DIY PaaS – running apps with a DEA

Perhaps the best way to feel confident using Cloud Foundry is to know how it works. And perhaps the best way to learn how it works is to rebuilt it from the ground up. In the DIY PaaS articles, we will re-build Cloud Foundry from the ground up piece-by-piece. This is article number 1, and we’re going to start with the good bit: running applications.


At the core of Cloud Foundry, or your own DIY PaaS, is a way to run arbitrary applications – Ruby, Java, PHP. A great pattern is to run an agent process on every server being allocated to running these applications. In Cloud Foundry this agent is called the Droplet Execution Agent. More commonly known as the DEA.

It’s an agent that executes droplets. The Droplet Execution Agent. You’re feeling educated already.

Skip the tutorial, just run something quickly for me!

You’re busy. You want to see something shiny. Here, run this and you’ll see the DEA running, and being told to run a Ruby/Sinatra application via a message bus:

cd /tmp
git clone git://
cd deploying-to-a-cloudfoundry-dea
git submodule update --init
rake bundle_install
foreman start
22:58:31 deploy.1 | New app registered at:

You can now open that URL, such as, in your browser and see your running application.

Now, if you’d like to learn more about what just happened, then let’s get started!


Everything in this tutorial can be done on your local computer. The wonders of cloud computing are for another day. I already have Ruby 1.9.3 installed on my laptop and available in my $PATH.

As I go along, I’ll clone/submodule the Cloud Foundry repositories that I need and show the bare minimum configuration files. The final product is available in a git repository as a demonstration of the minimum parts of Cloud Foundry required to run an application.


If you should know anything about Cloud Foundry its that applications are run via a Droplet Execution Agent (DEA). This is a small process that runs on any server or hardware where you want to deploy applications.

A DEA is like a process manager with an API. You tell a DEA which application to run, it validates that its server is capable of running the application, it fetches that application, and it starts the application.

$ mkdir -p /tmp/deploying-to-a-cloudfoundry-dea
$ cd /tmp/deploying-to-a-cloudfoundry-dea
$ mkdir config             # for all configuration files to come
$ mkdir -p var/dea         # for staging & running apps
$ mkdir -p var/log         # for logs
$ mkdir -p var/run         # for pid files
$ git clone git://
$ cd dea
$ bundle
$ cd /tmp/deploying-to-a-cloudfoundry-dea
$ ./dea/bin/dea
Config file location not specified. Please run with --config argument or set CLOUD_FOUNDRY_CONFIG_PATH

Ahh, introduction to running Cloud Foundry lesson 1 – YAML configuration files. Lots of YAML configuration files.

Ignore the out-of-date example dea config file, and look at the dea.yml.erb from Cloud Foundry’s own cf-release BOSH release. I created config/dea-laptop.yml as below.

# Base directory where all applications are staged and hosted
base_dir: /tmp/deploying-to-a-cloudfoundry-dea/var/dea
pid: /tmp/deploying-to-a-cloudfoundry-dea/var/run/
  - ruby19
multi_tenant: true
max_memory: 1024
# Optional as of
  level: debug

Running dea again whilst using this configuration file is a lot more successful!

$ ./dea/bin/dea -c config/dea-laptop.yml
Starting VCAP DEA (0.99)
Pid file: /tmp/deploying-to-a-cloudfoundry-dea/var/run/
Using ruby @ /Users/drnic/.rvm/rubies/ruby-1.9.3-p286/bin/ruby
Using network:
Socket Limit:256
Max Memory set to 4.0G
Utilizing 1 cpu cores
Restricting to single tenant
Using directory: /tmp/deploying-to-a-cloudfoundry-dea/var/dea/
Initial usage of droplet fs is: 0%
File service started on port:
EXITING! NATS error: Could not connect to server on nats://localhost:4222

And we move on to the next piece of Cloud Foundry, the messaging bus called NATS!


NATS is a simple pub-sub messaging system used by Cloud Foundry for communication between services.

By default, the DEA looks for a NATS server on http://localhost:4222. Port 4222 is coincidently the default port that nats-server (see below) binds too. NATS is currently a rubygems, but let’s fetch its source code via git into our project folder:

$ git clone git://
$ cd nats
$ bundle
$ cd ..
$ ./nats/bin/nats-server -d
["Starting nats-server version 0.4.28 on port 4222"]
["Switching to daemon mode"]

Now when we re-run the DEA it runs successfully in the foreground, listening to NATS for instructions.

