Overlay Networks are created using the Docker CLI, similar to creating a bridge network for connecting between hosts. When creating the network, a driver type of overlay is used. When new services are deployed via Swarm Mode, they can utilise this network allowing containers to communicate.

Task

To create the Overlay Network, use the CLI and define the driver. Networks can only be created via a Swarm Manager node. The network name would be app1-network.

docker network create -d overlay app1-network

All the networks can be viewed using:

docker network ls

Note: It's expected for the network not to appear on the worker nodes. The managers node handles network creation and services being deployed.

docker network ls

Once the network has been created, services can be deployed and able to communicate with other containers on the network.

Task

The following will deploy a Redis service using the network. The name of the service will be redis that can be used for discovery via DNS.

docker service create --name redis --network app1-network redis:alpine

The next step will deploy a web app on a different node that will interact with Redis over the network.

With the overlay network and Redis deployed, it's now possible to deploy a Web App to use Redis to persist data. The application is configured to look up Redis via DNS. The app is configured to listen on port 3000, but the service will be exposed to the public on port 80.

Task

Create the new service will the command below:

docker service create \ --network app1-network -p 80:3000 \ --replicas 1 --name app1-web \ katacoda/redis-node-docker-example

With a two-node deployment, each container will be deployed onto different hosts.

docker ps

They'll use the overlay network and DNS discovery to communicate.

Test

Sending a HTTP request will persist the IP of the client in Redis.

curl host01

As the service has been configured and deployed using Swarm Mode, it will take advantage of the load balancing discussing in our scenario Load Balance and Service Discover in Swarm Mode

curl host01