Chapter 4 Documenting Information Systems Yong Choi BPA CSUB

System Concept A system exits by taking input from the environment , transforming (processing) input, and release an output A system may be decomposed (exploded) into subsystems Output of one subsystem may become the input of other subsystems

System Concept (con’dt)

System Decomposition (General) No additional notes

Decomposition Diagram (Hierarchy Chart)

Example of Decomposition Diagram No additional notes. SoundStage Entertainment Club

Decomposition: Use-Case Diagram

Separating data and processes Characteristic System focus Design stability Data organization State of data Process-orientation what, when & how to do Limited due to change in business processes Designed for individual applications unstructured Data-orientation Operation of Data More stable as data needs change slowly Designed for enterprise Controlled

Separating data and processes Process-oriented approach An IS development strategy that focuses on how and when data are moved through and transformed by an IS Sequence of steps in the process-oriented approach: define problem -> identify necessary decisions -> describe information needs -> determine necessary processing -> specify data needs Data-oriented approach An IS development strategy that focuses on the optimal organization of data, not on where or how data are used Sequence of steps in the data-oriented approach: define problem -> identify requirements -> develop data model for each user's data -> integrate all the data models into an overall model

Process Modeling with DFD Process Modeling is a technique for organizing and documenting the structure and flow of data through a system’s processes, and the logic, policies, and procedures to be implemented by a system’s processes. A Data Flow Diagram (DFD) is a graphical modeling tool to depict the flow of data through a system and the work or processing performed by that system. What’s the system doing

Data Flow Diagrams (DFD) Key points for modeling How data moves through the organization Relationships between various data flows Storage of data There are no FIXED rules about how a DFD should be developed… There is no such a DFD call “CORRECT DFD”… Expert SAs may not need much…..but definitely need for YOU!!

Developing a DFD

Simple Data Flow Diagram for Banking Teaching Notes We have found it useful to walk through this first DFD. Don’t be alarmed if students take exception to some of the oversimplification of the illustrated problem—it can actually contribute to the learning experience.

Process Logic DFDs are effective tools for identifying processes, but are not good at showing all the detail logic inside those processes. Not always easy to know what the lowest logical level is However, it is better than: Flowcharts and Pseudocode (difficult for users to understand). No additional notes.

Flowchart

Flowchart Limitations Complex logic: Sometimes, the program logic is quite complicated. In that case, flowchart becomes complex and clumsy. Alterations and Modifications: If alterations are required the flowchart may require re-drawing completely. Reproduction: As the flowchart symbols cannot be typed, reproduction of flowchart becomes a problem. The essentials of what is done can easily be lost in the technical details of how it is done.

Pseudocode This is the pseudocode for a Game of Monopoly, including one person's move as a procedure: Main Procedure Monopoly_Game Hand out each player's initial money. Decide which player goes first. Repeat Call Procedure Monopoly_Move for next player. Decide if this player must drop out. Until all players except one have dropped out. Declare the surviving player to be the winner. Procedure Monopoly_Move Begin one's move. Throw the dice. Move the number of spaces on the board shown on the dice. If the token landed on "Go to Jail," then go there immediately. Else if the token landed on "Chance" or "Community Chest," then draw a card and follow its instructions. Else follow the usual rules for the square (buying property, paying rent, collecting $200 for passing "Go", etc.). End one's move.

Elements of a DFD External Entity Data Flow Process Data Store DE MARCO & YOURDON NOTATIONS (Textbook) External Entity Data Flow Process Data Store

Elements of a DFD External Entity Data Flow Process Data Store GANE & SARSON NOTATIONS (more universal) External Entity Data Flow 3 Process Pay Bill AP Clerk Data Store

External Entities SUPPLIER An External Entity is a provider (source) or receiver (sink) of data and information of the system External agents define the “boundary” or “scope” of a system being modeled.

External Entities … An external entity (agent) defines a person, organization unit, or other organization that lies outside of the scope of the project but that interacts with the system being studied. As scope changes, external agents can become processes, and vice versa. Almost always one of the following: Office, department, division inside the business but outside the system scope. An external organization or agency. Another business or another information system. One of system’s end-users or managers

Data Stores A data store is an inventory of data. Accounts Receivable A data store is an inventory of data. A data store means “data at rest.” A data flow means “data in motion.”

Data Stores … Almost always one of the following: Persons (or groups of persons): e.g., customer Places: e.g, cash register Objects: e.g., product Events (about which data is captured): e.g., sales Concepts (about which data is important): e.g., discount Data stores depicted on a DFD store all instances of data entities (depicted on an ERD)

Data Flows DELIVERY SLIP A Data Flow represents an input of data to a process, or the output of data from a process. A data flow may also be used to represent the creation, reading, deletion, or updating of data in a “data store.” A composite data flow (packet) is a data flow that consists of other data flows. A Data Flow does not represent a document or a physical good: it represents the exchange of information in the document or about the good

Processes 1 Pay Bill A Process is a work or action performed on input data flow to produce an output data flow Use a verb to label the action performed by the process (not the name of person or department who does it as in physical DFD) A Process must have at least one input data flow and at least one output data flow.

