dfdlisted

# DFD References: - <https://miro.com/diagramming/what-is-a-data-flow-diagram/> — components, levels, logical vs physical. - <https://www.visual-paradigm.com/guide/data-flow-diagram/what-is-data-flow-diagram/> — drawing rules, balancing. A DFD shows where data moves and what transforms it. Sequencing belongs in a flowchart, object structure in UML; a DFD carries neither. Control signals are not data — leave them out. ## Components Four, and only four: | Component | Is | Gane–Sarson | Yourdon/DeMarco | Mermaid | | --- | --- | --- | --- | --- | | External entity | A source or sink outside the system boundary | square | rectangle | `E[Name]` | | Process | Work that transforms input data into output data | rounded rectangle | circle | `P(1.0 Verb the thing)` | | Data store | Data at rest — a file, table, queue, keyring | open-ended rectangle | two parallel lines | `S[(Name)]` | | Data flow | Named data moving between the above | labelled arrow | labelled arrow | `A -->\|data name\| B` | Name every element. Entities and stores take noun phrases, processes take verb phrases, flows take the name of the data itself (`plaintext token`), never the mechanism carrying it. ## Levels Each level decomposes one process from the level above. - **Context** — the whole system as a single process numbered `0`, surrounded by every external entity it exchanges data with. No data stores; internal storage is inside the system. - **Level 0** — process `0` opened up into its major proces