The API Gateway Service is a key component in the Event-Sourcing + CQRS example application provided in the cer/event-sourcing-examples repository on GitHub. This project demonstrates how to build a microservices architecture driven by Event Sourcing, CQRS (Command Query Responsibility Segregation), and powered by Spring Boot and Docker.

 What Is This Example Application?

This repository provides a complete example application illustrating how modern distributed systems can be built using microservices, event-driven architecture, event sourcing, and CQRS. The application models a money transfer system (similar to a banking domain) where users can create customers, create accounts, and transfer money.

Multiple independent services make up the application:

• Customers Service — for creating and managing customers

• Accounts Service — for account creation and management

• Transactions Service — to manage money transfers

• View Services — read-optimized services for views of customers/accounts

• …and finally the API Gateway Service, which acts as the façade in front of all other services.

What Is the API Gateway Service?

The API Gateway Service is a Spring Boot-based application whose primary role is to act as a single entry point for clients (like front-end applications or API consumers) to interact with the entire microservices ecosystem.

In microservices architecture, the API Gateway:

✅ Provides a unified REST endpoint
✅ Routes requests to the appropriate downstream services
✅ Helps abstract service boundaries from clients
✅ Simplifies client logic by providing a consolidated API layer

In this example, the gateway acts as a Facade — encapsulating how internal services are composed and accessed.

 Why Use an API Gateway Here?

Modern microservices introduce complexity:

• Many independent services

• Each service can have its own API endpoints

• Services might evolve independently

An API Gateway centralizes:

✔ Routing
✔ Load balancing
✔ Potential cross-cutting concerns like authentication, logging, metrics (in more advanced setups)

It ensures clients don’t have to call individual microservices directly — they only talk to one gateway layer.

⚙️ How the API Gateway Fits With Event Sourcing & CQRS

To understand the gateway’s purpose in context:

🧩 Event Sourcing

Instead of persisting current state in a database, systems store every business change as events — immutable facts about what happened. This enables powerful capabilities like event replay, audit trails, and temporal queries.

🧩 CQRS

Read and write operations are separated:

• Writes (Commands) affect state and generate events

• Reads (Queries) are served from optimized “view” databases or projections

The API Gateway directs clients to appropriate endpoints based on the type of request — whether it’s for write operations or retrieving read models.

Typical Responsibilities of the API Gateway in This Project

While the specific source files in the api-gateway-service folder weren’t fully visible from the link, an API Gateway in such a project commonly includes:

🔹 Routing Configuration

A set of routes that map external API endpoints to internal services (e.g., customer service, account service, transaction service).

🔹 HTTP Controller Endpoints

Controllers that expose REST APIs such as:

Clients invoke these gateway APIs instead of calling each service directly. The gateway then proxies the requests internally.

🔹 Fallbacks and Resiliency

Gateways often integrate with patterns like:

• Circuit Breakers

• Timeouts

• Fallback responses

Though not always in simple examples, these improve fault tolerance in distributed systems.

🔹 Response Aggregation

In some setups, the gateway may combine responses from multiple microservices into one response for the client. This simplifies client logic.

 Running the Entire Example

To run the entire application:

1. Build the project using Gradle.

2. Launch all services together using Docker Compose.

3. The API Gateway starts and provides access to the whole system. Usually this is on port 8080 (configurable).

4. Open a browser or use a REST client to send requests to the gateway — it will route them internally.

 In Summary

What the API Gateway Service Does

✔ Acts as the front door to the entire application
✔ Routes client requests to the correct microservices
✔ Hides internal service complexity
✔ Makes interaction with the event-sourced system simple
✔ Helps support a scalable microservices architecture powered by Spring Boot

In essence, in this event-sourcing + CQRS microservices application, the API Gateway service improves modularity, reduces coupling for clients, and supports clean architectural separation