Today I wanted to set up my local environment with a load balancer for my tomcats. To get started, I took a look at the dockercloud-haproxy image and the provided example configuration for docker-compose:
version: '2' services: web: image: dockercloud/hello-world lb: image: dockercloud/haproxy links: - web volumes: - /var/run/docker.sock:/var/run/docker.sock ports: - 80:80
You can run this example right away and open http://localhost to see the
load balance hello-world page. To scale the web service, use
docker-compose scale web=2 and reload the page a couple of times. You
should see the hostname changing with each request, as haproxy is
using a roundrobin load-balancing strategy by default in this image.
To make this a bit more specific to my use case, I decided to replace the hello-world image with the official tomcat image. Because I like small images, I also decided to use tomcat:alpine.
version: '2' services: web: image: tomcat:alpine volumes: - ./index.jsp:/usr/local/tomcat/webapps/ROOT/index.jsp:ro lb: image: dockercloud/haproxy links: - web volumes: - /var/run/docker.sock:/var/run/docker.sock ports: - 80:80
I added a volume and a custom JSP to make this example a bit more my own:
<html> <body> This is host <strong><%= java.net.InetAddress.getLocalHost().getHostName() %></strong>. </body> </html>
If you run this example with
docker-compose up -d && docker-compose
scale web=2 you can again reload http://localhost a couple of times and
you should see a message like
This is host 76ce0ef9f152..
Congratulations, you now have (very simple) load balanced tomcat setup.
Next steps would be to replace the simple tomcat setup with a real cluster to get session replication.