The load balancer has now been configured to listen and proxy traffic to containers, based on the metadata of the container. This metadata is defined as labels.

Define HTTP Server

The container below uses the katacoda/docker-http-server image which returns the hostname of the container processing the request. The container will have two labels, one assigns it to the machine-echo backend and the other assigns the domain name. By defining the rule traefik.frontend.rule we can assign both public and internal domains the container should process.

machine:
  image: katacoda/docker-http-server
  labels:
    - "traefik.backend=machine-echo"
    - "traefik.frontend.rule=Host:machine-echo.example.com"

Labels can be used on containers to override default behaviour:

traefik.backend=foo assign the container to foo backend

traefik.port=80 register this port. Useful when the container exposes multiples ports

traefik.protocol=https override the default http protocol

traefik.weight=10 assign this weight to the container

traefik.frontend.rule=Host:test.traefik.io override the default frontend rule (Default: Host:{containerName}.{domain})

traefik.docker.network Set the Docker network to use for connections to this container

More details can be found at https://docs.traefik.io/toml/#docker-backend

In the next step, we'll create additional servers and send traffic to verify they're working as expected.

Like previously, Traefik can proxy traffic for multiple HTTP servers, each responding to the either the same or different domains. If two containers are assigned the same domain then Traefik will automatically load balance traffic between them.

Define Second HTTP Server

Create a second container which will respond to requests for echo-echo.example.com.

echo:
  image: katacoda/docker-http-server:v2
  labels:
    - "traefik.backend=echo"
    - "traefik.frontend.rule=Host:echo-echo.example.com"

In the next step we'll launch the containers and start sending traffic to verify the configuration.

With the docker-compose.yml file created it's possible start to the load balancer and the two HTTP servers with a single command.

Start Containers

Run Docker Compose to start the environment.

docker-compose up -d

Sending Traffic

The load balancing is based on the host header sent along with HTTP requests. By setting the host using cURL, it's possible to verify different configurations are responding as expected without having to configure DNS servers.

Issuing the two requests, you'll see that Traefik will proxy the request to the correct container.

curl -H Host:machine-echo.example.com http://host01

curl -H Host:echo-echo.example.com http://host01

In the next step, we'll scale our services to see the load balancing benefits.

As Traefik is listening to Docker Events, when new containers are started it can automatically update its configuration. This allows you to take advantage of a dynamic infrastructure which self-learns and heals based on what's running.

Scale

Docker Compose can manage scaling the number of containers running on a host. Run the command docker-compose scale machine=2 to launch two instances of the container.

Each request should be processed by a different container.

curl -H Host:machine-echo.example.com http://host01

The dashboard has two responsibilities. The first is to highlight which routes exist and which containers the traffic is being proxied too. The second is to show the health status of the cluster and if errors are being returned. If so, which container is causing the problem. In future scenarios, you'll learn how Traefik can use this health check to remove bad containers from the load balancer automatically.

View the dashboard on port 8080 via the URL https://[[HOST_SUBDOMAIN]]-8080-[[KATACODA_HOST]].environments.katacoda.com