$ ./dea/bin/dea -c config/dea-laptop.yml
Initial usage of droplet fs is: 0%
File service started on port:

When a DEA is launched it automatically joins the Cloud Foundry environment that it belongs to. The way it identifies itself is via NATS. When any part of Cloud Foundry launches, including each DEA, it publishes a vcap.component.announce message.

To see the NATS messages published by our DEA, add the following executable script at bin/nats_all

#!/usr/bin/env ruby
require "nats/client"
NATS.start do
  NATS.subscribe('>') { |msg, reply, sub| puts "Msg received on [#{sub}] : '#{msg}'" }

In a new terminal, run this script. In the original terminal, kill the DEA and restart it (Ctrl+C to kill it). The following output appears in the nats_all terminal.

$ ruby bin/nats_all
Msg received on [vcap.component.announce] :
"credentials":["USERNAME","PASSWORD"],"start":"2012-11-10 23:16:11 -0800"}'
Msg received on [dea.start] :
Msg received on [dea.advertise] :
Msg received on [dea.advertise] :

On the first message, vcap.component.announce, the DEA published its host ( and username/password credentials.

The last lines are the constant status announcement that the DEA publishes to inform Cloud Foundry that it is still available to deploy more applications; with 4G of RAM still available (the default amount).

All Cloud Foundry components can be polled for their health /healthz and their configuration data /varz.

$ curl http://USERNAME:[email protected]:62556/healthz
$ curl http://USERNAME:[email protected]:62556/varz
  "type": "DEA",
  "host": "",
  "credentials": [
  "mem": 42744,
  "cpu": 0.0,
  "apps_max_memory": 1024,
  "apps_reserved_memory": 0,
  "apps_used_memory": 0,
  "num_apps": 0,
  "running_apps": [

Our DEA thinks it has no runtimes for running applications (such as Ruby or Java), and no frameworks (such as Ruby on Rails, Sinatra or Play). So we’ll need to fix that before we can deploy an application.

But first, we need to discover a DEA to be able to talk to it; and second we need to deploy an application into the DEA. WE do both via NATS client calls.

Discovering a DEA

Each DEA listens (subscribes) for various messages on NATS.

A useful little script to discover all the available DEAs (on a NATS server) is below. It is in the tutorial repository at bin/dea_discover.

It broadcasts a NATS message and waits for a reply.

#!/usr/bin/env ruby
require "nats/client"
require "json"
NATS.start do
  NATS.subscribe('>') { |msg, reply, sub| puts "Msg received on [#{sub}] : '#{msg}'" }
  message = {
    'runtime_info' => {
      'name' => 'ruby19',
      'executable' => 'ruby',
      'version_output' => 'ruby 1.9.3p286'
    'limits' => {
      'mem' => 256
    'droplet' => 'DROPLET_ID_1234'
  NATS.request('', message.to_json) do |response|
    puts "Got response: #{response}"

Since NATS is independent of Ruby, you could also discover DEAs using the following Node.js script (bin/discover_dea.js)

var nats = require('nats').connect();
// Simple Subscriber
nats.subscribe('>', function(msg, reply, subject) {
  console.log('Msg received on [' + subject + '] : ' + msg);
var message = {
  'runtime_info': {
    'name': 'ruby19',
    'executable': 'ruby',
    'version_output': 'ruby 1.9.3p286'
  'limits': {
    'mem': 256
  'droplet': 'DROPLET_ID_1234'
nats.request('', JSON.stringify(message), function(response) {
  console.log("Got response: " + response);

The output from both includes the response from our one DEA.

Got response: {"id":"56a44db58ce330df22426c01b3c66b6c","ip":"","port":null,"version":0.99}

This response includes the UUID for each DEA in the id key. We now use this UUID to tell that DEA to run an application.

Using NATS to deploy to a DEA

An example script for deploying a local application is at bin/start_app. This section introduces that script.

To tell a DEA to deploy an application, you publish a NATS message that contains its UUID, dea.UUID.start. In a full Cloud Foundry, it is the Cloud Controller that publishes this message. The Cloud Controller is the public API for user requests – deploy new apps, update existing apps, and scaling existing apps. The bulk of the dea.UUID.start message is created in AppManager#new_message.

