Service Discovery & Load Balancing

What is Service Discovery?

Service discovery is the process where a service automatically detects the location (IP/port) of another service. In Docker Compose, this is done using DNS — services can use container names as hostnames.

Docker DNS-Based Service Discovery

Docker Compose creates a default network and assigns DNS names to containers. You can call one service from another like this:

fetch('http://auth-service:4000/login')

The DNS name auth-service resolves to the container’s internal IP address.

Load Balancing with Nginx

If you run multiple instances of a service, use Nginx to balance the load between them. Example:

http {
  upstream user_service {
    server user-service-1:5000;
    server user-service-2:5000;
  }

  server {
    listen 80;
    location / {
      proxy_pass http://user_service;
    }
  }
}

This setup distributes incoming requests evenly between two user-service containers.

Running Multiple Service Instances

In Docker Compose, scale services like this:

docker-compose up --scale user-service=3

Then configure Nginx or another reverse proxy to distribute requests to all instances.

Docker's networking and DNS make service discovery seamless. Combined with a load balancer like Nginx, you can distribute traffic across multiple replicas to improve reliability and performance.