Sequence diagrams in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the correct sequence diagram element for a synchronous message

In a sequence diagram, which element correctly represents a synchronous message between two objects?

A A dashed arrow with an open arrowhead pointing from the caller to the callee

B A solid arrow with a filled arrowhead pointing from the caller to the callee

C A solid line without arrowheads between two objects

D A dashed arrow with a filled arrowhead pointing from the callee to the caller

💡 Hint

Think about how synchronous calls are shown as direct requests expecting a response.

A synchronous message is shown as a solid arrow with a filled arrowhead pointing from the caller to the callee, indicating a direct call that waits for a response.

❓ Architecture

intermediate

Trace the request flow in a sequence diagram for a login process

Given a sequence diagram for a user login process involving User, Web Server, Authentication Service, and Database, which step correctly follows the user's login request?

A Database sends authentication result directly to User

B Authentication Service sends login credentials directly to User

C User sends login credentials to Web Server, which forwards them to Authentication Service

D Web Server sends login credentials to Database without Authentication Service

💡 Hint

Consider the typical flow of authentication in layered systems.

The user sends credentials to the Web Server, which then forwards them to the Authentication Service for verification before querying the Database.

❓ scaling

advanced

Estimate the impact on sequence diagram complexity when adding microservices

How does adding multiple microservices to a system affect the complexity of its sequence diagram?

A It increases the number of lifelines and messages, making the diagram more complex and harder to read

B It reduces the number of messages because microservices handle tasks internally

C It simplifies the diagram by grouping all microservices into a single lifeline

D It has no effect on the sequence diagram complexity

💡 Hint

Think about how each microservice is represented as a separate participant.

Each microservice adds a lifeline and messages, increasing diagram complexity and requiring careful organization to maintain clarity.

❓ tradeoff

advanced

Choose the best approach to handle asynchronous messages in sequence diagrams

Which option correctly represents asynchronous messages in a sequence diagram and explains its tradeoff?

A Use a dashed arrow with an open arrowhead; tradeoff is increased diagram complexity but clearer timing

B Use a solid arrow with an open arrowhead; tradeoff is less clarity on message timing

C Use a solid arrow with a filled arrowhead; tradeoff is confusion with synchronous messages

D Use a dashed arrow with a filled arrowhead; tradeoff is ambiguity in message direction

💡 Hint

Recall the standard UML notation for asynchronous messages.

Asynchronous messages are shown as dashed arrows with open arrowheads, which can make diagrams more complex but clearly indicate non-blocking calls.

❓ estimation

expert

Estimate maximum number of messages in a sequence diagram for a system with 5 components and 3 interactions each

If a system has 5 components and each component interacts with 3 others exactly once in a sequence diagram, what is the maximum number of messages shown?

A 25 messages

B 10 messages

C 5 messages

D 15 messages

💡 Hint

Multiply the number of components by the interactions per component.

Each of the 5 components sends 3 messages, so total messages = 5 * 3 = 15.

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