Continuous deployment with cloud formation and cfn-hup

Cloud formation lets you create application stacks from scratch in EC2. In my project I use cloud formation to do continuous deploys using cfn-hup with every git push. If you haven’t used cloud formation before, there are plenty of tutorials on the web to get you started.

Structure:

The application is bundled into a RPM using FPM with all the application dependencies and post install scripts to be executed when the RPM is installed.

Packaging and CFN-Script:

As a part of the CI build, RPM of the app is created and uploaded to S3 tagged as ‘latest’ and the cfn-script downloads the RPM and installs it. An example looks like:

curl ‘package_url’ > /tmp/earmark-app.rpm
yum erase -y earmark-app
yum install /tmp/earmark-app.rpm

CFN-HUP Configuration:

[main]
stack=stack-name
region=ap-southeast-2

Waiting on Resource Condition:

You can use ApplicationWaitHandle & ApplicationWaitCondition to signal cloud formation deploy to wait until the installation of the package to complete. You can then use cfn-signal to indicate cloud formation stack that the particular action is complete. If cfn-signal isn’t received within a specified time period, the stack creation will fail. Here is an example WaitCondition:

“ApplicationWaitCondition” : {
“Type” : “AWS::CloudFormation::WaitCondition”,
“Properties” : {
“Count” : “1”,
“Handle” : {“Ref” : “ApplicationWaitHandle”},
“Timeout” : “300”
}
},
“ApplicationWaitHandle” : {
“Type” : “AWS::CloudFormation::WaitConditionHandle”
}
</code>
You can then wait on this condition in your stack by using:
<code>
“LoadBalancer”: {
“DependsOn”: “ApplicationWaitCondition”,
}

Signalling Cloud Formation:

You can then signal cloud formation that the install was successful using:

"/opt/aws/bin/cfn-signal -e 0 -r "Application Installation Complete." ‘", { "Ref" : "ApplicationWaitHandle" }, "’\n"

Limitations:

Cfn-hup checks for updates on the stack every 15 minutes by default and there is no way to alter the duration.
Cfn-hup cannot be used to signal stack wait during stack updates.
Cfn-hup failures aren’t propagated back to the stack events which could result in a false positivies even when update fails.