Sequence Diagram

There are two categories of sequence diagrams. They serve a different purpose and have a different scope đŸ—ēī¸.

The first ones are System Sequence Diagrams (SSDs) đŸĒ›. They are used during the early stages of the design. They represent the high-level interactions between actors (ex: a person) and systems (ex: a shop).

System Sequence Diagram

On the other hand, a Detailed Sequence Diagram (DSD)is a visual representation of a use case with all its detailed interactions.

For instance, "Player engages in combat with an enemy character."

Detailed Sequence Diagram

💡 When we refer to a "Sequence Diagram", we usually refer to a Detailed Sequence Diagram.


Basic Overview

Sequence Diagrams focus on the chronological order interactions among systems or actors. They illustrate the lifetime of all entities âŗ.

The diagram is read from the top to the bottom, representing time progressing from top to bottom ⌛.

Each participant in the sequence can be:

This is determined by the level of abstraction needed. Below, we have an object diagram with two instances of Person.

Sequence Diagram Object Participants

The vertical dotted line is what we call a lifetime.

The interactions between participants are called messages 📮. You can view it as the same as function calls.

The solid line and arrowhead represents a message (ex: a function call). The rectangle is an activation bar. It means the participant is active (ex: executing our function). The dashed arrow is the reply (ex: return xxx).

Sequence Diagram Message

Some messages are asynchronous meaning we are not expecting a response meaning the timeline doesn't wait for a reply to continue. They are represented with a different arrowhead.

👉 For complex diagrams, we may add numbers before every message especially when it involves asynchronous messages.


Combined Fragments

Combined Fragments are used to add various types of control flow and interaction scenarios within a sequence diagram.

Conditions such as [isXXX] are called guards.

Most combined fragments with guards can have a variable number of blocks. Simply add a dotted line and a guard to the new block.


Combined Fragments: ALT and OPT

Both are used for branching. ALT is for alternative behavior (execute one of...) while OPT is optional behavior (execute if condition met).

ALT Combined Fragment OPT Combined Fragment


Combined Fragment LOOP

You can repeat some interactions multiple times.

LOOP Combined Fragment

Combined Fragments: PAR, SEQ/STRICT, CRITICAL

First, PAR is used to run instructions in parallel. SEQ or STRICT are used when we are calling asynchronous methods, and we need to ensure the previous one is done before the next one (STRICT doesn't propagate).

PAR Combined Fragment

đŸĢ I learned that CRITICAL means that the instructions mustn't fail but documentation says it doesn't mean that (it means an atomic instruction).


Combined Fragments "SD" and "REF"

I can't seem to find the documentation for both. Usually, sequence diagrams are wrapped in a combined fragment sd diagram_name.

This allows us to use the ref to import them.

SD Combined Fragment   REF Combined Fragment


đŸ‘ģ To-do đŸ‘ģ

Stuff that I found, but never read/used yet.

  • self messages
  • example with fully instantiated diagram
  • we can't get a reply without sending a message first
  • ensure target function existing on target ; link class diagram
  • "create", "destroy"...
  • cross at the end of a timeline = destroy
  • instantiated vs non-instantiated (cf scenarios)