There are three types of event:• External• Temporal• State

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External events

External events • occur outside the system and are usually

initiated by a person (often called an Actor).• The system is required to record details

about an external evente.g.) customer places an order.

• Or the system is required to produce something in response to the request: e.g.-customer rings to follow up/chase an order.

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Actors

The people who initiate these responses from the system are called Actors.

-They are carrying out a role (hence actor).

…When you place an order you are taking on the role of a customer.

…If you are requesting management information you are taking on the role of a particular job task.

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Typical external events• Actor wants some information - e.g.

customer enquires about an order they have placed.

• Data needs to be updated - e.g. employee changes address.

• Management wants some information-e.g. manager want to know who has holidays booked.

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Temporal Events• Temporal events are triggered by a specific

time being reached.• Many systems produce daily, weekly,

monthly and annual reports. These reports are triggered when a specific date and time are reached.

• Some events are triggered after a certain amount of time has passed (e.g.) reminder produced 2 weeks after bill was sent.

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Typical Temporal Events • Internal outputs required,

• Management reports,• e.g.) summary of all orders. • Operational reports (detail of daily

operation)• Internal statements and documents

(including payroll)• External outputs required,

• e.g.) Statements,bills and reminders

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State events

State events are triggered by a certain state/particular condition becoming true.(Similar to temporal events except that the time cannot be defined.)e.g.) Reorder stock because the number of items in stock have reached a specific minimum level (re-order level), ore.g.) Bank sends out a letter to their customer as their account has gone overdrawn.

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Identifying Events

…can be difficult!

• Following the process through can identify the sequence of events.

• If there is no time delay between two or more interactions then they are the same event, otherwise they are different events.

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Events we are not interested in

• The analyst is not interested in events that the do not normally concern the users. These are dealt with during design,

• Logging on• Passwords• Backups

• The other type of events we are not interested in just yet are the error routines.

• It helps to think that everything is perfect and concentrate on events that are required in a perfect world (Perfect World assumption).

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Example Look at the following sequence of occurrences at a hotel- identify the events

Look for things that occur together without any time delays.

What are the events ?What do they involve?What would you call the events?

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Example: hotel occurrences

1. Customer contacts the hotel to see if they have a room free for the following Saturday.

2. The clerk checks the room bookings and finds a free room.

3. Customer accepts the room and gives their personal details (e.g. name, address, etc.) to the clerk.

4. The customer pays the deposit by credit card.5. The clerk updates the system to show the room is no

longer free on Saturday and that it is booked to the customer.

6. The customer rings the following day to order flowers for the room. This is added to the room booking for inclusion on the bill.

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Example: hotel occurrences

4. The customer turns up on Saturday and gives their name.

5. The clerk uses the system to find the room number and gives the customer their room key.

6. The clerk records that the room is now occupied.7. On Sunday the customer is leaving the hotel.8. The bill is calculated by the clerk.9. The customer pays the bill.10. The clerk records that the room is no longer

occupied and that the customer has paid.

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Looking at each event

For each event the analyst must determine the: Trigger Source Activity/Use Case Response Destination

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Trigger

How does the system know the event has occurred?

For external events…data enters the system.

For temporal events…it is the date or time.

For state events… it is the condition that has been met.

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Source

Source is: for external events only usually the actor; in other words who is supplying the data which will be typed in.

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Activity or Use Case

What process does the system carry out in response to the event trigger?

Remember: Case when the system is used to produce…..

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Response and Destination

Response: What output (if any) is produced by the system?

Destination: What actor gets this output?

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Building an Events Table

• An Events Table can be built using one row for each event.

• We call the first event ‘customer makes booking’ we can call our Use Case this

• We can tell from our description that an event is happening and that we need the customer to trigger it and an Actor to perform the process.

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Events Table: hotel booking

Event Trigger

Source

Use Case

Response

Destination

Customermakes booking

customer enquiry

customer

create new booking

room booking confirm-ation

customer and clerk

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Events tables to Use Case diagrams

We can use the Events table to define the elements of a Use Case model.

Question: How does this work?

Answer : Our Events table has given us a Use Case -Create new booking

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Recap on Use Case model elements

The symbols we can use are as follows:

Actor

Association

Use CaseUse Case label

stereotype

<includes>

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Actors and Use Cases

Our Event table tells there are 2 Actors or ‘Inter-actors’ involved with the system for the first event.

1. The Booking Clerk2. The customer (who provides the

trigger and necessary input information)

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Event shown as a Use Case model

Create new booking

Booking Clerk

Customer

Remember this is just a fragment!!

Illustration

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Over to you..

• It is now your turn to complete the Hotel occurrences example.

• Complete the table, and then translate it into a Use Case diagram.

• Remember that practice makes perfect

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Answers1. Customer contacts the hotel for a free

room

2. Clerk checks room availability

3. Customer gives name and address

4. Customer pays deposit

5. Clerk updates system to show room booked

6. Customer rings to order flowers

7. Customer turns up

8. Clerk find room number and gives key

9. Clerk records room as occupied10. Customer leaves

11. Bill calculated

12. Customer pays bill

13 Clerk records room empty and customer paid.

Customer Makes Booking

Customer Orders service

Customer Checks in

Customer Checks out

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Events Table: hotel booking

Event Trigger

Source

Use Case

Response

Destination

Customermakes booking

customer enquiry

customer

create new booking

room booking confirm-ation

customer and clerk

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A simple business model might consist of an Event 3 business processes and a result

Could you also construct an events table and Use Case diagram from this example?

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SummaryYou should now know:• What is meant by an event.• The different types of events and be able

to give an example of each.• What is meant by a trigger, source, use

case, response and destination.• How to draw an events table.• How to draw a simple Use Case example

from your Events table.

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References• Systems Analysis & Design Shelley Cashman

and Rosenblat 6th Ed. Thomson (2006)

• Eva and Skidmore (2004). Introducing Systems Development. Palgrave Macmillan.

• Satzinger JW, Jackson RB and Burd SD (2004). Systems Analysis and Design in a Changing World. 3rd edition. Thomson.