Logical vs. Physical Data Flow Diagrams: What Sets Them Apart

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Logical vs. Physical Data Flow Diagrams: What Sets Them Apart

System Analysis and Design · Chapter 4: Requirements Validation, Modeling, and UMLSection 4.6 · lesson 29 of 697 min · free with a free account

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In this lesson

What a system does, and how it actually does it.

  • Explain what a logical DFD shows, and what a physical one adds
  • Describe the steps that convert a logical DFD into a physical one
  • Compare logical and physical DFDs of the same system, element by element

Transcript

Everything said in the video, section by section (998 words). Read along, or search it.

Requirements Validation, Modeling, and UML

Logical vs. Physical DFDs

A data flow diagram can be drawn at two different levels of detail. Both describe exactly the same process. But they answer different questions. This section separates the logical view from the physical view. We will see what each one shows, how to convert one into the other, and what changes, element by element, in a worked example.

2 Views of One Process

What It Does vs. How It Does It

A logical D F D shows what a system does, independent of how it is actually implemented. It names processes, data stores, and data flows in business terms. It does not mention specific software, hardware, file formats, or which department performs a step. A physical D F D shows how those same processes are actually carried out. It names the real technology, people, and formats involved. That could be a specific database table, a particular script, or a paper form. Both are usually needed, at different points in a project. The logical view lets the business agree on what should happen. The physical view lets the builders agree on how.

Side by Side

How the 2 Views Differ

Let's compare them across six aspects. The focus. A logical D F D shows what happens. A physical one shows how it happens. Process names. A logical process might be validate eligibility. Its physical counterpart might be, registrar runs eligibility check dot p y. Data store names. Logically, student records. Physically, the students table in an Oracle database called StudentDB. Data flow names. Logically, flows are named by meaning. Physically, by format, such as a JSON payload, or a printed form. A logical D F D mentions no technology at all. A physical one does. And the audiences differ. Business stakeholders review logical diagrams, while developers and IT staff work from physical ones.

Converting (1 of 2)

Start From a Validated Logical DFD

A physical D F D is not drawn from scratch. It is built by taking an already validated logical D F D, and deciding, process by process, how each part will be carried out. Step one. Start from the validated logical D F D. Its business processes, data stores, and data flows should already be agreed with stakeholders. Step two. Decide how each process will be carried out. Will it be a manual step, performed by a specific person or department? Or an automated script or program? And if so, which one? Step three. Decide how each data store will actually be implemented. A logical data store becomes a specific database table, a spreadsheet, a filing cabinet, or another concrete location.

Converting (2 of 2)

Formats, Additions, and a Feasibility Check

Step four. Decide the actual format of each data flow. A logical flow becomes a specific medium, such as a web form submission, an emailed file, or a printed report. Step five. Add anything the implementation needs that the logical model did not include. Error-handling steps, security checks, or backup processes, for example. Step six. Review the result with technical stakeholders, to confirm it is actually feasible to build, before development begins. Notice the order. Business agreement comes first, and implementation decisions second. That keeps technology from quietly reshaping what the business actually asked for.

Figure 1 - Course Registration

The Same System, Drawn Two Ways

Here is the conversion applied to a small course registration system. On the logical side, a student sends a registration request. Process one point zero, validate eligibility, checks student records and the course catalog. It passes an eligibility result to process two point zero, enroll student, which writes to enrollment records. And the student receives a confirmation. On the physical side, every element gets its implementation. The registration request becomes a web form, submitted over HTTPS. Validate eligibility becomes an eligibility check script, running on the registrar's PC. The three data stores become tables in an Oracle database called StudentDB. The eligibility result becomes an eligibility flag, in JSON. Enroll student becomes a PHP program on the application server. And the confirmation becomes an email, sent over SMTP.

