Unified Modeling Language (UML)

Unified Modeling Language (UML)

Free tools for creating UML diagrams:

Tool	Typical use
Excalidraw	Informal sketches / whiteboarding
Draw.io	General UML and other diagrams
Lucidchart	General UML and other diagrams
Mermaid	Text-based diagrams in Markdown
DB Diagram	Database schemas / ER diagrams only

Background

Prior to UML...

Different notations and diagrams
Everyone confused

Now, with UML...

Can take a diagram to anyone on earth in tech field and they will understand it

What is UML?

Not a programming language

Set of diagrams and notations used to model IT systems.
Universal, culture invariant, and so much fun!

Hooray standards!

Do We Really Need to Know All This?

Per IBM: An introduction to the Unified Modeling Language:

The most useful, standard UML diagrams are: use-case diagram, class diagram, sequence diagram, statechart diagram, activity diagram, component diagram, and deployment diagram.

Different domains use some more than others.

When to Use UML

Planning new systems / functionality
Documenting existing systems
Training new team members
Communicating with stakeholders
Safety critical systems
Regulatory compliance

Diagram Types

Use Case Diagram

Description

Represent functionality, actors, and how these relate to each other in the context of a system.
"High level" scope; not much detail.
Useful for planning new functionality.
Useful for communicating with stakeholders.

Key components

Actors
Use cases
Relationships between actors and use cases
- *Optionally, relationships between use cases (includes / extends)
System boundaries

Class Diagram

Description

Shows how entities (classes) in a system relate to each other.
Classes contain self-describing attributes and methods.
Connectors between classes show cardinality; how many of one class can be related to another class.
- https://www.uml-diagrams.org/multiplicity.html
- Also... Crow's feet notation.

Multiplicity	Option	Cardinality
0..0	0	Collection must be empty
0..1		No instances or one instance
1..1	1	Exactly one instance
0..*	*	Zero or more instances
1..*		At least one instance
5..5	5	Exactly 5 instances
m..n		At least m but no more than n instances

Concrete and abstract entities:

Entities can be concrete or abstract. From above:
- Concrete examples:
  - Person
  - ??? (what else?)
- Abstract examples:
  - Comment
  - ??? (what else?)

Key components

Classes
Attributes
Methods
Relationships between classes (cardinality)

For dog walking service, how would we show…

Dog walker and client (person)?
Dogs that he/she is scheduled to walk?
Relationships between classes?
Any other classes needed?

Sequence Diagram

Description

Shows interactions between objects in a system...
- over time
- for a specific use case

What has to happen before another thing happens? Who is involved?

Key components

Actors
Lifelines
Messages
Activation boxes

State Diagram

Description

Shows states of a system and how it transitions between them.

Key components

States
Transitions

Planning a user registration and login workflow:

Each page can be modeled as a state, shown here by a set of four variables:

Variable	Description
`uname`	The username of the user
`pwd`	The password of the user
`verified`	Whether the user has verified their email
`token`	The authentication token (a hidden field used for authentication)

Transitions show how users get from one page to another.

Activity Diagram

Description

Models the flow of control from one activity to another.
Shows logic without implementation detail.

Key components

Activities
Transitions
Branching / merging (indicates processes that may run concurrently, then meet back up)
Forking / joining (much like flowchart decision)

Hybrids

Swimlane Diagram

Like an activity diagram... with swimlanes.
Swimlanes show boundaries between actors / roles in system.

Custom

It’s useful to have a baseline knowledge of UML if you work with software at all. Even rough sketches can save valuable communication time.

Mix and match to suit your needs.