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System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

What difficulty level is this interview question?

This is a medium difficulty System Design question, commonly asked during Technical Screen rounds at Google.

What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Google during technical interviews.

Design at-least-once notification delivery

Quick Overview

This question evaluates expertise in reliable distributed systems design, including at-least-once delivery semantics, idempotency and deduplication strategies, retry and dead-letter handling, API and message-schema design, ordering and out-of-order handling, and scalability for high-throughput multi-tenant notification platforms.

System Design: At-Least-Once Notification System

Context

Design a multi-channel notification platform (email, SMS, push, in-app) that guarantees at-least-once delivery. Duplicates are possible, so the system must be idempotent and provide a good user experience without showing duplicates. Assume high throughput (e.g., thousands of notifications per second), multi-tenant services, and retries across transient failures.

Requirements

High-level architecture and flow that provides at-least-once delivery.
Producer interface (API/SDK) and consumer/worker interface (message schema, ack/nack semantics).
Deduplication strategy: keys, storage, TTL, and uniqueness constraints.
Retry policy with exponential backoff and jitter; failure handling and dead-letter queues.
Out-of-order handling across retries and parallelism; ordering guarantees per key.
User-facing idempotency: ensure no duplicate notifications are displayed to users across channels.

Deliverables

Define producer API (fields, idempotency, responses) and consumer message interface (fields, ack/nack).
Specify deduplication key design and how/where it is stored.
Describe retry algorithm (exponential backoff with jitter), limits, and DLQ behavior.
Describe how to handle out-of-order events (partitioning keys, sequence numbers, buffering windows).
Describe user-facing idempotency for UI and downstream providers (e.g., collapse/replace semantics).
Call out assumptions and trade-offs.

Quick Overview

Context

Requirements

High-level architecture and flow that provides at-least-once delivery.

Producer interface (API/SDK) and consumer/worker interface (message schema, ack/nack semantics).

Deduplication strategy: keys, storage, TTL, and uniqueness constraints.

Retry policy with exponential backoff and jitter; failure handling and dead-letter queues.

Out-of-order handling across retries and parallelism; ordering guarantees per key.

User-facing idempotency: ensure no duplicate notifications are displayed to users across channels.

Deliverables

Define producer API (fields, idempotency, responses) and consumer message interface (fields, ack/nack).

Specify deduplication key design and how/where it is stored.

Describe retry algorithm (exponential backoff with jitter), limits, and DLQ behavior.

Describe how to handle out-of-order events (partitioning keys, sequence numbers, buffering windows).

Describe user-facing idempotency for UI and downstream providers (e.g., collapse/replace semantics).

Call out assumptions and trade-offs.

Design at-least-once notification delivery

Quick Overview

System Design: At-Least-Once Notification System

Context

Requirements

Deliverables

Solution

Comments (0)

Design at-least-once notification delivery

Quick Overview

System Design: At-Least-Once Notification System

Context

Requirements

Deliverables

Solution

Comments (0)