An example dea.UUID.start JSON/Ruby message for deploying a Ruby/Sinatra application could be:

dea_app_start = {
  droplet: droplet_id,
  index: 0,
  services: [],
  version: '1-1',
  sha1: sha1,
  executableFile: '???',
  executableUri: "/staged_droplets/#{droplet_id}/#{sha1}",
  name: app_name,
  uris: ["#{app_name}"],
  env: [],
  users: ['[email protected]'],
  runtime_info: {
    name: 'ruby19',
    executable: 'ruby',
    version_output: 'ruby 1.9.3p286'
  framework: 'sinatra',
  running: 'ruby19',
  limits: { mem: 256 },
  cc_partition: 'default'

More interesting is to put this together with a sequence of NATS requests to discover a DEA, tell it to deploy an application, and watch for its internal host:port being announced.

We first request and when a DEA responds we the dea.UUID.start message which only that DEA subscribes. We watch for router.register messages to discover what host:port the application is running on.

#!/usr/bin/env ruby
require "nats/client"
require "json"
# see bin/start_app for creating local tarball
NATS.start do
  NATS.subscribe('>') { |msg, reply, sub| puts "Msg received on [#{sub}] : '#{msg}'" }
  dea_discover = {
    # from above
  NATS.request('', dea_discover.to_json) do |response|
    dea_uuid = JSON.parse(response)['id']
    dea_app_start = {
      # see example above
    # after "dea.UUID.start" below, wait for host:port to be announced
    NATS.subscribe("router.register") do |msg|
      new_app = JSON.parse(msg)
      host, port = new_app["host"], new_app["port"]
      puts "New app registered at: http://#{host}:#{port}"
    # Request deployment
    NATS.publish("dea.#{dea_uuid}.start", dea_app_start.to_json)

An example output of this script would be:

$ ./bin/start_app sinatra
New app registered at:

Now, the application that bin/start_app deploys is a Sinatra application. But its not just a Sinatra application, it is a specially "staged" application that is ready for the DEA.

What isn’t clear in the bin/start_app script is how the DEA knows how to run a Ruby/Sinatra application.

The short answer is it doesn’t. The DEA knows nothing about Ruby or Java or PHP.

I’ll try to write up the "staging" concept of Cloud Foundry, as implemented in vcap-staging. It is vcap-staging that knows how to launch a Ruby on Rails or Sinatra application on Ruby, not the DEA.

So when the DEA deploys an application, that application must be prepared in advance.

The DEA has three basic steps to deploy an application:

  • get a tarball (tgz) from a local cache, shared cache or via remote HTTP
  • unpack the tarball
  • run ./startup

In Cloud Foundry, vcap-staging creates the executable startup script specific to the type of application being deployed. That is, if you want to add a new programming language or a new framework, your first stop is vcap-staging.

In this tutorial, I have pre-staged the Sinatra application and the start_app script copies it from its location and runs ./startup.

The staged Sinatra application is at apps/sinatra and includes the startup script that vcap-staging would have generated for a simple Sinatra application that doesn’t use bundler.

DEA knows about its running an application

If you now ask the DEA for its varz status again, it will tell you about the application that it is running, and the runtimes and frameworks that it is feeling confident about:

$ curl http://USERNAME:[email protected]:62556/varz
  "running_apps": [
      "state": "RUNNING",
      "runtime": "ruby19",
      "framework": "sinatra",
  "frameworks": {
    "sinatra": {
  "runtimes": {
    "ruby19": {


Deploying an application to a DEA has a few simple requirements.

  • Run a NATS server
  • Run a DEA using a simple YAML configuration
  • Create a startup script and a copy of your application together in a folder
  • Publish NATS message dea.DEA_UUID.start to tell the application to deploy the application from its local cached/pre-staged version (or a remote tar)

Its not as simple as perhaps it could be; but it is relatively understandable as to how the pieces fit together. You use NATS to find and communicate with a DEA. You tell it what tarball to use to unpack and run via a startup script. Pretty simple.

Next, staging any application for the DEA

In the next DIY PaaS article, we will take the next logical step: what is a droplet and how did it get created?

If you want to skip ahead, look in the Cloud Foundry github account for the repositories related to the term "staging" or "stager".

Follow @starkandwayne for the release of the next article and other blog posts from the wonderful world Cloud Foundry.

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