Most examples on the deployment of Docker applications to CoreOS use a single docker application. But as soon as you have an application that consists of more than 1 unit, the number of commands you have to type soon becomes annoying. At Xebia we have a best practice that says "Three strikes and you automate" mandating that a third time you do something similar, you automate. In this blog I share the manual page of the utility called fleetappctl that allows you to perform rolling upgrades and deploy Consul Key value pairs of composite applications to CoreOS and show three examples of its usage.

fleetappctl is a utility that allows you to manage a set of CoreOS fleet unit files as a single application. You can start, stop and deploy the application. fleetappctl is idempotent and does rolling upgrades on template files with multiple instances running. It can substitute placeholders upon deployment time and it is able to deploy Consul key value pairs as part of your application. Using fleetappctl you have everything you need to create a self contained deployment  unit of your composite application and put it under version control.

The command line options to fleetappctl are shown below:

fleetappctl [-d deployment-descriptor-file]
            [-e placeholder-value-file]
            (generate | list | start | stop | destroy)

option -d

The deployment descriptor file describes all the fleet unit files and Consul key-value pair files that make up the application. All the files referenced in the deployment-descriptor may have placeholders for deployment time values. These placeholders are enclosed in  double curly brackets {{ }}.

option -e

The file contains the values for the placeholders to be used on deployment of the application. The file has a simple format:



starts all units in the order as they appear in the deployment descriptor. If you have a template unit file, you can specify the number of instances you want to start. Start is idempotent, so you may call start multiple times. Start will bring the deployment inline with your descriptor.

If the unit file has changed with respect to the deployed unit file, the corresponding instances will be stopped and restarted with the new unit file. If you have a template file, the instances of the template file will be upgraded one by one.

Any consul key value pairs as defined by the consul.KeyValuePairs entries are created in Consul. Existing values are not overwritten.


generates a deployment descriptor (deployit-manifest.xml) based upon all the unit files found in your directory. If a file is a fleet unit template file the number of instances to start is set to 2, to support rolling upgrades.


stops all units in reverse order of their appearance in the deployment descriptor.


destroys all units in reverse order of their appearance in the deployment descriptor.


lists the runtime status of the units that appear in the deployment descriptor.

Install fleetappctl

to nstall the fleetappctl utility, type the following commands:

curl -q -L | tar -xzf -
cd fleetappctl-0.25
brew install xmlstarlet
brew install fleetctl

Start the platform

If you do not have the platform running, start it first.

cd ..
git clone
cd coreos-container-platform-as-a-service
git checkout 029d3dd8e54a5d0b4c085a192c0ba98e7fc2838d
cd vagrant
vagrant up

Example - Three component web application

The first example is a three component application. It consists of a mount, a Redis database service and a web application. We generate the deployment descriptor, indicate we do not want to start the mount, start the application and then modify the web application unit file to change the service name into 'helloworld'. We perform a rolling upgrade by issuing start again.. Finally we list, stop and destroy the application.

cd ../fleet-units/app
# generate a deployment descriptor
fleetappctl generate

# do not start mount explicitly
xml ed -u '//fleet.UnitConfigurationFile[@name="mnt-data"]/startUnit' \
       -v false deployit-manifest.xml > \
mv deployit-manifest.xml{.new,}

# start the app
fleetappctl start 

# Check it is working

# Change the service name of the application in the unit file
sed -i -e 's/SERVICE_NAME=hellodb/SERVICE_NAME=helloworld/' app-hellodb@.service

# do a rolling upgrade
fleetappctl start 

# Check it is now accessible on the new service name

# Show all units of this app
fleetappctl list

# Stop all units of this app
fleetappctl stop
fleetappctl list

# Restart it again
fleetappctl start

# Destroy it
fleetappctl destroy

Example - placeholder references

This example shows the use of a placeholder reference in the unit file of the paas-monitor application. The application takes two optional environment  variables: RELEASE and MESSAGE that allow you to configure the resulting responses. The variable RELEASE is configured in the Docker run command in the fleet unit file through a placeholder. The actual value for the current deployment is taken from an placeholder value file.

cd ../fleetappctl-0.25/examples/paas-monitor
#check out the placeholder reference
grep '{{' paas-monitor@.service

ExecStart=/bin/sh -c "/usr/bin/docker run --rm --name %p-%i \
 <strong>--env RELEASE={{release}}</strong> \
# checkout our placeholder values
cat dev.env
# start the app
fleetappctl -e dev.env start

# show current release in status

# start is idempotent (ie. nothing happens)
fleetappctl -e dev.env start

# update the placeholder value and see a rolling upgrade in the works
echo 'release=V3' > dev.env
fleetappctl -e dev.env start

fleetappctl destroy

Example - Env Consul Key Value Pair deployments

The final example shows the use of a Consul Key Value Pair, the use of placeholders and envconsul to dynamically update the environment variables of a running instance. The environment variables RELEASE and MESSAGE are taken from the keys under /paas-monitor in Consul. In turn the initial value of these keys are loaded on first load and set using values from the placeholder file.

cd ../fleetappctl-0.25/examples/envconsul

#check out the Consul Key Value pairs, and notice the reference to placeholder values
cat keys.consul

# checkout our placeholder values
cat dev.env
message=Hi guys
# start the app
fleetappctl -e dev.env start

# show current release and message in status

# Change the message in Consul
fleetctl ssh paas-monitor@1 \
    curl -X PUT \
    -d \'hello Consul\' \

# checkout the changed message

# start does not change the values..
fleetappctl -e dev.env start


CoreOS provides all the basic functionality for a Container Platform as a Service. With the utility fleetappctl it becomes easy to start, stop and upgrade composite applications. The script is an superfluous to fleetctl and does not break other ways of deploying your applications to CoreOS.

The source code, manual page and documentation of fleetappctl can be found on