rack #8

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Rack provides a minimal, modular and adaptable interface
for developing web applications in Ruby.
By wrapping HTTP requests and responses in the simplest way possible,
it unifies and distills the API for web servers, web frameworks, and
software in between (the so-called middleware) into a single method call.


Rack provides a minimal, modular and adaptable interface for developing web applications in Ruby. This Charm will deploy Ruby on Rails, Sinatra or any other Rack application and connect it to supported services.


To deploy this charm you will need at a minimum: a cloud environment, working Juju installation and a successful bootstrap. Once bootstrapped, deploy Rack charm and all required services.

Ruby on Rails example

Configure your application, for example:


  repo: https://github.com/pavelpachkovskij/sample-rails

Deploy Rack application:

juju deploy rack sample-rails --config sample-rails.yml

Deploy and relate database

juju deploy postgresql
juju add-relation postgresql:db sample-rails

Now you can run migrations:

juju ssh sample-rails/0 run rake db:migrate

Seed database

juju ssh sample-rails/0 run rake db:seed

And finally expose the Rack service:

juju expose rack

Find the Rack instance's public URL from

juju status

MySQL setup

juju deploy mysql
juju add-relation mysql rack

Sinatra example

Configure your application, for example html2haml


  repo: https://github.com/twilson63/html2haml.git

Deploy your Rack service

juju deploy rack html2haml --config html2haml.yml

Expose Rack service:

juju expose html2haml

Source code updates

juju set <service_name> revision=<revision>

Executing commands

juju ssh <unit_name> run <command>

Restart application

juju ssh <unit_name> sudo restart rack

Foreman integration

You can add Procfile to your application and Rack to start additional processes or replace default application server:

Example Procfile:

web: bundle exec unicorn -p $PORT
watcher: bundle exec rake watch

Specifying a Ruby Version

You can use the ruby keyword of your app's Gemfile to specify a particular version of Ruby.

source "https://rubygems.org"
ruby "1.9.3"

Horizontal scaling

Juju makes it easy to scale your Rack application. You can simply deploy any supported load balancer, add relation and launch any number of application instances.


juju deploy rack rack --config rack.yml
juju deploy haproxy
juju add-relation haproxy rack
juju expose haproxy
juju add-unit rack -n 2


Apache2 is harder to start with, but it provides more flexibility with configuration options.
Here is a quick example of using Apache2 as a load balancer with your rack application:

Deploy Rack application

juju deploy rack --config rack.yml

You have to enable mod_proxy_balancer and mod_proxy_http modules in your Apache2 config:

apache2.yml example

  enable_modules: proxy_balancer proxy_http

Deploy Apache2

juju deploy apache2 --config apache2.yml

Create balancer relation between Apache2 and Rack application

juju add-relation apache2:balancer rack

Apache2 charm expects a template to be passed in. Example of vhost that will balance all traffic over your application instances:


<VirtualHost *:80>
  ServerName rack
  ProxyPass / balancer://rack/ lbmethod=byrequests stickysession=BALANCEID
  ProxyPassReverse / balancer://rack/

Update Apache2 service config with this template

juju set apache2 "vhost_http_template=$(base64 < vhost.tmpl)"

Expose Apache2 service

juju expose apache2

Logging with Logstash

You can add logstash service to collect information from application's logs and Kibana application to visualize this data.

juju deploy kibana
juju deploy logstash-indexer
juju add-relation kibana logstash-indexer:rest

juju deploy logstash-agent
juju add-relation logstash-agent logstash-indexer
juju add-relation logstash-agent rack
juju set logstash-agent CustomLogFile="['/var/www/rack/current/log/*.log']" CustomLogType="rack"
juju expose kibana

Monitoring with Nagios and NRPE

You can can perform HTTP checks with Nagios. To do this deploy Nagios and relate it to your Rack application:

juju deploy nagios
juju add-relation rack nagios

Additionally you can perform disk, mem, and swap checks with NRPE extension:

juju deploy nrpe
juju add-relation rack nrpe
juju add-relation nrpe nagios

MongoDB relation

Deploy MonogDB service and relate it to Rack application:

juju deploy mongodb
juju add-relation mongodb rack

Rack charm will set environment variables which you can use to configure your Mongodb adapter.

MONGODB_URL   => mongodb://host:port/database

Mongoid 2.x

Your mongoid.yml should look like:

  uri: <%= ENV['MONGODB_URL'] %>

Mongoid 3.x

Your mongoid.yml should look like:

      uri: <%= ENV['MONGODB_URL'] %>

In both cases you can set additional options specified by Mongoid.

Memcached relation

Deploy Memcached service and relate it to Rack application:

juju deploy memcached
juju add-relation memcached rack

Rack charm will set environment variables which you can use to configure your Memcache adapter. Dalli use those variables by default.

MEMCACHE_PASSWORD    => xxxxxxxxxxxx
MEMCACHE_SERVERS     => instance.hostname.net
MEMCACHE_USERNAME    => xxxxxxxxxxxx

Redis relation

Deploy Redis service and relate it to Rack application:

juju deploy redis-master
juju add-relation redis-master:redis-master rack

Rack charm will set environment variables which you can use to configure your Redis adapter.

REDIS_URL   => redis://username:password@my.host:6389

For example you can configure Redis adapter in config/initializers/redis.rb

uri = URI.parse(ENV["REDIS_URL"])
REDIS = Redis.new(:host => uri.host, :port => uri.port, :password => uri.password)

Known issues

Rack application didn't start because assets were not compiled

To be able to compile assets before you've joined database relation you have to disable initialize_on_precompile option in application.rb:

config.assets.initialize_on_precompile = false

If you can't do this you still can join database and compile assets manually:

juju ssh rack/0 run rake assets:precompile

Then restart Rack service (while you have to replace 'rack/0' with your application name, e.g. 'sample-rails/0', 'sudo restart rack' is a valid command to restart any deployed application):

juju ssh rack/0 sudo restart rack


Deploy from Git

Sample Git config:

  repo: <repository_url>
  revision: <revision_number>

To deploy from private repo via SSH add 'deploy_key' option:

deploy_key: <private_key>

Deploy from SVN

Sample SVN config:

  scm_provider: svn
  repo: <repository_url>
  revision: <revision_number>
  svn_username: <username>
  svn_password: <password>

Install extra packages

Specify list of packages separated by spaces:

extra_packages: 'libsqlite3++-dev libmagick++-dev'

Set ENV variables

You can set ENV variables, which will be available within all processes defined in a Procfile:

env: 'AWS_ACCESS_KEY_ID=aws_access_key_id AWS_SECRET_ACCESS_KEY=aws_secret_access_key'


(string) A deploy key is an SSH key that is stored on the server and grants access to a repository (git only).
(string) List of environment variables (e.g. SECRET_TOKEN=secret_token). These variables will override those which were set by Rack charm (like DATABASE_URL, REDIS_URL)
(string) Extra packages to install before bundle install
(string) Both RACK_ENV and RAILS_ENV environment variables.
(string) Application repository URL
(string) The revision to be checked out. This can be symbolic, like HEAD or it can be a source control management-specific revision identifier.
(string) The name of the source control management provider to be used (git or svn).
(string) The user name for a user that has access to the Subversion repository (svn only).
(string) The password for the user that has access to the Subversion repository (svn only).