Step-by-Step Development of a DFD Model - 8.2 | Course Module: Software Design Principles and Structured Analysis | Software Engineering Micro Specialization
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8.2 - Step-by-Step Development of a DFD Model

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Introduction to DFDs

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Teacher
Teacher

Today, we'll start discussing Data Flow Diagrams, or DFDs. Who can tell me what a DFD represents in software design?

Student 1
Student 1

DFDs show how data flows through a system, indicating how it's processed, stored, and output.

Teacher
Teacher

Exactly! They visually represent the movement of data, helping us understand system functionality without technical jargon.

Student 2
Student 2

So, they can help us in identifying what's happening in the system, right?

Teacher
Teacher

Absolutely! They also aid in identifying missing or redundant data flows. Think of it as a roadmap for data within the system!

Developing the Context Diagram

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Teacher
Teacher

Let's move on to drawing the context diagram. What is the first thing we need to identify?

Student 3
Student 3

We should identify the system boundary to understand what is included?

Teacher
Teacher

Correct! And once we have that, we represent the entire system as a single process. Can someone give me an example of what we might label that process?

Student 4
Student 4

It could be 'Order Processing System' or 'Student Registration System.'

Teacher
Teacher

Perfect! Now remember to ensure all external entities and data flows are clearly represented in the diagram. This sets the stage for our next DFD levels.

Level 1 DFD and Balancing Principles

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Teacher
Teacher

Moving on to Level 1 DFDs: After breaking down our single process, why is balancing important?

Student 1
Student 1

Balancing ensures that data flows from our Level 0 context diagram match those in Level 1, right?

Teacher
Teacher

Yes! It maintains consistency and coherence across the diagrams. So, can anyone summarize what we have to check for this balancing?

Student 2
Student 2

We need to match all data inputs and outputs from the parent process to the sub-processes!

Teacher
Teacher

Exactly! And if anything changes, we must ensure that adjustments are reflected in both diagrams.

Drawing Lower-Level DFDs

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Teacher
Teacher

As we draw lower-level DFDs, what's one crucial step to remember?

Student 3
Student 3

We need to decompose our selected process into more details and show the granular data flows.

Teacher
Teacher

Correct! And when we do this, we must keep applying the balancing principle. Any thoughts on how we can maintain flow consistency?

Student 4
Student 4

We should make sure that inputs to our sub-processes are the same as those from the parent process.

Teacher
Teacher

Great point! Remember, it reinforces the integrity of our diagrams and simplifies future updates.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section outlines the process of creating Data Flow Diagrams (DFDs) by systematically decomposing a system into multiple levels, starting from a high-level context diagram.

Standard

In this section, the methodology for developing Data Flow Diagrams (DFDs) is detailed, emphasizing a step-by-step approach that begins with the context diagram and progresses to more detailed levels, ensuring that all processes and data flows are accurately represented and balanced across levels.

Detailed

Step-by-Step Development of a DFD Model

The development of Data Flow Diagrams (DFDs) is a crucial aspect of representing how data moves through a system. This section of the chapter elaborates on the step-by-step approach to creating these diagrams to facilitate system analysis and design. The key stages in the development of a DFD model include:

Step 1: Draw the Context Diagram (Level 0 DFD)

  • Identify the System Boundary: Clearly define what is inside the system and what is outside it.
  • Identify the Primary System Process: Represent the system as a single, central process.
  • Identify External Entities: Determine entities that interact with the system.
  • Identify Major Data Flows: Draw arrows representing primary data inputs and outputs, ensuring meaningful labeling.
  • Review: Confirm that the context diagram precisely reflects the system's overall interface with its environment without showing internal data stores.

Step 2: Draw the Level 1 DFD

  • Decompose the Single Process: Break down the context diagram's process into major sub-processes.
  • Add Internal Data Stores: Identify data stores that the sub-processes will interact with.
  • Connect Processes, Data Stores, and External Entities with Data Flows: Maintain consistency in data flow names and ensure clear representations.
  • Number Processes: Use sequential numbering for clarity.
  • Review for Consistency and Clarity: Maintain adherence to DFD rules for logical representation.

Step 3: Draw Lower-Level DFDs (Level 2, Level 3, etc.)

