Reading System Overview Descriptions

ShopCore Platform — System Overview

ShopCore is a three-tier e-commerce platform. The presentation tier consists of a React single-page application (SPA) served via a CDN, and a separate React Native mobile app. Both clients communicate exclusively with the application tier through a single entry point: an API Gateway that handles authentication, rate limiting, and request routing.

The application tier is decomposed into five microservices: Product Catalogue, Cart, Order, Payment, and Notification. Each service owns its data and exposes a REST or gRPC API. Inter-service communication uses an event bus (Kafka) for asynchronous workflows and direct gRPC calls for synchronous queries.

The data tier holds each service's dedicated datastore: PostgreSQL for transactional data (Orders, Cart), MongoDB for the Product Catalogue (flexible schema for varied product attributes), and Redis for session state and rate-limit counters.

The application tier is decomposed into five microservices: Product Catalogue, Cart, Order, Payment, and Notification. Each service owns its data and exposes a REST or gRPC API. Inter-service communication uses an event bus (Kafka) for asynchronous workflows and direct gRPC calls for synchronous queries.

The data tier holds each service's dedicated datastore: PostgreSQL for transactional data (Orders, Cart), MongoDB for the Product Catalogue (flexible schema for varied product attributes), and Redis for session state and rate-limit counters.

Request flow: placing an order

1. The React SPA sends a POST /orders request to the API Gateway with a JWT in the Authorization header.
2. The API Gateway validates the JWT (auth check), applies rate limiting, and forwards the request to the Order Service.
3. The Order Service validates the cart contents by calling the Cart Service via gRPC (synchronous).
4. The Order Service calls the Payment Service via gRPC to authorise the charge (synchronous).
5. On success, the Order Service publishes an order.created event to Kafka.
6. The Notification Service consumes the order.created event and sends a confirmation email (asynchronous).
7. The Order Service returns HTTP 201 to the API Gateway, which forwards it to the client.

Service boundaries and data ownership

Each microservice in ShopCore owns its data exclusively. No service reads directly from another service's database. Instead, services expose APIs or publish events to share information.

For example, when the Order Service needs to display an order history page including product names and images, it does NOT query the Product Catalogue database directly. Instead, it stores a snapshot of the relevant product data (name, image URL, price at time of order) within its own Order database at the time the order is placed. This is called data denormalisation.

Architecture trade-offs

The microservices architecture introduces operational complexity that a monolithic application would not have: service discovery, distributed tracing, multiple deployment pipelines, and eventual consistency between services. The team chose this approach because ShopCore is maintained by five independent engineering teams — one per service — and the ability to deploy services independently (without coordinating releases) outweighs the operational overhead.

A monolithic architecture would be simpler to operate and debug but would require all teams to coordinate every release, creating a bottleneck as the organisation scales.