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Multi-channel Support: The system must support sending notifications through various channels including email, SMS, push notifications, and in-app messages.
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Multiple Notification Types: Support transactional (e.g., order confirmation), promotional (e.g., discount offers), and system-generated alerts (e.g., password reset).
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Scheduled Delivery: Support scheduling of notifications for future delivery.
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Rate Limiting: Ensure that users receive only a limited number of promotional messages in a given day to prevent spam.
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Retry Mechanism: Handle notification delivery failures, retrying when necessary (e.g., failed SMS or email).
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Scalability: The system should handle millions of notifications per minute, supporting millions of concurrent users.
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High availability: Ensure minimal downtime so that notifications are delivered even in case of failures.
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Reliability: Guarantee at-least-once delivery of notifications, with the possibility of exactly-once semantics for certain use cases.
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Low latency: Notifications should be sent as quickly as possible to ensure timely delivery.