DATA FLOW DIAGRAMS

Systems Analysis Focus is the logical view of the system, not the physical “What” the system is to accomplish, not how Tools: data flow diagrams data dictionary process specification entity-relationship diagrams

Data Flow Diagram: "a network representation of a system. The system may be automated, manual, or mixed. The DFD portrays the system in terms of its component pieces, with all interfaces among the components indicated." - Tom DeMarco hence DFDs: focus on the movement of data between external entities and processes, and between processes and data stores

Example Data Flow Diagram data store process data flow external entity

Data Flow Diagrams are: Used to perform structured analysis to determine logical requirements A graphical tool, useful for communicating with users, managers, and other IS personnel Useful for analyzing existing as well as proposed systems A relatively simple technique to learn and use

Why Conduct Process Modeling? Understand components of current logical or physical system for purpose of rebuilding in a different physical form/technology, possibly with some changed functionality Find inefficiencies in current system Re-engineer current system The reason we do process modeling is to improve the operation of a system, not just to change its physical form. Start with understanding Then analyze for improvements or where to add new functionality Then specificy re-engineered system BE SURE TO QUESTION ALL ASSUMPTIONS, ESPECIALLY THOSE WHICH ARE IMPLICIT IN CURRENT SYSTEM

Sources/Sinks (external entities) Any class of people, an organization, or another system which exists outside the system you are studying. Form the boundaries of the system. The system and external entities exchange data in the form of data flows. Must be named, titles preferred to names of individuals - use a noun source/ sink

Data Flows data in motion marks movement of data through the system - a pipeline to carry data connects the processes, external entities and data stores Unidirectional originate OR end at a process (or both) name as specifically as possible - reflect the composition of the data - a noun do not show control flow! Control flow is easy to identify- a signal with only one byte - (on/off). HINT: if you can't name it: either it's control flow, doesn't exist or you need to get more information!

Processes transform incoming data flows into outgoing data flows represent with a bubble or rounded square name with a strong VERB/OBJECT combination; examples: create_exception_report validate_input_characters calculate_discount process

Data Stores data at rest represents holding areas for collection of data, processes add or retrieve data from these stores name using a noun (do not use ‘file’) only processes are connected to data stores show net flow of data between data store and process. For instance, when access a DBMS, show only the result flow, not the request data store

Data Flow Diagram Don’ts 1. BLACK HOLES 2. MIRACLES 3. Let it get too COMPLEX: 7 ± 2 processes 4. Leave things UNLABELED (corollary: labels should have meaning) 5. Data stores that are “SOURCES” or “SINKS” 6. Data flows that are UNASSOCIATED with a PROCESS 7. Expect your diagram to be “perfect” the first time!

Data Flow Diagram Don’ts process stuff 1. ‘Black Hole’ process stuff 2. ‘It’s a Miracle’

Data Flow Diagram Don’ts ds-1 A.1 A.2 data 4. Leave Things Unlabeled Corollary: Labels Should Have Meaning

Data Flow Diagram Don’ts data store 5. Miracle data source data store 5. Black hole data source

Data Flow Diagram Don’ts 6. Data Flows Unassociated With a Process entity to entity data store to entity - or reverse data store to data store

Diagramming A System multiple DFDs are required to represent a system DFDs are created at increasing levels of detail

Different Types of DFDs Context diagram Level-0 diagram (system diagram) Level-n diagram Primitive diagram Context: - One process - Shows interactions with environment - No internal details Level-0 - Major processing steps - Processes similar to “main menu” items Level-n - Detail of one process from next highest level Primitive - Lowest level - Each process usually has a single output - Further detail of each process shown by a logic model

Context Diagram defines the scope of the system by identifying the system boundary contains: one process (which represents the entire system) all sources/sinks (external entities) data flows linking the process to the sources and sinks (external entities)

Example Context Diagram student course selections Registration System schedule Registration details business office

Constructing a Context Diagram identify and list sources/sinks (external entities) identify and list inputs to and outputs from sources/sinks (external entities) create context diagram

Level-0 Diagram describes the overall processing of the system show one process for each major processing step or functional requirement data flows from the context appear on system diagram also (level balancing) can show a single data store to represent all data in aggregate at this level can draw duplicate sources, sinks and data stores to increase legibility

Drawing a Level-0 Diagram list the major data stores list major business steps draw a segment for each business step assemble into single DFD re-organize until satisfied number processes

Functional Decomposition similar to a series of more detailed maps iterative process of breaking the description of a system into finer and finer detail to create a set of charts in which one process on a given chart is explained in greater detail on another chart referred to as exploding, partitioning, or leveling must use your judgment to decide what goes on each level show error and exception handling on lower levels (if at all)

Lower Level Diagrams explode the processes shown on the level-0 diagram each process is represented by its own DFD balance data data flows on upper level appear on lower level, or data flows on upper level are broken into component pieces with components shown on lower level each lower level shows greater and greater detail follow numbering convention

Balancing DFDs conserve data from level to level - inputs and outputs on the higher level must appears somewhere on the lower level

Advanced Rules Composite data flow on one level can be split into its component data flows on the next level - but new data cannot be added and all data in the composite must be included in the sub-flows The inputs to a process must be sufficient to produce the outputs. Lowest level DFDs may add new data flows to represent exception handling, i.e., error messages May repeat data stores or sources/sink to avoid crossing lines

Additional Guidelines the inputs to a process are different from the outputs of that process objects in a set of DFDs have unique names do not change data flow names on lower levels unless you are decomposing a data flow into component pieces. never explode a single process into another single process. If you cannot partition the process, then the lower level DFD is not needed. expect to iterate, put down the DFD and go back to it a few times to create something satisfactory.

Other Questions about Lower level diagrams 1. How deep? (how many levels?) if the process has only one input or one output, probably cannot partition further; can you describe the process in English in about 1/2 page? 2. How broad? (how many processes on a level?) 7 ± two is a reasonable heuristic may temporarily place much of the system on a single diagram then re-draw into separate levels

Quality Guidelines Completeness Consistency Timing considerations all components included & in project dictionary Consistency between levels: balancing, leveling Timing considerations assume system never starts and never stops Iterative nature revisions are common Drawing primitives (lowest level) when to stop?