Sequence diagrams in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the lifeline for the User actor in a sequence diagram.

LLD

[1] User

Drag options to blanks, or click blank then click option'

Aactor

Bentity

Cobject

Dparticipant

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'actor' instead of 'participant' causes syntax errors.

In sequence diagrams, participant is used to define lifelines for actors or objects.

2fill in blank

medium

Complete the code to show a message from Client to Server in a sequence diagram.

LLD

Client -> [1]: Request

Drag options to blanks, or click blank then click option'

ACache

BServer

CUser

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'Database' or 'Cache' instead of 'Server'.

The message is sent from Client to Server, so the arrow points to Server.

3fill in blank

hard

Fix the error in the return message syntax from Server to Client.

LLD

Server --> [1]: Response

Drag options to blanks, or click blank then click option'

ADatabase

BServer

CClient

DCache

Attempts:

3 left

💡 Hint

Common Mistakes

Pointing the return message to Server instead of Client.

The return message goes back to the Client, so the arrow points to Client.

4fill in blank

hard

Fill both blanks to show a synchronous call and its return in a sequence diagram.

LLD

Client -> [1]: ProcessData
[2] --> Client: Result

Drag options to blanks, or click blank then click option'

AServer

BClient

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Using Client as the receiver of the call or sender of the return.

The synchronous call goes from Client to Server, and the return message comes back from Server to Client.

5fill in blank

hard

Fill all three blanks to define lifelines and a message flow in a sequence diagram.

LLD

participant [1] as Client
participant [2] as Server
[3] -> Server: Authenticate

Drag options to blanks, or click blank then click option'

AUser

BClient

CServer

DDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing participant names or message sender names.

The lifeline names are Client and Server. The message is sent from Client to Server.

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