Designing a fault-tolerant order processing system (payment gateway, logistics). Migration should be gradual without stopping the service. SCALE - Total users: 8 million - Daily active users (DAU): 3 million - Monthly active users (MAU): 12 million - Peak load: 25k reads / 5k writes per second - New data volume: 10 TB per year - Peak parallelism: 120k simultaneous requests - Annual user growth: 30% - Target p99 latency: <150 ms for reads, <400 ms for writes - Target availability: 99.95%. Your task is to develop an architectural solution that...

To design a fault-tolerant order processing system with specified parameters and migration requirements without service downtime, the following architectural solution can be proposed:

1. Microservices Architecture

   • Divide the system into separate services: order reception, payment processing, logistics, notifications.
   • Each service is responsible for its part, which simplifies scaling and updating.

2. Database and Data Storage

   • Use distributed databases supporting horizontal scaling (e.g., CockroachDB, Cassandra, or NewSQL solutions).
   • For orders and payments — ACID-compliant database supporting transactions.
   • For logistics — possibly NoSQL for flexibility.

3. Load Handling

   • Load balancing with API Gateway and load balancers (e.g., Nginx, Envoy).
   • Caching hot data (Redis, Memcached) to reduce latency.

4. Fault Tolerance and Scalability

   • Data replication and automatic failover to backup nodes.
   • Use message queues (Kafka, RabbitMQ) for asynchronous processing and peak load management.
   • Horizontal scaling of services and databases.

5. Migration Without Service Downtime

   • Use blue-green deployment or canary releases for gradual updates.
   • Stage data migration supporting two schema versions (backward compatible).
   • Use feature flags to enable new features.

6. Monitoring and Logging

   • Implement monitoring system (Prometheus, Grafana).
   • Centralized logging (ELK stack).

7. Performance and Latency

   • Optimize database queries and indexes.
   • Use CDN and edge services to reduce latency.

8. Security

   • Encrypt data at rest and in transit.
   • User authentication and authorization.

This solution will ensure scalability, high availability (99.95%), low latency, and smooth migration without downtime.