Load-balance the service

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To load-balance your application, you need to deploy a load-balancing service. This service distributes incoming requests to all of the available containers in the application.

In this example, you need a load balancer that will forward incoming requests to both container #1 (web-1) and container #2 (web-2). For this tutorial, you’ll use Docker Cloud’s HAProxy image to load balance, but you could also use other custom load balancers.

You can configure and run the haproxy load balancer service from the command line using a command like the example below. (If you are using the Go quickstart, edit the link-service value before running the command.)

$ docker-cloud service run \
-p 80:80/tcp \
--role global \
--autorestart ALWAYS \
--link-service web:web \
--name lb \

-p 80:80/tcp publishes port 80 of the container, and maps it to port 80 of the node in which it will be deployed.

–role global grants API access to this service. You can use this to query the Docker Cloud API from within the service.

–autorestart ALWAYS tells Docker Cloud to always restart the containers if they stop.

–link-service web:web links your load balancer service haproxy with the web service, and names the link web. (Learn more about Service Linking here.)

–name lb names the service lb (short for load balancer).

dockercloud/haproxy specifies the public image that we’re using to make this service.

Run the service ps command to check if your service is already running.

$ docker-cloud service ps
NAME                 UUID      STATUS     IMAGE                                          DEPLOYED
web                  68a6fb2c  ▶ Running  my-username/quickstart-python:latest           2 hours ago
lb                   e81f3815  ▶ Running  dockercloud/haproxy:latest                     11 minutes ago

Now let’s check the container for this service. Run docker-cloud container ps.

$ docker-cloud container ps
NAME                   UUID      STATUS     IMAGE                                          RUN COMMAND          EXIT CODE  DEPLOYED        PORTS
web-1                  6c89f20e  ▶ Running  my-username/quickstart-python:latest           python app.py                   2 hours ago     web-1.my-username.cont.dockerapp.io:49162->80/tcp
web-2                  ab045c42  ▶ Running  my-username/quickstart-python:latest           python app.py                   33 minutes ago  web-2.my-username.cont.dockerapp.io:49156->80/tcp
lb-1                   9793e58b  ▶ Running  dockercloud/haproxy:latest                           /run.sh                   14 minutes ago  443/tcp, lb-1.my-username.cont.dockerapp.io:80->80/tcp

You should notice an URL endpoint in the PORT column for haproxy-1. In the example above this is lb-1.my-username.cont.dockerapp.io:80. Open the lb-1 URL in your browser or curl from the CLI.

If you refresh or run curl multiple times you should see requests distributed between the two containers of in the web service. You can see which container responds to your request in the Hostname section of the response.

$ curl lb-1.$DOCKER_ID_USER.cont.dockerapp.io
Hello Friendly Users!</br>Hostname: web-1</br>Counter: Redis Cache not found, counter disabled.%
$ curl lb-1.$DOCKER_ID_USER.cont.dockerapp.io
Hello Friendly Users!</br>Hostname: web-2</br>Counter: Redis Cache not found, counter disabled.%

You can learn more about dockercloud/haproxy, our free open source HAProxy image here.

Next: Provision a data backend for your service

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