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How are data flow diagrams and UML diagrams used to model a system's processes, data and objects?

System modelling with data flow diagrams and UML diagrams (use case, class and activity), and the role of the data dictionary.

A CCEA A-Level Software Systems Development answer on system modelling: data flow diagrams showing processes and data stores, and UML diagrams (use case, class and activity), plus the data dictionary, and why models are used.

Generated by Claude Opus 4.813 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

CCEA expects you to model a system using data flow diagrams (DFDs) and UML diagrams (use case, class and activity), to know the components of each and what they show, and to explain why models are used. You should also know the data dictionary that records the meaning of every data item. Describing DFD components and the purposes of the main UML diagrams are examined directly.

The answer

Data flow diagrams

DFDs can be levelled: a context (level 0) diagram shows the whole system as one process with its external entities, and lower levels decompose each process into more detail. They give a clear, technology-independent view that helps communicate with users and check that all required data is accounted for.

UML diagrams

The data dictionary

A data dictionary is a catalogue describing every data item in the system: its name, data type, size/length, format, validation rules and description. It ensures everyone uses each item consistently and underpins both the DFD's data flows and the database design.

Worked example: modelling a small system three ways

Examples in context

Example 1. A hospital records system. A use case diagram captures that Doctors view records and prescribe, while Receptionists book appointments, agreed with staff before any code is written. A DFD then shows patient data flowing from the reception entity through a register patient process into a Patients data store, and a class diagram models Patient, Appointment and Prescription classes with their relationships, guiding the build.

Example 2. A data dictionary catches an inconsistency. Two designers describe a phone field, one as a number and one as text with spaces and a plus sign. The data dictionary forces a single agreed definition (text, 15 characters, a format mask), so the DFD's data flows, the database column and the validation all match, preventing a costly mismatch later.

Try this

Q1. Name the four components of a data flow diagram. [4 marks]

  • Cue. External entity (source/sink), process, data store, and data flow.

Q2. State what a UML use case diagram shows and from whose point of view. [2 marks]

  • Cue. It shows the system's functionality (actors and use cases) from the user's point of view, capturing requirements.

Q3. State two pieces of information recorded about a data item in a data dictionary. [2 marks]

  • Cue. Any two of name, data type, size/length, format, validation rules, description.

Exam-style practice questions

Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

CCEA 20196 marksDescribe the components of a data flow diagram (DFD), and explain why an analyst uses one during systems development.
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A data flow diagram models how data moves through a system. Its components are:

External entities (sources or sinks) are people or systems outside the system that send or receive data, drawn as a labelled rectangle (or square).

Processes transform data, taking input data flows and producing output data flows, drawn as a circle or rounded rectangle and named with a verb phrase.

Data stores hold data at rest, such as a file or database table, drawn as an open-ended rectangle.

Data flows are arrows showing data moving between entities, processes and stores, labelled with the data carried.

An analyst uses a DFD because it gives a clear, graphical view of what data the system handles, where it comes from, how it is processed and where it is stored, independent of the technology. This helps communicate with users, check that all required data is accounted for, and form the basis of the design.

Markers reward the four components (external entity, process, data store, data flow) with their meanings, and a valid reason for using a DFD (clear communication, technology-independent view of data movement).

CCEA 20215 marksDescribe the purpose of a UML use case diagram and a UML class diagram, and state what each shows.
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UML (Unified Modelling Language) is a standard notation for modelling object oriented systems.

A use case diagram shows the functionality of a system from the user's point of view. It shows actors (the users or external systems, drawn as stick figures) and use cases (the things the system lets them do, drawn as ovals), connected by lines. Its purpose is to capture and communicate the system's requirements: who uses it and what they can do with it.

A class diagram shows the static structure of the system: the classes, each with its attributes and methods, and the relationships between them (such as association, inheritance and aggregation). Its purpose is to model the object oriented design, showing what classes exist, what data and behaviour each has, and how they are related, which guides the implementation.

Markers reward the use case diagram (actors and use cases, the user's view of functionality/requirements) and the class diagram (classes with attributes and methods, and their relationships, the static structure), each with its correct purpose.

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