Communication/Collaboration Diagrams

What is a Communication Diagram?

A Communication Diagram (formerly called a Collaboration Diagram) shows the same information as a sequence diagram — object interactions — but organized around the objects and their links rather than along a time axis. It emphasizes the structural relationships between objects that participate in an interaction.

If a sequence diagram is a "movie script" (ordered by time), a communication diagram is a "network diagram" (organized by relationships). Both show the same messages, just arranged differently.

Communication Diagram Notation

Objects are shown as named instances:
  ┌──────────────┐
  │  :ClassName   │  ← object name (or :ClassName for unnamed)
  └──────────────┘

Links connect objects (solid lines):
  ┌────────┐          ┌────────┐
  │  :A    │──────────│  :B    │
  └────────┘          └────────┘

Messages are numbered to show sequence:
  ┌────────┐          ┌────────┐
  │  :A    │──1.0────▶│  :B    │
  └────────┘          └────────┘
      │                   │
      │  1.1              │ 1.2
      ▼                   ▼
  ┌────────┐          ┌────────┐
  │  :C    │◀─────────│  :D    │
  └────────┘          └────────┘

Numbering scheme:
  1.0    — first message
  1.1    — first sub-message of 1.0
  1.2    — second sub-message of 1.0
  2.0    — second top-level message

Example: Login Interaction

Let's compare the same login scenario in both diagram types:

Communication Diagram:

┌────────┐      1: enterCredentials()      ┌──────────┐
│ :User  │────────────────────────────────▶│ :LoginForm│
└────────┘                                 └─────┬────┘
                                                 │
                           2: validate()         │
                      ┌──────────────────────────┘
                      ▼
                ┌──────────┐    3: findUser()    ┌────────┐
                │  :Auth   │────────────────────▶│   :DB  │
                └─────┬────┘                     └────┬───┘
                      │                               │
                      │     4: userData                │
                      │◀──────────────────────────────┘
                      │
                      │     5: result
                ┌─────▼────┐
                │ :LoginForm│
                └─────┬────┘
                      │
                      │     6: showResult()
                ┌─────▼────┐
                │  :User   │
                └──────────┘

Sequence Diagram (same scenario):

┌──────┐   ┌──────────┐   ┌──────┐   ┌────┐
│ :User│   │ :LoginForm│   │ :Auth│   │ :DB │
└──┬───┘   └────┬─────┘   └──┬───┘   └──┬─┘
   │            │             │          │
   │ 1.enter    │             │          │
   │ Credentials│             │          │
   │───────────▶│             │          │
   │            │ 2.validate()│          │
   │            │────────────▶│          │
   │            │             │3.findUser│
   │            │             │─────────▶│
   │            │             │          │
   │            │             │4.userData│
   │            │             │◀─────────│
   │            │ 5.result    │          │
   │            │◀────────────│          │
   │ 6.show     │             │          │
   │ Result     │             │          │
   │◀───────────│             │          │

Communication vs Sequence Diagrams

┌────────────────────┬──────────────────────────────────────┐
│ Feature            │ Comparison                           │
├────────────────────┼──────────────────────────────────────┤
│ Organization       │ Sequence: time-ordered (vertical)    │
│                    │ Communication: structure-based        │
├────────────────────┼──────────────────────────────────────┤
│ Emphasis           │ Sequence: WHEN messages are sent     │
│                    │ Communication: WHO talks to WHOM     │
├────────────────────┼──────────────────────────────────────┤
│ Message ordering   │ Sequence: implicit (top-to-bottom)   │
│                    │ Communication: explicit (numbered)   │
├────────────────────┼──────────────────────────────────────┤
│ Loops/Conditions   │ Sequence: easy (combined fragments)  │
│                    │ Communication: harder to represent   │
├────────────────────┼──────────────────────────────────────┤
│ Object links       │ Sequence: not shown                  │
│                    │ Communication: explicitly shown      │
├────────────────────┼──────────────────────────────────────┤
│ Best for           │ Sequence: algorithm flows, workflows │
│                    │ Communication: structural analysis   │
└────────────────────┴──────────────────────────────────────┘

When to Use Communication Diagrams

Use Communication Diagrams When:
  ✓ You want to see which objects interact directly
  ✓ You need to understand the structural relationships during an interaction
  ✓ You're analyzing object responsibilities in a specific scenario
  ✓ You want a compact view of an interaction
  ✓ You're doing object-oriented analysis (discovering relationships)

Use Sequence Diagrams Instead When:
  ✓ The order of messages is critical
  ✓ You need to show loops, conditions, or timing constraints
  ✓ The scenario is long or complex
  ✓ You're communicating with non-technical stakeholders
  ✓ You're documenting detailed behavior for developers

Practical Tip:
─────────────
Use BOTH diagrams for the same scenario:
  • Communication diagram during analysis → discover relationships
  • Sequence diagram during design → define exact interaction order

The communication diagram helps you think about structure.
The sequence diagram helps you think about behavior.
Together, they give you a complete picture.