How do you read a Data Flow Diagram? What does the "Reserve Hotel Room" DFD example show?

You follow the arrows: every data flow shows a named piece of data moving between an external entity, a process, and a data store — each piece must have a traceable source.

* The "Reserve Hotel Room" data-flow diagram — every arrow is a named piece of data with a traceable source. *

In the hotel reservation example:

1. Guest (source) sends Name, Dates, Location, CC Profile to the Reserve Hotel Room process
2. The process queries the Reservation System with Dates, Location, CC Profile
3. Reservation System returns Availability, Price, Confirmation
4. Process sends Name, Dates of Stay to Hotel (sink)
5. Process returns Availability, Price, Confirmation to the Guest

Key insight: Data doesn't "magically appear" - every piece of data must have a source and flow through the system. If data appears without a source, the DFD is incomplete.

Security perspective: DFDs are the foundation for threat modeling. Each element can be analyzed:

• Processes: Can they be compromised? Input validation?
• Data Stores: Encryption at rest? Access control?
• Data Flows: Encryption in transit? Authentication?
• External Entities: Can they be trusted? Spoofed?

Go deeper:

• Data-flow diagram (Wikipedia) — reading flows, the "data doesn't magically appear" rule, and the context-diagram / DFD-level hierarchy.