  • Select a Process for Decomposition: Choose one process for further breakdown.
  • Decompose the Chosen Process: Show more detailed data flows and potentially new data stores relevant only to that process.
  • Apply DFD Balancing: Ensure that the data flows entering and exiting processes balance perfectly across levels by reviewing data flow continuity.

Step 4: Verify DFD Balancing

  • Principle: The input and output data flows of a parent process must match the aggregated input and output flows of its sub-processes.
  • Checklist for Verification: Ensure all flows from parent diagrams are represented in child diagrams without introducing new flows unnecessarily.

Conclusion

This structured approach not only aids in visualizing complex systems but also ensures that they are accurately documented and easier to understand for stakeholders involved in system design.

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Step 1: Draw the Context Diagram (Level 0 DFD)

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  1. Identify the System Boundary: Clearly define what is inside the system and what is outside.
  2. Identify the Primary System Process: Represent the entire system as a single, central process (e.g., "Order Processing System," "Student Registration System").
  3. Identify External Entities: Determine all entities (people, organizations, other systems) that interact directly with the system by sending or receiving data. Draw them as rectangles outside the system boundary.
  4. Identify Major Data Flows: Draw arrows representing the primary data inputs from external entities to the system and data outputs from the system to external entities. Label these data flows meaningfully.
  5. Review: Ensure the Context Diagram accurately reflects the system's overall interface with its environment. It should not show any data stores internally.

Detailed Explanation

In step 1, we start by defining what the system is and what it isn't, which is crucial for understanding its boundaries. The main process that the system handles is identified, along with the external entities interacting with it, like users or other systems. Next, we map out how data moves between these external entities and the system, ensuring to give meaningful names to these data flows for clarity. Finally, the diagram must be checked to ensure it provides a cohesive overview of interactions without unnecessary complexity such as internal data stores.

Examples & Analogies

Think of the Context Diagram like a map of a city (the system). The boundaries of the city are established (what's inside as the system and what's outside as external entities). The central landmark (the main process) is identified, such as the city hall or central business district. People, like residents and visitors, represent external entities that interact with this central area. Roads signify the data flows allowing us to see how information moves in and out of the city.

Step 2: Draw the Level 1 DFD

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  1. Decompose the Context Diagram's Single Process: Break down the single process from the Context Diagram into its major sub-processes. Typically, 3-7 main processes are a good number.
  2. Add Internal Data Stores: Identify the major data stores that these sub-processes read from or write to.
  3. Connect Processes, Data Stores, and External Entities with Data Flows: Ensure that all data flows that entered/exited the single process in the Context Diagram are now shown entering/exiting the appropriate sub-processes at Level 1, maintaining the same names. Show data flows between internal processes and internal data stores. Show data flows between processes and external entities.
  4. Number Processes: Use sequential numbering (e.g., 1.0, 2.0, 3.0).
  5. Review for Consistency and Clarity: Ensure the Level 1 DFD logically represents the system's major functions and adheres to DFD rules.

Detailed Explanation

In step 2, we take the overarching process from the Context Diagram and break it down into several main sub-processes, which are the core functions that contribute to the overall system's purpose. We also add data stores that these sub-processes will utilize to keep information. It's essential that the data flows entering or exiting the system in the previous diagram are accurately reflected here in relation to each sub-process and data store. Processes are numbered for clarity, and a review ensures that the diagram meets DFD rules and accurately depicts the system's functions.

Examples & Analogies

Imagine that the city from our previous example is hosting an event. The Level 1 DFD is like breaking down this single event into distinct activities such as ticketing, food stalls, and information booths. Each activity (or sub-process) operates simultaneously but contributes to the event. The food stalls gather supplies (internal data stores) and share information with the ticketing booth (data flows) to ensure everything runs smoothly. Numbering these activities would keep everything organized, much like a festival schedule.

Step 3: Draw Lower-Level DFDs (Level 2, Level 3, etc.)

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  1. Select a Process for Decomposition: Choose a complex process from the current DFD level that needs further detail.
  2. Decompose the Chosen Process: Break it down into its sub-processes, showing more granular data flows and potentially new data stores relevant only to that sub-process.
  3. Apply DFD Balancing: Ensure the inputs and outputs of the decomposed sub-process (e.g., process 2.0) are precisely the same as the inputs and outputs of its child DFD (e.g., Level 2 DFD for process 2.0).
  4. Number Processes: Use hierarchical numbering (e.g., if decomposing process 2.0, its sub-processes might be 2.1, 2.2, 2.3).
  5. Continue Decomposition: Repeat the process until all processes are "elementary" or "primitive" processes, whose logic can be fully described by a short textual specification (Process Spec).