What Changed

One-to-One, Element by Element

Now compare the two diagrams. The number of processes, data stores, and data flows is identical on both sides. Two processes, three data stores, and the same flows between them. And every element lines up one to one. That is the key point. Converting a logical D F D into a physical one does not change what the system does. It only adds the implementation detail needed to actually build it. If an element appears on one side with no partner on the other, something has been lost, or invented, in the conversion.

Check Yourself

Logical or Physical?

Let's check that the distinction has landed. For each name, decide which view it belongs to. Validate eligibility. That is logical. It describes a business action, and mentions no technology. Enroll dot p h p, on the app server. That is physical. It names a specific program, on specific hardware. Course catalog. Logical again. It is named by what it contains. Confirmation email, sent over SMTP. Physical. It names the medium, and even the protocol. Here is a simple test you can apply to any element. If the name mentions a product, a format, a person, or a machine, it belongs to the physical view. If it only describes what the business needs, it is logical.

Key Terms

The Vocabulary of This Section

Let's lock in the key terms. A logical D F D describes what a system does, in business terms, independent of implementation. A physical D F D describes how its processes are actually carried out, including specific technology, people, and formats. And a data store is where data is held. In a logical D F D it is named by its content. In a physical one, by its actual implementation.

Key Takeaways

What to Carry Forward

To pull this together. A logical D F D answers what a system does. A physical D F D answers how it is actually carried out. A physical D F D is built from a validated logical D F D, by deciding the specific implementation of every process, data store, and flow. The two views describe the same process, so every logical element should have a matching physical element. And logical D F Ds suit business stakeholders reviewing requirements, while physical D F Ds suit the developers and IT staff who build the system.

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Universities that work with NCCRS

System Analysis and Design carries a NCCRS credit recommendation. These universities work with NCCRS; some are UPI Study's signed partners. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

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FAQ

Questions students ask us

About Logical vs. Physical Data Flow Diagrams: What Sets Them Apart

What is the difference between a logical and a physical data flow diagram?
A logical DFD shows what a system does in business terms, independent of implementation, while a physical DFD shows how those same processes are actually carried out. A logical process might be validate eligibility and a logical store student records. The physical version names the real technology, people, and formats, such as an eligibility check script on the registrar's PC or a students table in a specific database.
How do you convert a logical DFD into a physical DFD?
To convert a logical DFD into a physical one, start from a logical DFD already validated with stakeholders, then decide how each process will be carried out, manually or by a specific program. Next decide how each data store is implemented, such as a database table or filing cabinet, and the format of each data flow. Then add implementation needs like error handling or security checks, and review feasibility with technical staff.
Who uses logical DFDs and who uses physical DFDs?
Logical DFDs are mainly reviewed by business stakeholders, while physical DFDs are mainly used by developers and IT staff who build the system. The logical view lets the business agree on what should happen without getting distracted by technology. The physical view lets builders agree on how it will happen. Because business agreement comes first, technology choices are less likely to quietly reshape what the business asked for.
How can you tell whether a DFD element is logical or physical?
A simple test is that an element is physical if its name mentions a product, a format, a person, or a machine, and logical if it only describes what the business needs. Validate eligibility and course catalog are logical names. A PHP program on the application server or a confirmation email sent over SMTP are physical, because they name specific software, hardware, media, or protocols.

About the System Analysis and Design course

How many college credits is System Analysis and Design?
System Analysis and Design (Computer Science 390) carries a NCCRS credit recommendation for 3 semester credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.
How much does System Analysis and Design cost and what is included?
$250, paid once, with lifetime access: all 69 video lessons, a practice quiz after each, the graded work and final exam, and an official transcript when you finish. Students who want several courses choose UPI Study PRO instead.
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UPI Study has signed partner agreements with universities such as Thomas Edison State University, University of the People and Charter Oak State College, and works with universities such as Penn State, SNHU and SUNY. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities. Use "Find my college" on this page to check yours.
How long does System Analysis and Design take?
It is self-paced with no deadlines. The 69 lessons total about 9 hours of video; most students add reading and practice time and finish in a few weeks to a few months, on their own schedule.
How are the exams done?
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