Communication Between Multiple Docker-Compose Projects

As your microservices architecture grows, you might split it into multiple independent Docker Compose projects—each responsible for a different part of the application. But what happens when services from Project A need to talk to services in Project B?

In this blog post, you’ll learn:

  • How Docker Compose handles networking
  • Ways to enable communication between separate Compose projects
  • Best practices for cross-project service interaction

🧠 Understanding Docker Compose Networks

Each docker-compose project creates its own default bridge network. For example, starting a docker-compose project in directory app1/ might create a network like:

app1_default

Services within the same network can resolve each other by container name. But services from different projects cannot communicate by default unless explicitly configured to share a network.


✅ Option 1: Use a Shared External Network

The cleanest way to connect multiple projects is by defining a common external Docker network that all projects attach to.

🔧 Step-by-Step

  1. Create a shared network manually:
docker network create shared-network
  1. In each docker-compose.yml, declare the external network:
networks:
  shared:
    external: true
    name: shared-network
  1. Attach services to that network:
services:
  service-a:
    image: my-image
    networks:
      - shared

networks:
  shared:
    external: true
    name: shared-network

Now, any container on shared-network can communicate using service names.


✅ Option 2: Manually Connect Containers to Another Project’s Network

If both projects are already running, you can attach containers to another network using the docker network connect command:

docker network connect app2_default container_name

This is useful for ad-hoc connections but not ideal for permanent setups.


🧪 Example: Service A from Project 1 Communicates with Service B in Project 2

Suppose:

  • Project 1 has a service frontend
  • Project 2 has a service api

Both are connected to the external shared-network. From frontend, you can now do:

curl http://api:5000/endpoint

Assuming api is listening on port 5000, this request will work!


⚠️ Common Pitfalls

ProblemCauseSolution
Service not reachable by nameDifferent networksUse shared external network
Connection refused errorWrong port or host bindingCheck exposed/internal port and hostname
No route to hostHost network issues or firewallEnsure both containers are on same network

💡 Best Practices

  • Use meaningful network names when defining shared networks.
  • Avoid relying on localhost inside containers—use container names.
  • Keep networks loosely coupled to preserve project independence.
  • Define networks outside of compose files to manage lifecycle clearly.

📝 Conclusion

Connecting multiple Docker Compose projects is easy once you understand how Docker networks work. By using shared external networks, you can build modular, scalable, and independently managed microservices that talk to each other securely.


🔑 Recap

GoalMethod
Cross-project communicationUse shared external Docker network
Temporary container linkingdocker network connect
Container-to-container accessUse service names, not localhost
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