System Design - Webhook Delivery System

In an OpenAI system design interview, the Webhook Delivery System problem statement typically asks you to design a reliable, scalable, and secure platform that allows source applications to automatically notify third-party subscribers when specific events occur. 2

Core Problem Statement

"Design a webhook delivery system that can handle high-volume event fan-out (e.g., 1 billion events per day) with reliable delivery guarantees and multi-tenant isolation." 4 5

Functional Requirements

• Registration: Users must be able to register, update, and delete callback URLs associated with specific event types.
• Event Ingestion: The system must listen for internal events (e.g., a payment success or a completed AI batch job).
• Reliable Delivery: The system must provide at-least-once delivery guarantees.
• Filtering: Support for event subscriptions so clients only receive the data they care about. YouTube +3

Non-Functional Requirements

• High Scalability: Ability to handle massive spikes in event volume (e.g., 100k events/sec at peak).
• Fault Tolerance & Retries: Automatic retries with exponential backoff for failed delivery attempts.
• Security: Implement payload signing (HMAC) so recipients can verify the request came from your platform.
• Idempotency: Ensuring that multiple deliveries of the same event do not cause duplicate actions on the receiver's end.
• Multi-tenant Isolation: Prevent a single "noisy neighbor" (one client's failing or slow server) from clogging the entire delivery pipeline. YouTube +6

Key Components to Discuss

1. Ingestion Service: Validates incoming internal events and pushes them to a message queue.
2. Message Queue/Bus: Used for decoupling and buffering high-volume traffic (e.g., Kafka or SQS).
3. Delivery Workers: Consume events from the queue and perform the actual HTTP POST requests to third-party endpoints.
4. Dead Letter Queue (DLQ): Stores events that have failed all retry attempts for later manual inspection.
5. State Store/Database: Tracks subscription metadata and delivery history for observability. Hello Interview +2

Would you like me to walk through a high-level architecture diagram or dive deeper into the retry strategy for this system?

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