Chapters 7 & 9 System Scope DFDs, Use Cases, ERDs

Agenda Summary Documenting System Scope Context and Level-0 DFDs Use Case Diagrams Enterprise Entity-Relationship Diagrams Summary

Scope What’s in and what’s out: boundary Constraints and givens What can change Who / what is affected

Process Modeling Graphically represent the processes that capture, manipulate, store, and distribute data between a system (what’s in) and its environment (what’s out) and among system components (functionalities) Utilize information gathered during requirements determination Processes and retained data are modeled

Data Flow Diagram (DFD) A picture of the movement of data between external entities and the processes and data stores within a system Difference from system flowcharts: DFDs depict logical data flow independent of technology Flowcharts depict details of physical systems

DFD Symbols

DFD Symbols (cont.) Process: work or actions performed on data (inside the system) Data store: data at rest (inside the system) Source/sink: external entity that is origin or destination of data (outside the system) Data flow: arrows depicting movement of data

Context Diagram Context diagram shows the system boundaries, external entities that interact with the system, and major information flows between entities and the system. NOTE: only one process symbol, and no data stores shown.

Level-0 DFD Level-0 DFD shows the system’s major processes, data flows, and data stores at a high level of abstraction. Processes are labeled 1.0, 2.0, etc. These will be decomposed into more primitive (lower-level) DFDs.

Use Cases Depiction of a system’s behavior or functionality under various conditions as the system responds to requests from users What are tasks in which each actor (external entity) is involved Alternative to DFDs (especially Level-0)

UML Use Case Diagram Symbols Actor Boundary Connection Include relationship Extend relationship <<include>> <<extend>>

What is an Actor? Actor is an external entity that interacts with the system. Most actors represent user roles, but actors can also be external systems. An actor is a role, not a specific user; one user may play many roles, and an actor may represent many users.

What is a Boundary? A boundary is the dividing line between the system and its environment. Use cases are within the boundary. Actors are outside of the boundary.

What is a Connection? A connection is an association between an actor and a use case. Depicts a usage relationship Connection does not indicate data flow

What is an <<extend>> Relationship? A connection between two use cases Extends a use case by adding new behavior or actions Specialized use case extends the general use case

What is an <<include>> Relationship? A connection between two use cases Indicates a use case that is used (invoked) by another use case Links to general purpose functions, used by many other use cases

Conceptual Data Modeling A detailed model that captures the overall structure of data in an organization Independent of any database management system (DBMS) or other implementation considerations Enterprise data model: the major categories of data and the relationships between the categories

Process of Conceptual Data Modeling Develop a data model for the current system Develop a new conceptual data model that includes all requirements of the new system In the design stage, the conceptual data model is translated into a physical design Project repository links all design and data modeling steps performed during SDLC

Gathering Information for Conceptual Data Modeling Two perspectives Top-down Data model is derived from an intimate understanding of the business. Bottom-up Data model is derived by reviewing specifications and business documents.

Requirements Determination Questions for Data Modeling What are subjects/objects of the business? Data entities and descriptions What unique characteristics distinguish between subjects/objects of the same type? Primary keys What characteristics describe each subject/object? Attributes and secondary keys How do you use the data? Security controls and user access privileges

Requirements Determination Questions for Data Modeling (cont.) Over what period of time are you interested in the data? Cardinality and time dimensions Are all instances of each object the same? Supertypes, subtypes, and aggregations What events occur that imply associations between objects? Relationships and cardinalities Are there special circumstances that affect the way events are handled? Integrity rules, cardinalities, time dimensions

Summary Scope: What may change (in vs. out) What interactions must be preserved What are major functionalities Who / what will be affected Processes, data, people, systems