UML Diagrams That Every Software Engineer Should Know

Introduction

UML (short for Unified Modelling Language) is a standardised, general-purpose modelling language that encompasses a set of diagrams for detailing and visualising a broad range of system designs and activities. It offers software development teams, designers, and other stakeholders a unified language to collaborate and communicate efficiently and successfully. Using UML diagrams is like having a visual guide for building and understanding software.

UML Diagrams Categories

UML Diagrams are categorised into two main types:

• Structure Diagrams: This category addresses the fixed aspects of the system. The goal of this category is to ensure a consistent structure of the system that is being modelled. This category includes:
  • Class Diagram
  • Component Diagram
  • Deployment Diagram
  • Object Diagram
  • Package Diagram
  • Composite Structure Diagram
  • Profile Diagram
• Behaviour Diagrams: This category covers the dynamic aspects of the system, and it demonstrates the series of changes to a system over time. The goal of this category is to show how the state of an object or system will be affected throughout a sequence of steps. This category includes:
  • Activity Diagram
  • State Diagram
  • Use Case Diagram
  • Interaction Diagrams
    • Communication Diagram
    • Interaction Overview Diagram
    • Sequence Diagram
    • Timing Diagram

Now, let's dive into the most used diagrams with their order of usage during the development lifecycle.

Use Case Diagram

The use case diagram is a visual representation to illustrate and organise interactions between various system actors and the system itself. It identifies and outlines different roles, systems, and interactions that collectively define a user's function within a software system. By showcasing how external entities (actors) interact with the system to achieve specific goals or tasks, the use case diagram enhances understanding and communication among stakeholders during the early stages of system design. It offers a high-level overview, emphasising the essential functionalities and relationships within the system.

Following is an example of a use case diagram with an admin role extending a guest role:

(Figure 1): Use case example

Activity Diagram

The activity diagram is a visual representation that illustrates and elaborates on the sequence and flow of activities within a system or a specific use case. Serving as a complement to the use case diagram, it plays a crucial role in the early to middle stages of system development. This diagram provides a more detailed view of how tasks and actions unfold, showcasing the dynamic aspects of the system. By mapping out the step-by-step flow of activities, the activity diagram aids in a deeper understanding of the system's behaviour and interactions, facilitating effective communication and design decisions.

Following is an example of an activity diagram that contains condition and loop with counter:

(Figure 2): Activity diagram example with counter and condition

Class Diagram

The class diagram is a visual representation employed to illustrate the critical building blocks of a system, known as objects or classes, and their relationships. It serves as a foundational tool in system design, offering insights into the static structure of the software. By showcasing the various classes and their associations, the class diagram helps developers understand how different system parts collaborate and contribute to overall functionality. It is a fundamental element used throughout the software development lifecycle, aiding in conceptualising, designing, and communicating the architecture of a system.

ERD vs Class Diagram
It's worth mentioning that ERD (Entity Relation Diagram) is different. The purpose of an ERD is to represent entities, attributes, and relationships in database design and data modelling.

An example of a class diagram demonstrates two classes, Person and Employee, where the Employee class extends the Person class.

(Figure 3): Class diagram example with extension

Sequence Diagram

The sequence diagram is a visual representation that illustrates the dynamic sequence of interactions among various system components over time. It is a powerful tool to elaborate the chronological order of steps required to accomplish an activity or function within a software system. By depicting the flow of messages between objects or components, the sequence diagram offers a detailed insight into the system's dynamic behaviour, aiding in the analysis and design of complex processes. It is precious for visualising the temporal aspects of system interactions, facilitating clear communication among developers and stakeholders.

Following is an example of a sequence diagram, which demonstrates several components and interactions amongst those components.

(Figure 4): Sequence diagram example

State Machine Diagram

The state machine diagram visually represents the distinct states a system or object can inhabit, along with the transitions between these states. For modelling systems' dynamic behaviour, state machine diagrams capture how entities respond to various events over time. These diagrams vividly analyse the lifecycle of a system, providing a clear understanding of its behavioural patterns. By illustrating states, events, and transitions, state machine diagrams become indispensable tools for system design, enabling developers to comprehend, design, and optimise the intricate behaviours within a software system.

The following is an example of a state machine diagram demonstrating the Build state and the Testing state.

(Figure 5): State Machine diagram example

Component Diagram

A component diagram, a structural diagram in UML, visualises the organisation and relationships among system components. It is pivotal in breaking down intricate systems into manageable and understandable components, elucidating their interdependencies. These diagrams are instrumental in ensuring efficient system design and architecture by providing a comprehensive view of how components collaborate and contribute to the overall functionality. By showcasing the modular structure of a system, component diagrams facilitate effective communication among stakeholders, aiding in system comprehension, development, and maintenance.

A component diagram demonstrates three different components, Identity Service, Carts Service and Orders Service. The Identity Service exposes an interface where the Carts Service requires it (or consumes it), and the Orders Service exposes an interface where the Carts Service requires it.

(Figure 6): Components diagram example

Deployment Diagram

A Deployment Diagram, a specific type of diagram within UML, serves as a visual representation to illustrate the architecture of systems, focusing on how software components are deployed onto hardware infrastructure. This diagram offers a clear and insightful portrayal of the distribution of components across different nodes, including servers, workstations, and devices. By mapping the physical deployment of software artefacts, Deployment Diagrams aid in system architecture planning, ensuring efficient utilisation of hardware resources and facilitating a comprehensive understanding of the system's deployment topology. They are crucial in communicating deployment strategies, optimising performance, and supporting decision-making throughout the software development lifecycle.

Following is a deployment diagram that demonstrates two nodes, one node hosting three services and another node hosting a MySQL database. The diagram also highlights the network connectivity between the services and nodes.

(Figure 7): Deployment diagram example

Conclusion

To conclude, UML diagrams serve as essential blueprints in software design. Through detailed examples, we've navigated class structures, dynamic behaviours, and more. These visual aids facilitate clear communication and collaborative development. By leveraging these sophisticated yet accessible tools, we enhance our capacity for precise and efficient software construction.

UML extends beyond the realm of software development, finding utility in diverse fields. Business analysts employ UML to model processes, using tools like activity diagrams akin to flow charts to visualise workflows and optimise operations. Project managers utilise UML to plan and communicate project structures, employing tools such as Gantt charts. In engineering, UML aids in system design, while even educators leverage it to explain complex concepts visually.