Processes are usually used to perform different actions, such as: - Take some data and sort it out. - Alter an existing data by applying filters on a database. - Taking input and doing some calculation. - Decision making based on existing data.

CRUD

Performing manipulation on stored data is called CRUD operations.
CRUD stands for: - Create - Read - Update - Delete

Layers of Abstractions

Within the DFD, we hide the abstractions and complexities of each process or part of the diagram to keep it readable.
Otherwise the diagram becomes way too big to read.
Then each part can be explained in details in other diagrams.

Context Level Diagram

In this level, we probe out the system and the agents it interacts with.
At this level, the system is a black box (keeping the details hidden).
The flow of information and which outputs are generated from which inputs are also drawn.

Level One Diagram

This level describes the system as major processes and which outputs are produced by each process against the inputs provided by the external entities.

Level Two Diagram

This level describes each process in detail.

Note: The number of external agents, inputs and outputs should remain the same throughout the levels to keep consistency.

Example

Patient Monitoring System

Context Level Diagram

The patient might show some vital signs which should be sent to nurse as an alter message.
Other than that, the nurse should be able to get reports about the patient, stored as logs.

Level One Diagram

Level Two Diagram

Example of Mistake

Pay close attention, if you perform the source-and-sink-analysis,
you will notice that freeze member account process is not accepting any inputs.
Similarly, the create a new member account process is not generating any output.
The process Generate an employee bank statement takes 2 inputs which are irrelevant to the output generated because we need account number and time of request to generate the bank statement.

Illegal Data Flows

1. an external entity cannot directly interact with another external entity, it has to be done through a process.
2. A data-store cannot directly interact with another data-store, it has to be done through a process.
3. An external entity cannot directly interact with a data-store directly, it has to be done through a process.