Detailed Explanation

Step 3 involves taking one of the processes from the Level 1 DFD that appears complex and breaking it down further. This decomposition reveals the sub-processes that together fulfill the task, and it allows us to better detail the data flows and what data stores they utilize. It's important to maintain balance, ensuring that the flow of data remains consistent between the higher level and the lower level DFDs. The processes are numbered hierarchically based on their relationship to help keep track of them. This method continues until we reach elementary processes that require no further break down and can be described simply.

Examples & Analogies

If we consider a company that has a complex department like HR, the Level 2 DFD is akin to detailing various sub-functions such as recruitment, payroll, and employee benefits. Each sub-function (or process) can be broken down further into its tasks, like posting a job ad or calculating payroll taxes. As we can’t forget to maintain balance, just as in managing a project, every task must be aligned with the bigger goal of HR's functions. Eventually, we break each task down until we have simple actions that can be directly discussed.

Step 4: Verify DFD Balancing

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  1. Principle: The input and output data flows of a parent process must exactly match the sum of the input and output data flows of its child DFD.
  2. Checklist: For every process you decompose:
  3. Are all inputs to the parent process shown as inputs to one or more sub-processes in the child DFD?
  4. Are all outputs from the parent process shown as outputs from one or more sub-processes in the child DFD?
  5. Are there any new data flows introduced at the child level that were not present at the parent level, or vice-versa, without a clear reason?
  6. Consequences of Unbalanced DFDs: Incomplete requirements, difficulty in tracing data, potential for design errors.

Detailed Explanation

Step 4 is critical to ensuring that our DFDs maintain integrity across levels. Balancing refers to the principle that all data entering a parent process must be reflected in some form in its child processes. This ensures coherent scaling of the diagrams down to the most granular level. We utilize a handy checklist to ensure that all inputs and outputs have been properly accounted for in their decomposed forms. Failure to balance results in potential gaps in requirements and could lead to design errors which can complicate further development.

Examples & Analogies

Think of balancing like making a recipe. If a cake recipe requires 2 cups of sugar, that exact amount must be balanced by the output of the cake, ensuring its sweetness matches expectations. If we decide to add new ingredients (child process) or forget to note any of the original required ingredients (parent process), the final product may not taste right. Just like in baking, ensuring all components of our DFDs are balanced is essential for success.

Example Walkthrough (e.g., Online Order Processing System)

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  1. Context Diagram:
  2. External Entities: Customer, Warehouse, Bank.
  3. System Process: Online Order Processing System.
  4. Data Flows: Order Request (from Customer), Order Details (to Warehouse), Payment Request (to Bank), Payment Confirmation (from Bank), Order Confirmation (to Customer).
  5. Level 1 DFD (Decomposition of "Online Order Processing System"):
  6. Processes: 1.0 Accept Order, 2.0 Process Payment, 3.0 Fulfill Order, 4.0 Notify Customer.
  7. Data Stores: Orders, Products, Customer Accounts.
  8. Show how Order Request goes to 1.0 Accept Order, which then stores New Order in Orders and sends Payment Details to 2.0 Process Payment. 2.0 Process Payment interacts with Bank via Payment Request/Payment Confirmation and updates Orders with Payment Status. 3.0 Fulfill Order reads Paid Order from Orders and sends Order Details to Warehouse. 4.0 Notify Customer reads Order Status from Orders and sends Order Confirmation to Customer. (Demonstrate balancing by ensuring external entity connections match Level 0).
  9. Level 2 DFD (Decomposition of a selected process, e.g., 1.0 Accept Order):
  10. Processes: 1.1 Validate Customer, 1.2 Validate Products, 1.3 Calculate Order Total, 1.4 Create Order Record.
  11. Data Stores: Customer Accounts, Products.
  12. Show granular data flows like Customer ID to 1.1 Validate Customer (which reads from Customer Accounts), Product List to 1.2 Validate Products (reading from Products), etc., ensuring all inputs and outputs of 1.0 Accept Order are reflected.

