Data Flow Diagrams: Logical & Physical Working Together

Data Flow Diagrams: Logical & Physical Working Together


Data flow diagrams can be quite useful when utilized within the systems development lifecycle because they assist the analyst in gaining comprehensive views of a system’s processes, inputs, outputs, data stores, and external entities at varying elevations.

Data flow diagrams are put together through a top-down method starting from a bird’s-eye-view, then gradually decreasing the altitude, thus narrowing our view of the system. By narrowing our view we are in effect slowly moving from a macro view of the system to an ever increasing micro view of the system’s process and subprocesses. It is through this process that an analyst can eventually document and describe a system down to its most primitive processes, creating a very specific set of instructions which can eventually be translated into a working system.

As with most every man-made creation it is paramount we start at a logical level, a level at which we can focus on the design of our creation rather than the actual physical manifestation. Once our creation has been completely conceptualized we can then focus on how we intend to physically create it.

For this reason data flow diagrams are separated into two types, the logical or conceptual and the physical.

The Logical Diagram

As hinted at above, the logical data flow diagram is kept at what is called the conceptual level or a level at which we are only considering the concepts making up our system, not its physical embodiment. By remaining at a conceptual level we are better able to focus on entities, processes, data, and data stores in an attempt to identify them all and verify they are documented as accurately as possible.

The reason this is important is that once the physical aspect of something is introduced we tend to give it more focus and attention. This is because we are physical beings and we like dealing with what is real; we can see it, touch it, and work with it with our bare hands. At the moment the physical is introduced we are no longer concerned with the design, or not as much as we should be.

While are are in the conceptual space our diagrams will focus more on the following important aspects of design:

  • The business and how it operates
  • The business events
  • The data involved in those events
  • The data created by those events


The conceptual space also provides us with specific advantages:

  • Better communication with users
  • A more stable system
  • Better business understanding
  • Design flexibility
  • More effective maintenance
  • Elimination of redundancies
  • Easier creation of the physical model due to our understanding of the logical model to start

The Physical Diagram

Once we have great grasp on the concept of a new system we are better prepared to decide on its physical implementation. With this knowledge we can create what is called the physical data flow diagram.

In this diagram our focus changes from the conceptual to the actual way in which we will develop the system. With this change in focus we begin to consider the items we will need such as the hardware, software, files, and resources.

When at the physical level we need to focus on the following items:

  • How the system will be implemented
  • Hardware, software, files, and people involved in the system
  • Business triggers which spawn activities
  • Actual data inputs required by those activities
  • Actual data outputs created by those activities
  • Data repositories to house said data
  • Data structures to store structured data


The physical level also provides us with several advantages:

  • Clarified processes base on partitions such as manual vs. automated
  • Processes are described in more detail than in logical for ease of development
  • Processes are sequenced
  • Temporary data stores are identified
  • Data stores actually named and technologies selected
  • Process controls are identified
  • CRUD operations identified

Order of Operation

These two types of data flow diagrams are extremely useful alone or in combination with one another. This efficacy is greatly enhanced when they are utilized within a very specific sequence as outlined below.

  • Logical data flow diagram of current system (one to be replaced)
    • Provides a clear understanding of the current system
    • Provides a good starting point for replacement system
  • Logical data flow diagram of proposed system
  • Physical data flow diagram for system implementation


In the above order, each data flow diagram helps to enhance each subsequent one. When used together the resulting system will provide a much better solution to the business needs.

Sources/References

1. Kendall, K.E. (2011). Systems Analysis and Design.
 Pearson Education, Inc., publishing as Prentice Hall

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