Cloud Computing Basics 7

There's been so much talk about Consul-template and I've never shown you any examples. Consul-template is what gives Docker container the knowledge when something is changed in Consul. It does it by having a template for configuration file filled with Consul data and command to run when template is generated. Let's take a look at my Dockerfile for NginX:

FROM nginx:latest
MAINTAINER Goran Mekić <meka@lugons.org>

ENV DEBIAN_FRONTEND noninteractive
ADD consul /app/consul
ADD consul-template /usr/bin/consul-template
ADD run.sh /run.sh

CMD /run.sh

It's a bit dirty as I get consul-template out of the sky, but kids, don't be like me. :o) Yeah, always works!

I have my run.sh:

1
2
3
4
5
6
7
8

#!/bin/bash

set -e

rm /etc/nginx/conf.d/*
consul-template -config /app/consul/nginx.conf
sleep 1
tail -f /var/log/nginx.log

Great. So Consul-template does all the work. Let's see the config.

consul = "172.17.42.1:8500"

template {
    source = "/app/consul/nginx.tmpl"
    destination = "/etc/nginx/conf.d/onelove.conf"
    command = "service nginx restart"
}

Remember I've told you that Consul is available on 172.17.42.1 on all hosts? This is where it comes handy. It is achieved through docker0 interface. Ensure you have the same IP on all hosts and that Consul is bind to it, and that's it.

Second part is the template. What, where and how are the questions it answers. What source template I should use? Where to put the output? How to notify the service. Yeah, I'm aggressive, I restart instead of reload because of simplicity. The problem I had was with the run.sh. If you look closely, only thing I really call is consul-template. I never start nginx by hand or automatically inside the container, so if I have used reload, there would be nothing to reload initially (read: container doesn't boot).

There are two thing you can do to ensure that your load balancers don't reboot all at the same time. First one is to explore (wait parameter)[https://github.com/hashicorp/consul-template#usage] and restart with random delay. Second one is to initialize the config, start services, and watch for changes. When there is a change, reinitialize config and reload service. Example of it is One Love API.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20

#!/bin/bash

set -e

export COMMAND="consul-template -config /app/consul/api.conf"

echo -n "Waiting for initial config "
until $COMMAND -once; do
    echo -n "."
    sleep 3
done
echo " done"

uwsgi --ini /app/uwsgi.ini
python /app/manage.py migrate --noinput
python /app/manage.py collectstatic --noinput

$COMMAND &
sleep 1
tail -f /var/log/uwsgi.log

Now you know everything I know about the cloud computing. This is my way of doing things, and my view. You don't have to agree on everything. I'm not going to agree with these posts in a year! What I didn't describe is Docker HUB, but go to site, register, and add your repository. Play with it, it's dead simple. I gave my best to give you posts that have no bull shit talk, because I was mad on todays media representing the cloud. I hope I did it. Happy dockering!

previous