Detailed Explanation

In the example walkthrough, we apply the concepts of DFD modeling to a real-world scenario: an online order processing system. Initially, we define the main components in the context diagram, identifying the essential external entities (the customer, warehouse, and bank) and the primary system process. Moving on, we create the Level 1 DFD, breaking the primary process into sub-processes like accepting orders, processing payments, fulfilling orders, and notifying customers, while illustrating how critical data flows relate those components together. Lastly, we delve into a more detailed Level 2 DFD to effectively break down the process of order acceptance into validation and recording actions, ensuring that our flows maintain proper balance throughout.

Examples & Analogies

Consider ordering a pizza online. The context diagram would show the customer (external entity) wanting to place an order, with data flows representing order details sent from the customer to the pizza shop system. The Level 1 DFD would break down this main process into components like taking the order, processing payment, and delivering the pizza. Each of these steps can be further detailed in a Level 2 DFD, such as validating the customer’s information and calculating the total amount. Each sub-task must lead back to the same flow of data, ensuring that every piece of information is accounted for.

Best Practices for DFD Development

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  1. Start with the Context Diagram.
  2. Identify all external entities first.
  3. Use clear, concise, active names for processes.
  4. Use meaningful nouns for data flows and data stores.
  5. Avoid crossing data flow lines unnecessarily.
  6. Aim for 3-7 sub-processes at each level of decomposition.
  7. Regularly review DFDs with users and stakeholders for accuracy and clarity.
  8. Maintain strict DFD balancing across all levels.
  9. Do not introduce control flow (decision points, loops) into DFDs; focus solely on data flow. Control flow is for program logic, often captured in process specs or structure charts.

Detailed Explanation

The best practices for developing DFDs are established to streamline the process and ensure clarity. It's advised to start with the context diagram to have a solid foundation before building more detailed DFDs. Clear naming conventions help ensure that the labels communicate the purpose of processes and data effectively. Avoiding unnecessary line crossings keeps the diagram clean and easy to follow. Ensuring a manageable number of sub-processes at each level maintains coherence. Frequent reviews with stakeholders help catch potential errors early. Finally, keeping strict DFD balancing prevents inconsistencies across diagrams.

Examples & Analogies

Imagine you are organizing an event and drawing a layout of the venue. Best practices for that event layout would begin with outlining the entire venue first (context diagram) before placing booths and stages. Clear labeling helps everyone understand where each activity is happening. Ideally, you'd want to avoid crowding pathways between booths (no unnecessary crossing of lines). Having a good number of vendors makes the event lively but not overcrowded (3-7 sub-processes rule). Finally, you’d want to walk through the layout with your team to ensure everyone knows their roles and responsibilities, much like the importance of reviews in DFD development.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Data Flow Diagrams (DFDs): Visual representations showing data movement through systems.

  • Context Diagram: Highest-level DFD representing the system as a single process.

  • Balancing: Ensuring input and output flows match across different levels of DFDs.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • An example of a context diagram for an 'Online Shopping System' illustrating external entities such as 'Customer', 'Payment Gateway', and their interactions.

  • A Level 1 DFD that breaks down the 'Order Processing System' into subprocesses like 'Accept Order', 'Process Payment', and 'Notify Customer'.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎡 Rhymes Time

  • When data flows, let’s take a look, in diagrams, it’s the best book!

πŸ“– Fascinating Stories

  • Imagine a library where the librarian, represented as a process, interacts with books stored in the shelves (data stores) and patrons at the circulation desk (external entities), showing how the system operates.

🧠 Other Memory Gems

  • Remember 'C-B-L' for DFDs: Context, Breakdown, Leveling!

🎯 Super Acronyms

DFD

  • Data Flows Diagrammatically!

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Data Flow Diagram (DFD)

    Definition:

    A graphical representation of data flows and processes within a system.

  • Term: Context Diagram

    Definition:

    The highest-level DFD that depicts the entire system as a single process.

  • Term: Level 1 DFD

    Definition:

    A DFD that breaks down the single process of the context diagram into sub-processes.

  • Term: Balancing

    Definition:

    The principle that input and output data flows of a parent diagram must match those of its child diagram.

  • Term: Hierarchical Decomposition

    Definition:

    The process of breaking down a system into hierarchical sub-processes.