Build a multi-tenant Email Pipeline

Question

Design a multi-tenant email system that handles millions of requests. Discuss trade-offs in consistency, availability, and performance.

Codemia · Accepted Answer

- **Functional Requirements:**
  - Handle sending and receiving emails for multiple tenants (hosts and guests) on Airbnb.
  - Support for transactional emails (booking confirmations, reminders) and marketing emails.
  - Provide a dashboard for tenants to track email metrics (open rates, click rates).
  - Enable email template management and A/B testing features.
  - Multi-language support for emails.

- **Non-Functional Requirements:**
  - High availability: 99.9% uptime to ensure critical emails are delivered.
  - Scalability: Support millions of emails per day as Airbnb grows.
  - Low latency: Email delivery should happen within seconds.
  - Security: Ensure data encryption in transit and at rest, comply with GDPR requirements.
  - Monitoring and alerting for delivery failures, performance bottlenecks, and email queue lengths.

- **Estimating QPS (Queries Per Second):**
  - Assuming Airbnb sends about 500 million emails per month, this averages to approximately 16.67 million emails per day.
  - With a 24-hour operational window, that results in around 192,000 emails per second (QPS).

- **Storage Requirements:**
  - Each email might average around 5 KB (including metadata).
  - Therefore, for 500 million emails, the total storage required would be approximately 2.5 TB (500 million * 5 KB).
  - Additional storage for logs and metrics may double this requirement, estimating around 5 TB total.

- **Bandwidth Considerations:**
  - At 16.67 million emails per day, assuming an average size of 5 KB, we'd need around 80 GB of bandwidth per day. With peaks, we can estimate up to 1-2 TB of bandwidth monthly.

![Component Diagram](https://example.com/component_diagram.png)

- **Components:**
  - **API Gateway:** Handles incoming requests, authenticates users, and routes requests to appropriate services.
  - **Email Service:** Core service responsible for composing and sending emails.
  - **Template Engine:** Manages email templates and supports A/B testing.
  - **Queue System (e.g., RabbitMQ):** Buffers email requests to ensure smooth processing under load.
  - **Database (e.g., PostgreSQL):** Stores email logs, templates, and user preferences.
  - **Analytics Service:** Collects and analyzes metrics on email performance.
  - **Monitoring (e.g., Prometheus, Grafana):** Monitors system health and performance metrics.

- **Email Log Table:**
  - `email_id` (UUID, Primary Key)
  - `tenant_id` (UUID, Foreign Key)
  - `recipient` (VARCHAR)
  - `subject` (VARCHAR)
  - `body` (TEXT)
  - `status` (ENUM: 'SENT', 'FAILED', 'PENDING')
  - `created_at` (TIMESTAMP)
  - `updated_at` (TIMESTAMP)

- **Template Table:**
  - `template_id` (UUID, Primary Key)
  - `tenant_id` (UUID, Foreign Key)
  - `template_name` (VARCHAR)
  - `template_body` (TEXT)
  - `created_at` (TIMESTAMP)

- **Access Patterns:**
  - Frequent inserts for sending emails, and reads for fetching email logs and templates. Strong indexing on `tenant_id` and `created_at` to optimize query performance.

- **Consistency vs Availability:**
  - Opt for eventual consistency in email delivery for better availability. Use a queue system to ensure emails are processed even during high load, accepting that some emails may be delayed.

- **Database Choices:**
  - Choosing PostgreSQL provides strong consistency and ACID properties but may require partitioning strategies to handle scaling. Consider sharding by tenant_id to distribute load.

- **Caching Strategy:**
  - Implement Redis for caching frequently accessed data (e.g., templates). This improves read performance but introduces complexity in cache invalidation.

- **Monitoring and Logging:**
  - Set up comprehensive logging to track email delivery statuses and performance. Utilize alerts for failure rates exceeding a threshold to quickly address issues.

Build a multi-tenant Email Pipeline

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