Sequence diagrams in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a sequence diagram?

A sequence diagram shows how objects or parts of a system talk to each other over time. It uses arrows to show messages sent between them in order.

beginner

What do vertical lines represent in a sequence diagram?

Vertical lines represent the lifelines of objects or components, showing their existence over time during the interaction.

beginner

What is the purpose of arrows in a sequence diagram?

Arrows show messages or calls sent from one object to another, indicating the flow of communication and the order of events.

intermediate

How do sequence diagrams help in system design?

They help visualize how parts of a system interact step-by-step, making it easier to understand, design, and debug complex processes.

intermediate

What is an activation bar in a sequence diagram?

An activation bar is a thin rectangle on a lifeline that shows when an object is active or processing during the interaction.

In a sequence diagram, what does a horizontal arrow represent?

A The lifespan of an object

B A data store

C A decision point

D A message sent between objects

Horizontal arrows show messages or calls sent from one object to another.

What does a lifeline in a sequence diagram show?

A The user interface layout

B The data stored in a database

C The timeline of an object during interaction

D The network connection status

Lifelines are vertical lines that represent the existence of an object over time.

Which element indicates when an object is active in a sequence diagram?

A Activation bar

B Lifeline

C Message arrow

D Note

Activation bars show when an object is performing an action or processing.

Sequence diagrams are mainly used to:

A Show the order of interactions between system parts

B Design database schemas

C Create user interface layouts

D Monitor server health

Sequence diagrams focus on the order and flow of messages between components.

What does a dashed arrow usually represent in a sequence diagram?

A New object creation

B Return message or response

C Error message

D User input

Dashed arrows typically show return messages after a call.

Explain the main components of a sequence diagram and their roles.

Think about what shows objects, communication, and timing.

Describe how sequence diagrams help in understanding system behavior.

Consider how seeing steps in order helps design and debugging.

Practice

1. What is the main purpose of a sequence diagram in system design?

easy

A. To list all system hardware requirements

B. To display database table structures

C. To show how components interact over time

D. To describe user interface colors

Solution

1. Step 1: Understand sequence diagram focus

   Sequence diagrams illustrate the order and flow of messages between system parts over time.

2. Step 2: Compare options with purpose

   Only To show how components interact over time matches this purpose; others describe unrelated system aspects.

3. Final Answer:

   To show how components interact over time -> Option C

4. Quick Check:

   Sequence diagram = interaction over time [OK]

Hint: Sequence diagrams show message flow over time [OK]

Common Mistakes:

• Confusing sequence diagrams with database schemas
• Thinking sequence diagrams show UI design
• Assuming sequence diagrams list hardware

2. Which of the following is the correct syntax to represent a message from Client to Server in a sequence diagram?

easy

A. Client == Server: Request

B. Client <-> Server: Request

C. Client -- Server: Request

D. Client -> Server: Request

Solution

1. Step 1: Identify standard message arrow

   In sequence diagrams, a solid arrow (->) shows a message sent from one object to another.

2. Step 2: Match options to syntax

   Client -> Server: Request uses '->' correctly; others use incorrect or non-standard symbols.

3. Final Answer:

   Client -> Server: Request -> Option D

4. Quick Check:

   Message arrow = '->' [OK]

Hint: Use '->' for messages in sequence diagrams [OK]

Common Mistakes:

• Using double arrows '<->' which imply bidirectional but not standard
• Using dashes '--' which are not message arrows
• Using '==' which is not a valid arrow

3. Given this sequence diagram snippet:

Client -> Server: LoginRequest
Server -> DB: QueryUser
DB -> Server: UserData
Server -> Client: LoginResponse

What is the correct order of messages received by the Server?

medium

A. LoginRequest, UserData

B. UserData, LoginRequest

C. LoginRequest, LoginResponse

D. QueryUser, LoginRequest

Solution

1. Step 1: Identify messages sent to Server

   Server receives 'LoginRequest' from Client and 'UserData' from DB.

2. Step 2: Determine order of reception

   First, Server gets 'LoginRequest', then after querying DB, it receives 'UserData'.

3. Final Answer:

   LoginRequest, UserData -> Option A

4. Quick Check:

   Server messages received = LoginRequest then UserData [OK]

Hint: Trace arrows pointing to Server in order [OK]

Common Mistakes:

• Confusing messages sent by Server with received messages
• Mixing order of messages
• Including messages Server sends as received

4. In this sequence diagram, the message arrow is drawn as Server --> Client: Response. What is the likely issue?

medium

A. The arrow should be solid '->' not dashed '-->'

B. The message label 'Response' is missing

C. The direction of arrow is reversed

D. The lifeline for Client is missing

Solution

1. Step 1: Check arrow style correctness

   Sequence diagrams use solid arrows '->' for messages; dashed arrows '-->' usually represent return messages or asynchronous calls.

2. Step 2: Identify problem in given arrow

   The arrow is dashed '-->' but labeled as a normal message, which is incorrect syntax.

3. Final Answer:

   The arrow should be solid '->' not dashed '-->' -> Option A

4. Quick Check:

   Message arrow must be solid '->' [OK]

Hint: Use solid arrow '->' for normal messages [OK]

Common Mistakes:

• Confusing dashed arrows with normal message arrows
• Ignoring arrow direction
• Assuming label absence is the issue

5. You want to model a system where a Client sends a request to Server, which then calls two services ServiceA and ServiceB in parallel before responding. How should the sequence diagram represent the parallel calls?

hard

A. Use a loop fragment around calls to ServiceA and ServiceB

B. Use combined fragments with 'par' to show parallel calls to ServiceA and ServiceB

C. Draw sequential arrows to ServiceA then ServiceB without any special notation

D. Show only one service call to simplify the diagram

Solution

1. Step 1: Understand parallel calls in sequence diagrams

   Parallel interactions are shown using combined fragments labeled 'par' to indicate concurrent execution.

2. Step 2: Match options to parallel representation

   Use combined fragments with 'par' to show parallel calls to ServiceA and ServiceB correctly uses 'par' fragment; others either show sequential calls or omit parallelism.

3. Final Answer:

   Use combined fragments with 'par' to show parallel calls to ServiceA and ServiceB -> Option B

4. Quick Check:

   Parallel calls = 'par' combined fragment [OK]

Hint: Use 'par' fragment for parallel calls in sequence diagrams [OK]

Common Mistakes:

• Drawing parallel calls as sequential arrows
• Using loop fragments instead of parallel
• Omitting one service call to simplify