Master UML: Visualizing System Design with Sequence, Activity, and Component Diagrams

Sequence Diagram

Content

Describes the time order of messages passed between objects.

Used to represent the behavior order of a use case.

Emphasizes the chronological sequence of messages.

Models interactions between classes as message exchanges.

Elements

1. Object – represents the role of an object in the interaction.

2. Lifeline – a dashed line indicating the existence of an object over time.

Each object has a lifeline at its bottom center.

The lifeline extends from the top to the bottom of the diagram.

Its length reflects the duration of the interaction.

Combines the object icon with its lifeline.

3. Activation – the period during which an object performs an operation, analogous to a pair of braces {} in C.

Indicates when an object is active to complete a task.

Deactivation means the object is idle, waiting for a message.

4. Message – defines the information exchanged between entities.

Models communication content between entities.

Can trigger operations, signals, or object creation/destruction.

Flows from sender to receiver as a control information stream.

Symbol

Meaning

Draws a synchronous call between two objects.

Shows a message between two objects.

Displays the return message of a call.

Draws a self‑call message.

Draws an asynchronous message between two objects.

Data‑Center Billing System Sequence Diagram

Collaboration Diagram

Content

A dynamic representation emphasizing the structure of participating objects.

Shows object roles and their relationships, similar to a class diagram but with interaction details.

Can be viewed as an extension of an object diagram.

Elements

1. Object – role of an object in the collaboration, similar to sequence diagram objects.

2. Link – a solid line connecting two roles, representing communication paths.

Symbol

Meaning

Creates a communication path between objects.

Shows an object invoking its own operation.

Adds a message between two objects or within the same object.

Adds a reverse‑direction message between objects.

Shows information flow between two objects.

Shows reverse‑direction information flow.

Data‑Center Billing System Collaboration Diagram

Activity Diagram

Overview

An activity represents a state of ongoing work, either in real life or within a software system.

It is a non‑atomic execution composed of a series of actions that cause state changes or return values.

Elements

1. Action State – an atomic, indivisible unit that cannot be interrupted.

Instantaneous and non‑interruptible.

May have incoming and outgoing transitions.

Differs from state‑chart states: no entry/exit actions or internal transitions.

2. Activity State – can be decomposed into sub‑activities or actions; it may be interrupted.

Can contain nested activity diagrams.

Has entry/exit actions and internal transitions.

When it contains only one action, it is equivalent to an action state.

3. Action Flow – the transition from one action or activity state to the next, occurring automatically without a trigger.

4. Branch and Merge – represent decision points and convergence of parallel flows.

5. Fork and Join – split a control flow into concurrent threads and synchronize them.

6. Swimlane – groups activities by responsible organizational unit.

Data‑Center Billing System Activity Diagram

State Diagram

State Machine

A state machine consists of states linked by transitions. A state represents the condition of an object while performing an activity or waiting for an event. A transition connects two states, is triggered by an event, and may execute actions.

State Chart

A state chart describes the dynamic behavior of an entity based on events, showing how the entity reacts to different events in each state.

Elements

Name

Description

Entry/Exit Action

Action executed when entering or leaving a state.

Internal Transition

Transition that does not cause a state change.

Delay Event

Event that is postponed until the object reaches another state.

Substate

A state that contains other states.

Transitions

Element

Description

Source State

State before the transition.

Target State

State after the transition.

Trigger Event

Event that initiates the transition.

Guard Condition

Condition that must be true for the transition to occur.

Action

Operation executed when the transition fires.

Data‑Center Billing System State Diagram

Component Diagram

Overview

Describes software components and their relationships; components are physical modules of code.

Can include packages and subsystems.

Shows dependencies among components.

Elements

1. Component – a replaceable physical unit with defined interfaces.

2. Interface – a set of operations offered by a class to another class; can be provided (export) or required (import).

Export Interface – provided by the component offering operations.

Import Interface – used by a component to access services.

3. Relationship – includes dependency, generalization, association, and realization.

Dependency:

Realization:

Data‑Center Billing System Component Diagram

Deployment Diagram

Overview

Shows the physical hardware on which the software system runs and how software is deployed onto that hardware.

Can contain packages and subsystems to group model elements.

Includes components placed on nodes, representing the hardware topology.

Often used to model static configuration views.

Nodes

A node is a physical element representing a computational resource with memory and processing capability.

1. Name – displayed inside the node icon.

2. Node Types

Processor – a node capable of executing software (servers, workstations, etc.).

Device – a node without processing power, providing services via external interfaces (e.g., printers, scanners).

Components in Deployment

Components can be placed inside node symbols to indicate they run on the same node.

Relationships

Deployment diagrams typically include dependency and association relationships.

Relationship

Graphic

Association

Dependency

Data‑Center Billing System Deployment Diagram