We need to build for scale: 500 million transactions.
💡 Model Answer
Handling 500 million daily transactions requires a layered architecture. Ingest data via a high‑throughput streaming platform like Kafka, partitioned by key to ensure order per entity. Stateless microservices consume the stream, perform validation, and write to a distributed transactional store (e.g., CockroachDB or a sharded MySQL cluster). For analytics, materialize aggregates in a columnar store (ClickHouse or BigQuery) using incremental ETL jobs. Use a CDN‑backed cache for read‑heavy endpoints. Autoscale compute resources based on Kafka lag and request latency. Implement back‑pressure by limiting producer rates and using consumer groups. Ensure idempotent writes to avoid duplicates. For fault tolerance, replicate Kafka brokers and database replicas. Monitoring with Prometheus, tracing with OpenTelemetry, and log aggregation with ELK stack provide observability. This design keeps per‑transaction latency low while scaling horizontally to meet the 500 million transaction target.
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