How to find each one… External Entities (EE) Processes (P) people/organizations/things that supply information to or use information from the system Processes (P) actions/doing words/verbs Data Flows (DF) movement/exchange of information/data between external entities to processes, and processes to processes Data Stores (DS) store/record information/data

DFD using Use-Case The actor that initiates the event The use cases External entities The use cases Will be handled by a process Inputs (or triggers) & outputs and responses Will become a flow of data ** data store is a result of data flow..thus, need naming creativity… **

DFD Naming Guidelines External Entity  Noun Data Flow  Names of data Process  verb phrase a system name a subsystem name Data Store  Noun

Guidelines for Developing DFD Develop a list of business activities Similar to business rules of ERD Create the Context Diagram (we know this) shows external entities and data flows to and from the system: conceptual DFD Create level 0 DFD (or Diagram 0 in textbook) shows general processes at the highest level Create level 1 DFD (or Child Diagram in textbook) shows more detailed processes

Apply logical sequence of the system.. A student (EE / DS) sends in an application form (DF) containing their personal details, and their desired course The university checks (P) that the course is available. If the course (DS) is available the student is enrolled (P) in the course, and the university confirms (P) the enrolment by sending a confirmation letter (DF) that they are registered for the course to the student. Or if the course is unavailable the student is sent a rejection letter (DF).

Level 0 Tips Generally move from top to bottom, left to right Minimize crossed lines Iterate as needed The DFD is often drawn many times before it is finished, even with very experienced systems analysts

Level 0 and 1 DFD Building Procedure Level-0 DFD Identify what is being done between each input and its corresponding output Identify the processes Identify external data flows between external entities and processes Identify internal data flows between processes and data stores

Tips for Level 1 and Below Level-1 DFD’s Sub-processes (primitive processes) of Level-0 processes Sources for inputs and outputs listed at higher level List source and destination of data flows to processes and stores within each DFD Depth of DFD depends on overall system complexity Two processes generally don’t need lower level More than seven processes become overly complex and difficult to read

Context diagram for the example External entity: Student Process: Student Administration process application Data Flows: Application Form, Confirmation/Rejection Letter

Apply logical sequence of the system.. A student (EE / DS) sends in an application form (DF) containing their personal details, and their desired course The university checks (P) that the course is available. If the course (DS) is available the student is enrolled (P) in the course, and the university confirms (P) the enrolment by sending a confirmation letter (DF) that they are registered for the course to the student. Or if the course is unavailable the student is sent a rejection letter (DF).

Level-0 DFD External entity: Student Processes: Check course available, Enroll student, Confirm Registration Data Flows: Application Form, Course Details, Course Enrolment Details, Student Details, Confirmation/Rejection Letter Data Stores: Courses, Students. Also see the “Lemonade Stand” example from the class website

Rules in DFD Building Rule 1 : Use only DFD notations to avoid confusion Rule 2 : Use an action VERB to label a process (because a process is an action !!!)

Rules in DFD Building ... Rule 3 : Must be one process associated with each data flow … M M

Detail Rules in DFD Building ... Rule 3 : Must be one process associated with each data flow … M N M N

Detail Rules in DFD Building ... Rule 3 : Must be one process associated with each data flow.

Detail Rules in DFD Building ... Rule 4 : Shaded corner must appear in ALL occurrences of a duplicated symbol in a same diagram CUSTOMER D3 Accounts Receivable CUSTOMER D3 Accounts Receivable

Detail Rules in DFD Building ... Rule 5 : No process without output data flow (black hole !!!)

Detail Rules in DFD Building ... Rule 6 : No process without input data flow (miracle !!!)

Detail Rules in DFD Building ... Rule 7 : No need for routing (without transforming) a data flow with a process (non value-added activities !!!) Info A Info A

Detail Rules in DFD Building ... Rule 8 : Identical input, output data flows for parent and child processes (but the child processes can have their own throughputs) See the picture in next slide

Detail Rules in DFD Building ... M Context Diagram P N M 1 2 P 3 N Level-0 Diagram

Detail Rules in DFD Building ... Rule 9 : Data flows cannot split by themselves

Detail Rules in DFD Building ... Rule 9 : Data flows cannot split …

Detail Rules in DFD Building ... Rule 10 : A data packet can combine many data elements being transmitted at the same time to the same destination

Detail Rules in DFD Building ... Rule 11 : Double-headed arrows are forbidden [in- flow (update) and out-flow (extract info) of a data store are different]

Common Process Errors on DFDs Teaching Notes Idea: Correct this diagram as an in-class exercise. 3.1.1: To correct the diagram, a data flow, ACCOUNTING DATA, should be added from the data store, MEMBER ACCOUNTS, to process 3.1.1. 3.1.2: To fix the black hole, we might add an output data flow called NEW MEMBER ACCOUNT from process 3.1.2 to the data store MEMBER ACCOUNTS. 3.1.3: To fix the miracle, you would need to at least add a data flow such as ACCOUNTING DATA from the data store, MEMBER ACCOUNTS, to process 3.1.3. In all likelihood, you also need some type of triggering data flow, such as ACCOUNT FREEZE AUTHORIZATION, from a new external agent, such ACCOUNTING DEPARTMENT, to process 3.1 3.