JSD NetworkingBuilding the System

Specification

2SSPD - Week 7

What are we building?

• Not a DFD, or an ERM

• But a network of concurrently running processes

3SSPD - Week 7

The Network Diagram

• Shows all entity model processes

• Their inputs

• Their outputs

• Their relationships

• All viewed from the client’s point of view - remember, this is a real world model.

4SSPD - Week 7

Diagram Components

TGM Time Grain Marker

DS Data Stream

SV State Vector

CustomerEntity/Process

Fixed Merge

Rough Merge

Clock Clock Process

Connection 1: Many

Connection 1:1

5SSPD - Week 7

Key Concepts

• Datastream connection

• State vector connection

• Time grain marker

6SSPD - Week 7

Datastreams

• Imagine a drainpipe– Into one end of which you put coloured

balls– In what order do they come out the other

end?– How do you retrieve the fifth ball?

• Essentially this is a FIFO queue

7SSPD - Week 7

Datastream Rules

• All messages must be read in the order they were written

• Writer dependent

• Writing process can cause blocking of reading process

• Imagine washing dishes...

8SSPD - Week 7

State Vectors

• A means of interrogating the current state of a process

• Provide a snap shot

• Initiative lies with inspecting process

• Cannot cause blocking, always available

9SSPD - Week 7

Time Grain Marker

• A special form of the datastream

• Contains timing signals generated by a clock process

• Used to trigger the execution of real-world-time-dependent processes

10SSPD - Week 7

Datastream vs State Vector

• Datastream long term connection, state vector loosely coupled

• Datastream connection controlled by writing process, state vector by reading process

• Datastream messages are queued, state vector info must be fetched at right time otherwise it is lost

11SSPD - Week 7

Metaphor

• State vector

• Datastream

• What happens in each case if you need to get up at 4.30 a.m. to catch early train to job interview?

12SSPD - Week 7

An Example• See handout for entity models

CUSINP CUSTOMER

CUSHIRE

HIREDTLS

HIRING VEHICLE

VEHSV

ACQDTLS

SRVDTLS

PLACEDTLS

RTNDTLS

KEYACQ DTLS - Aquisition details

RTN DTLS - Return & amend detailsSRV DTLS - Service details

13SSPD - Week 7

Initial Model

• This initial model provides a view of the real world

• without having considered functional requirements

• Now we need to specify the system

• At last, time think of the functional requirements

14SSPD - Week 7

Elaborating the Model

• If the model is a simulation of the real world

• Then information outputs (functional requirements) should simply be the result of inspecting the state of the model

• e.g., how many hirings were there?

15SSPD - Week 7

Adding functions

• We can add function to the model in two ways:– embedded functions: information

requests that can be satisfied by existing process logic

– imposed functions: requests that do not map onto existing model process, e.g. time-dependent functions. I.F. basically result from identifying a structure clash.

16SSPD - Week 7

Requirements List

Any system produced must allow for the provision of output in a timely manner, in respect of cars due for replacement or servicing. Also, management information to allow the regulation of the servicing and replacement policies may well be required. The hire fees collection and accounting is not to be considered at this time.

17SSPD - Week 7

Distills to...

• Report1: Generated weekly to list all vehicles that have been serviced or replaced that week

• Report2: Generated upon user request to list all cars in need of servicing & replacement

• Report3: Additionally, management wants to know each time customer returns a car

18SSPD - Week 7

Embed or Impose?

• Report 1 must be imposed as model processes are not structured around notion of a week

• Report 2 must be imposed as its execution is triggered ad-hoc

• Report 3 can be embedded into existing CUSTOMER process

19SSPD - Week 7

Elaborated Network

CUSINP CUSTOMER

CUSHIRE

HIREDTLS

HIRING VEHICLE

VEHSV

SRVDTLS

PLACEDTLS

RTNDTLS

SERVICE &REPLACE

SERV/REPPOLICY

VEHSV

MANINPT

POLOUT

TGMWEEK

S&ROUT

TGMDATE

ACQDTLS

REP 3

20SSPD - Week 7

Complete the Model

• Draw structure diagrams and write structure text for new imposed functions

• Update existing model processes to include any new embedded functions

• Now we have a system specification

• It remains now to design the implementation