Design market price change notifications

Q: Design market price change notifications

This question evaluates expertise in designing scalable, real-time data ingestion and fan-out notification systems, covering competencies such as stream processing, pub/sub architectures, subscription management, backpressure handling, rate limiting, deduplication, ordering, and reliable delivery.

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Question

Design a system that notifies users when market prices change for the symbols they track.

Context and requirements:

Users maintain a watchlist of symbols (e.g., AAPL, TSLA).
You have a 3rd-party market data vendor API that supports both REST polling and WebSocket streaming.
The system should push near-real-time price change notifications to users (e.g., via mobile push, email, or in-app/WebSocket).

Design goals to address:

Ingesting market price updates efficiently (prefer streaming when possible).
Fan-out: delivering updates to only the users watching the symbol.
Scalability (many users, many symbols), reliability, backpressure, and rate limits.
Handling subscription changes (users add/remove symbols) without excessive vendor calls.
Deduplication/ordering expectations and retry strategy.

Provide a high-level architecture, key components, data model, and the main flows for (a) ingesting price updates and (b) notifying users.

Design market price change notifications

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