Logical Design and Physical Design
CMGT 555/ Systems Analysis and Development

Throughout the whole process of system development, there are designs that take place before any coding or setup takes place. During this time, phrases like logical design and physical design get tossed around a lot, but what are they and what do they mean? In short, the logical design defines what must take place, not how it is accomplished. The logical design is like a set of blue prints, it describes the actual processes of entering, verifying, and storing data. In this paper we will explain when logical designs and physical designs are used, what design information a logical design and physical design contain and any similarities or differences.

Figure 1. Example of Logical Model and Physical Model for an ERD Relationship Diagram
(Compare Logical and Physical ERD, 2009)
The direct definition of a logical design “is the Conceptual Blueprint of a software application, illustrating entities, relationships, rules, and processes (Thibeault, 2011)”. So what is the logical design is used for? The logical design contains all the business entries, what each entries attributes are, and relationships among entries. Now the logical design to some is misleading because they often confuse it with detailed technical design, even though the goals for these two are not similar at all. Now when do we use the logical design? That question is simpler to answer; it usually starts during the requirements definition phase of the project and after the physical design. When working with the logical design, several pieces of important information needs to be included in the design. This information would include graphical diagrams that help when the business users and the developers sit down to discuss the system; it helps work through business problems and test out ideas.
During the logical design phase, there are deliverables that are provided in the design. Here is a list of these deliverables: * Full Object Model, System Level * Update Object Life Cycle Models * Object Interaction Diagram * System Wide Behavior Model * External Specifications * Initial Draft of Test Plan * Use Cases * Documentation Plan * Initial Draft of Test Scripts * Updated Project Plan * Methodology Compliance Form * Recommendation Form
The logical side represents the inherent nature of a business and only changes if the business changes, which is considerably less volatile than the physical.
Now that we have discussed the logical design, let’s move on to the physical design. To help fully understand the physical design we need to know what it is. The logical design of a system pertains to an abstract representation of the data flows, inputs and outputs of the system. This is often conducted via modeling, using an over-abstract (and sometimes graphical) model of the actual system. The physical components describe "how" it will be implemented. For example, companies have been implementing payroll systems for many years prior to the advent of the computer. We have seen payroll implemented manually, then with time clocks and punch cards, followed by automation on mainframes, minis and PC's. We have also seen such innovations as "direct deposit" to expedite paying employees, and electronic government reporting. All of these devices are simply a physical variation of a theme (Thibeault, 2011).
The physical design has many components what work within the design; it is very easy to get lost scanning all data that is involved. Physical design involves generating the schema for tables, indexes, default and check constraints, and views for a specific physical structure such as a database, file, or XML document (Physical vs. Logical Design). The one thing to keep in mind about the physical design is that it is all about the hardware and it is ultimately based off of the available technology and because of this, it changes drastically from time to time when hardware changes. The physical models can change as well because of hardware changes, even well after the application is deployed in the production environment.
The deliverable in the physical design is: * Full Object Model Technical Level * Internal Specifications * Transaction Plan * Updated Test Scripts * Updated Test Plan * Updated Project Plan * Methodology Compliance Form * Recommendation Form
In conclusion, the logical design and physical design can seem similar on paper when looking at them but they are different when you look at it more closely. The logical design is the blueprints or layout and the physical design is about the hardware and how it plays out in the system. We have also learned that the logical design does not change even though the physical design does change when hardware changes. The hardware my change, evolving the physical design but the processes and functions stay the same.

References

Compare Logical and Physical ERD. (2009, April 20). Retrieved December 04, 2011, from Visual Paradigm: http://www.visual-paradigm.com/highlight/compare_logical_physical_erd.jsp
Physical vs. Logical Design. (n.d.). Retrieved December 04, 2011, from SQLServerPedia: http://sqlserverpedia.com/wiki/Physical_vs._Logical_Design
Thibeault, K. M. (2011, February 14). Definition of a Logical Design. Retrieved 12 04, 2011, from Logical Design Database Solutions : http://www.logicaldesigndbs.com/definition-of-a-logical-design-2/