API Gateway and Service Discovery: Seamless Microservices Integration
Published: 2024-02-07 08:25:49

In the digital era, Application Programming Interfaces (APIs) have become the cornerstone of communication between different systems and services.

With the proliferation of microservices architecture, the importance of API gateways and service discovery is increasingly prominent. The API gateway serves as the entry of the microservices architecture, responsible for handling all external requests and coordinating communication between internal services; whereas service discovery ensures the system can dynamically find and connect to the correct service instances.

Together, they build an efficient, flexible, and scalable distributed system. This article delves into the connections between API gateways and service discovery, and their crucial roles in modern distributed systems.

## API Gateways and Service Discovery

As the entry of the [microservices](https://api7.ai/blog/what-are-microservices) architecture, the API gateway handles all external requests and facilitates communication with internal services. Its main functionalities include:

- **Request routing and forwarding**: Routing requests to the corresponding services based on request characteristics.
- **Access control and security authentication**: Ensuring only authorized users or systems can access the API.
- **Load balancing and caching**: Optimizing service performance by evenly distributing requests to various service instances and providing caching to reduce requests to backend services.
- **Logging and analysis**: Collecting and analyzing API usage data to support business decision-making.

[Service discovery](https://api7.ai/blog/service-discovery) is a mechanism used to automatically find available service instances in a distributed system. When the number of services is large and dynamically changing, service discovery becomes crucial. The principles of service registration and discovery are as follows:

- **Service registration**: When a service starts, it registers its information (such as IP address, port number, etc.) with the service registry.
- **Service discovery**: When other services need to call this service, they query the service registry to find available service instances and select one for communication.

### Connections between API Gateways and Service Discovery

The API gateway serves as the "gatekeeper" handling all external requests and intelligently routing them to the correct service instances. API gateways and service discovery are interdependent in [microservices](https://api7.ai/blog/what-are-microservices) architecture, providing stability and scalability.

Before introducing service discovery, the API gateway relies on manual configuration to specify the upstream service addresses. Once service instance addresses change, such as due to upgrades, emergency switches to backup instances, or scaling out more service instances due to increased traffic, administrators must manually update the API gateway configuration in a timely manner. This approach is not only inefficient but also difficult to ensure accuracy and timeliness.

Through the service discovery mechanism, the API gateway can dynamically connect to the service registry in real time, automatically obtaining the latest instance addresses of all upstream services, and ensuring requests are accurately routed to new services.

At the same time, the service discovery mechanism can promptly notify the API gateway of fault conditions, effectively avoiding routing requests to unavailable services. By configuring the service registry's address and service names, the API gateway has made significant progress in reusing configurations across environments, reducing configuration workload, and improving configuration reuse and accuracy.

![Service Discovery](https://static.apiseven.com/uploads/2024/02/06/J8fcy2Wd_customer-service-faqs-illustration.jpg)

## Practical Application of API Gateway Integration with Kubernetes Service Discovery

Suppose we have a microservices architecture system based on Kubernetes, deploying multiple services such as user services, order services, payment services, etc. Each service has multiple replicas (Pods), and the IP addresses and port numbers of these replicas change dynamically as containers are created and destroyed.

In this scenario, we want to use the API gateway to handle external requests and intelligently route them to the correct Kubernetes services. At the same time, we want to leverage Kubernetes' service discovery mechanism to dynamically discover and manage service instances.

**Step 1: Service Deployment and Registration**

In Kubernetes, we use Deployments or StatefulSets to deploy services and create corresponding Services as service abstractions. Kubernetes Services automatically provides load balancing for the backend Pods and maps service names to stable cluster internal IP addresses and port numbers.

When Pods are created or destroyed, Kubernetes' Service Controller automatically updates the Endpoint objects of services, which contain the IP addresses and port numbers of all related Pods. These Endpoint objects are stored in Kubernetes' API server for querying by other components.

**Step 2: API Gateway Deployment and Configuration**

Taking [API7 Enterprise](https://api7.ai/enterprise) as an example, we connect to the address of the Kubernetes service registry in the gateway group. When we publish a service to the gateway group, instead of manually entering the instance address upstream, we select this Kubernetes registry and specify the namespace and corresponding Service within it. API7 Enterprise automatically retrieves the Endpoint information of the corresponding service from Kubernetes' API server as the upstream address of the service.

**Step 3: Request Handling**

When the API gateway receives an external request, it determines which service to route the request to based on the configured routing rules. Then, it queries Kubernetes' API server to obtain the latest upstream information. The API gateway selects an instance from this information (based on different load-balancing strategies) and forwards the request to that instance. Since Endpoint information is dynamically updated, the API gateway can automatically handle the creation, destruction, and migration of Pods, ensuring requests are always routed to available upstreams.

**Step 4: Service Discovery and Updates**

In Kubernetes, service discovery is automatically performed. When the status of Pods changes (such as creation, destruction, or migration), Kubernetes' Service Controller automatically updates the corresponding service's Endpoint objects.

The API gateway regularly retrieves the latest Endpoint information from Kubernetes' API server and updates its upstream based on this information. This enables the API gateway to instantly perceive changes in service status and automatically adjust routing rules to ensure requests are correctly routed to available service instances.

## Conclusion

Through the integration of API gateway and Kubernetes service discovery, our system can achieve dynamic service registration and discovery, intelligent routing, and load balancing. This approach simplifies system configuration and management, enhances system reliability and scalability, and fully utilizes the powerful features provided by Kubernetes, making the deployment and operation of microservices architecture simpler and more efficient.