Message delivery guarantees in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding At-Least-Once Delivery

Which of the following best describes the behavior of a messaging system with at-least-once delivery guarantee?

A Messages are delivered exactly once, with no duplicates or losses.

B Messages are delivered one or more times, possibly causing duplicates but never lost.

C Messages may be lost but never duplicated.

D Messages are delivered at most once, possibly lost but never duplicated.

💡 Hint

Think about whether duplicates are allowed and if messages can be lost.

At-least-once delivery means the system ensures messages arrive but may deliver duplicates if acknowledgments are lost.

❓ Architecture

intermediate

Choosing a Delivery Guarantee for a Payment System

You are designing a payment processing system. Which message delivery guarantee should you choose to avoid charging customers twice while ensuring no payment is lost?

A At-most-once delivery without retries.

B At-least-once delivery with idempotent payment processing.

C Exactly-once delivery with transactional support.

D No delivery guarantee, relying on manual reconciliation.

💡 Hint

Consider the importance of avoiding duplicates and losing messages in payments.

Exactly-once delivery with transactions ensures each payment is processed once and only once, preventing duplicates and losses.

❓ scaling

advanced

Scaling a Messaging System with Exactly-Once Guarantee

What is the main challenge when scaling a distributed messaging system that provides exactly-once delivery guarantee?

A Ignoring acknowledgments to speed up delivery.

B Allowing message loss to reduce storage requirements.

C Dropping messages to maintain throughput.

D Coordinating distributed state to avoid duplicate processing increases latency and complexity.

💡 Hint

Think about what exactly-once delivery requires in a distributed environment.

Exactly-once delivery needs coordination to track message processing state, which adds latency and complexity when scaling.

❓ tradeoff

advanced

Tradeoffs Between At-Most-Once and At-Least-Once Delivery

Which statement correctly describes a tradeoff between at-most-once and at-least-once message delivery guarantees?

A At-most-once avoids duplicates but risks message loss; at-least-once avoids loss but may cause duplicates.

B At-most-once guarantees no message loss and no duplicates; at-least-once allows duplicates and loss.

C Both guarantees always deliver messages exactly once.

D At-least-once delivery is faster and simpler than at-most-once.

💡 Hint

Consider which guarantee prioritizes avoiding duplicates versus avoiding loss.

At-most-once may lose messages but never duplicates; at-least-once never loses messages but may duplicate them.

❓ estimation

expert

Estimating Storage for Exactly-Once Delivery State

A messaging system processes 1 million messages per hour with exactly-once delivery. Each message requires 100 bytes of metadata to track processing state. Estimate the storage needed to keep 24 hours of state for deduplication.

A Approximately 2.4 GB

B Approximately 1.2 GB

C Approximately 24 GB

D Approximately 240 MB

💡 Hint

Calculate total messages in 24 hours and multiply by metadata size.

1 million messages/hour × 24 hours = 24 million messages. 24 million × 100 bytes = 2.4 billion bytes = 2.4 GB.

Practice

1. Which message delivery guarantee ensures that a message is never delivered more than once, but some messages might be lost?

easy

A. Exactly once

B. At most once

C. At least once

D. None of the above

Solution

1. Step 1: Understand 'At most once' guarantee

   This guarantee means messages can be lost but never duplicated.

2. Step 2: Compare with other guarantees

   'At least once' may duplicate messages, 'Exactly once' avoids loss and duplication but is complex.

3. Final Answer:

   At most once -> Option B

4. Quick Check:

   At most once = no duplicates, possible loss [OK]

Hint: No duplicates but possible loss means 'At most once' [OK]

Common Mistakes:

• Confusing 'At most once' with 'At least once'
• Thinking 'Exactly once' is simple
• Assuming 'At most once' never loses messages

2. Which of the following is the correct description of the 'Exactly once' message delivery guarantee?

easy

A. Messages are delivered once without loss or duplication

B. Messages may be lost but never duplicated

C. Messages may be duplicated but never lost

D. Messages are delivered at least once, possibly duplicated

Solution

1. Step 1: Define 'Exactly once' guarantee

   This guarantee ensures each message is delivered once, no loss, no duplication.

2. Step 2: Eliminate other options

   'Messages may be lost but never duplicated' is 'At most once', 'Messages may be duplicated but never lost' and 'delivered at least once, possibly duplicated' describe 'At least once'.

3. Final Answer:

   Messages are delivered once without loss or duplication -> Option A

4. Quick Check:

   Exactly once = no loss, no duplicates [OK]

Hint: Exactly once means no loss and no duplicates [OK]

Common Mistakes:

• Mixing 'Exactly once' with 'At least once'
• Believing 'Exactly once' allows duplicates
• Confusing 'At most once' with 'Exactly once'

3. Consider a message queue system that uses 'At least once' delivery. If a message is sent and the receiver crashes before acknowledging, what is the likely outcome?

medium

A. The message is lost and never delivered

B. The message is delivered once without duplication

C. The message is delivered at most once

D. The message may be delivered multiple times

Solution

1. Step 1: Understand 'At least once' behavior on receiver crash

   If receiver crashes before ack, sender retries, causing possible duplicates.

2. Step 2: Analyze options

   The message is not lost but sender retries on no ack, so may be delivered multiple times to ensure at least once.

3. Final Answer:

   The message may be delivered multiple times -> Option D

4. Quick Check:

   At least once = possible duplicates [OK]

Hint: Receiver crash before ack causes duplicates in 'At least once' [OK]

Common Mistakes:

• Assuming message is lost on crash
• Confusing 'At least once' with 'At most once'
• Ignoring possibility of duplicates

4. A system claims to provide 'Exactly once' delivery but sometimes messages are duplicated. What is the most likely cause?

medium

A. The system uses 'At least once' delivery without deduplication

B. The system uses 'At most once' delivery internally

C. The system does not handle acknowledgments properly

D. The system drops messages on network failure

Solution

1. Step 1: Identify cause of duplicates in 'Exactly once'

   Duplicates usually happen if 'At least once' is used without deduplication.

2. Step 2: Evaluate other options

   'At most once' causes losses not duplicates, improper ack handling or dropping messages cause losses or retries but exactly once needs deduplication atop at least once.

3. Final Answer:

   The system uses 'At least once' delivery without deduplication -> Option A

4. Quick Check:

   Duplicates mean missing deduplication in 'At least once' [OK]

Hint: Duplicates in 'Exactly once' mean missing deduplication [OK]

Common Mistakes:

• Assuming 'At most once' causes duplicates
• Ignoring deduplication step
• Confusing message loss with duplication

5. You are designing a payment processing system that must never lose or duplicate transactions. Which message delivery guarantee should you choose and why?

hard

A. At most once, because it avoids duplicates

B. At least once, because it ensures no message loss

C. Exactly once, because it guarantees no loss and no duplicates

D. None, because message delivery guarantees are not relevant

Solution

1. Step 1: Identify system requirements

   Payment processing must avoid losing or duplicating transactions.

2. Step 2: Match guarantee to requirements

   'Exactly once' ensures messages are delivered once without loss or duplication, fitting the need.

3. Final Answer:

   Exactly once, because it guarantees no loss and no duplicates -> Option C

4. Quick Check:

   Payment systems need exactly once delivery [OK]

Hint: No loss or duplicates means choose 'Exactly once' [OK]

Common Mistakes:

• Choosing 'At most once' and risking loss
• Choosing 'At least once' and risking duplicates
• Ignoring delivery guarantees in critical systems