E-Commerce Order Platform

Topology Notes

• ·The checkout write path is the most operationally critical path in the system. It touches PostgreSQL (inventory reservation + order creation + outbox INSERT), issues an external payment API call, and publishes a Kafka event: all within a single user-visible latency budget. Every added step in this path is a latency and failure surface. The saga orchestrator must have a timeout for each step that is shorter than the user-facing checkout timeout.
• ·PostgreSQL owns the authoritative inventory counts and order records. Inventory reservations use SELECT FOR UPDATE with NOWAIT on the inventory row: a checkout that cannot acquire the lock immediately returns a 409 (conflict) rather than waiting and holding a connection open. This converts lock contention from a latency problem into a retry problem, which is more controllable.
• ·Redis stores cart state keyed by session_id with a TTL of 7 days. Cart contents are serialized as a single JSON blob to minimize Redis round-trips. Cart reads are always cache-hits for active sessions; the cart service falls back to PostgreSQL only on cache miss (session resumed after TTL expiry or Redis failure). Redis must be configured with AOF persistence to survive node restarts.
• ·Elasticsearch serves product search and order history queries. Product index is populated via Kafka CDC from the PostgreSQL catalog. Order history index is populated from order_created and order_updated Kafka events. Both indexes are eventually consistent: the checkout service must never trust Elasticsearch prices or inventory availability; it must always read from PostgreSQL before finalizing a reservation.
• ·RabbitMQ carries fulfillment notifications (order_confirmed → warehouse), shipping updates (shipment_created → customer), and return processing (return_requested → warehouse, return_completed → payment). Each notification type has its own exchange and queue. Queue depth and consumer lag are primary operational metrics. Dead-letter queues capture exhausted-retry messages for manual review.