Design an automated email marketing system

Guidance

Design an automated email marketing system

to send emails to a target audience of 1 million people without slowing down user login processes.

System Design

The Asynchronous Communication Playbook

Designing asynchronous systems is about acknowledging that not everything needs to happen right now. It is the strategic decoupling of services using queues and background jobs to protect system availability, trading absolute real-time consistency for massive scalability.

Step 1

Identify Synchronous Bottlenecks

Start by separating the critical path from background tasks. If an action doesn't require an immediate response to the user, move it off the main thread. For example, when a user registers, creating their DB record is synchronous (needs immediate confirmation), but sending the welcome email, generating recommendations, or syncing data to a data warehouse should be handled asynchronously.

Step 2

Choose the Right Messaging Pattern (Queue vs. Pub/Sub)

Step 3

Define Delivery Guarantees

Step 4

Handle Failures (Retries & Dead Letter Queues)

Step 5

Bridge Eventual Consistency with UX

Additional Tips

Idempotency is the Anchor: Because asynchronous systems use "at-least-once" delivery, network partitions will cause messages to be re-delivered. You must state that all async workers and API endpoints are designed to be Idempotent (processing the same event twice will not change the final state).
The Message Ordering Trap: Do not assume queues maintain strict global order. In Kafka, ordering is only guaranteed within a specific partition, not globally. If event order matters (e.g., "Account Created" must precede "Profile Updated"), explain how you would partition the data (e.g., by User_ID).
The Observability Tax: Acknowledge the operational complexity. Asynchronous architectures are notoriously difficult to debug because a single user request cascades across multiple decoupled systems. Highlight the need for Distributed Tracing (e.g., Jaeger, OpenTelemetry) by passing a unique trace_id through the entire message payload.

Design an automated email marketing system

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