Docker Cookbook

10.3 ELB: Creating a Dynamic Load-Balancer with Confd
and Registrator
Problem
You want to build a dynamic load-balancer that gets dynamically reconfigured when
containers come and go.
Solution
The solution is based on 
registrator
(see 
Recipe 7.13
), which acts as a service-
discovery mechanism, and 
confd
, which gets information from the key-value store
used by 
registrator
and writes configuration files based on templates.
To illustrate this, you will build a simple one-node setup. A simple 
hostname
applica‐
tion will run in multiple containers. An Nginx load-balancer will front these contain‐
ers to distribute the load among the containers. These containers will get automati‐
cally registered in a Consul key-value store, thanks to 
registrator
. Then 
confd
will
pull information from Consul to write an Nginx configuration file. The load-balancer
(i.e., Nginx) will then get restarted using the new configuration. 
Figure 10-3
illus‐
trates this example.
308 | Chapter 10: Application Use Cases

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Figure 10-3. Dynamic load-balancing schematic
To get started, you will reproduce the steps explained in 
Recipe 7.13
. You will start a
Consul-based key-value store via a single container.
In production deployments, you will want to use a multinode
key-value store running separately from the nodes running your
application.
This is done easily with the Docker image 
progrium/consul
, like so:
$ docker run -d -p 8400:8400 -p 8500:8500 -p 8600:53/udp
-h cookbook progrium/consul -server
-bootstrap -ui-dir /ui
Then you will start the 
registrator
container and set the registry URI to 
consul://
192.168.33.10:8500/elb
. The IP address of your Docker host will be different.
$ docker run -d -v /var/run/docker.sock:/tmp/docker.sock
-h 192.168.33.10 gliderlabs/registrator
-ip 192.168.33.10 consul://192.168.33.10:8500/elb
Next you will start your toy application. You can use your own and pull 
runseb/host
name
, which is a simple application that returns the container ID. Start two of them at
first:
$ docker run -d -p 5001:5000 runseb/hostname
$ docker run -d -p 5002:5000 runseb/hostname