Data Flow Diagram (Level 1 Decomposition)

CSC-326 · Semester V · System Analysis and Design

intermediate25 minAvailable

DFD Level 1 — Decomposition

Process 0 exploded into four sub-processes and stores.

Click a process

Data stores (D1, D2) persist state across processes. Each sub-process can itself be exploded further (level 2), always preserving the parent's input and output flows.

Lab brief

Objective

Decompose the context diagram into its main processes, data stores and internal data flows.

Theory & concepts

A level 1 DFD expands the single context process into the primary functional processes of the system. Data stores appear here for the first time, and data flows now connect processes to stores as well as to external entities.

Rendering diagram…

Level 1 DFD decomposes the context process into numbered processes

Every flow that entered or left the context diagram must reappear at level 1, attached to the correct process - this is the balance rule. Processes are labeled with imperative verbs (Validate, Update, Generate), and each process will itself be decomposed further at level 2 if needed.

Identify the main functions the system performs.
Draw each function as a process bubble.
Add data stores for data that must persist between functions.
Connect processes with the flows from the context diagram plus internal flows.
Check the balance rule: nothing is added or dropped between levels.

Prerequisites

Lesson: Data flow diagrams
Lab: Context Diagram (DFD Level 0)

Instructions

1Start from your context diagram.
2Name 3 to 6 primary processes such as Register Member, Issue Book, Return Book.
3Add the data stores (Members, Catalogue, Loans).
4Draw the flows between processes and stores.
5Verify that every context-level flow still appears at level 1.

Tasks

Decompose the system
Split your system into its main processes with imperative names.
Place the stores
Add the data stores and connect each process that reads or writes them.
Apply the balance rule
Check every context flow against the level 1 diagram.

Self-check

The level 1 diagram preserves all context-level flows (balance rule).
Processes have clear imperative labels.
Data stores are connected to the processes that access them.
The student can explain when to decompose a process further.

Environment

diagram canvasprocess and store shapesflow labelingbalance rule checklist