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.