Docker Swarm Load Balancer

One of the key feature from Docker Swarm Mode is load balancing.


Precondition, you're running this on a manager node (when running this on your machine, simple run docker swarm init)

Given this simple Dockerfile

FROM alpine:3.6

RUN echo "red" > /index.html

ENTRYPOINT busybox httpd -p 80 -h / && tail -f /dev/null

and build it as demo: docker build -t demo .

Now we create a new Docker overlay network named dev for this demonstration.

docker network create -d overlay --attachable dev

It's important to make it attachable, so we can demonstrate all features from docker swarm load balancing.

  • round robin load balancing
  • resolving names

We can start now our demo service

docker service create \
--network dev \
--name demo \

and change the "red" in index.html to "blue" by attaching the service.

docker exec -ti \
$(docker ps --filter "Name=demo" --format {{.Names}}) \
sed -i 's/red/blue/' /index.html

It's time to scale our demo service

docker service scale demo=2

When both services are running (docker service ls), we can start our container to testing the DNSRR feature.

docker run -ti --network dev alpine:3.6

We need to install curl

/ # apk --update add curl

And finaly we can request for http://demo

/ # curl http://demo
/ # curl http://demo
/ # curl http://demo
/ # curl http://demo

As you see, one time the swarm resolver requested our first demo service where we changed the index.html from "red" to "blue".
The second request resolves for the 2nd demo service which contains the original image we've build at the beginning with "red" in index.html.

It's such simple to scale (load balance) services with Docker Swarm Mode ;)