Support for userData Script

[cfndsl]

Support for userData Script

[howech]

cfndsl implements a very crude templating system through its FnFormat call. It takes a string, followed by a hash of key-value pairs, and it takes care of generating cfn json templates that contain the replacements.

For instance...

FnFormat( <<<EOF,
#!/bin/bash
echo "%{greeting} world!"
EOF
           { :greeting => 'hello' } )

will actually generate template json that looks like this

{"Fn::Join": [ "", "#!/bin/bash\necho \", "hello", "\" world"\n"]]}

which ends up resolving at template invocation time to the script you want.

Note that if you are using Resource(...) { Property(...) } you are missing out on some of the coolness that cfndsl provides. Instead you could say:

CloudFormation {
  Instance(:instance1) {
    UserData FnBase64("userdata here")
  }
  Instance(:instance2) {
    UserData FnBase64( FnFormat( <<EOF,
#!/bin/sh
cat
echo "instance1 has instance id %{instance1}" > %{file}
EOF
        { :file => '/tmp/inst1_id'}) 
  }
}

Ok, so there is a lot going on here. First, you are invoking the deeper part of the dsl, where the
"Instance(:resource_name)" method takes care declaring a resource of type AWS::EC2::Instance. The block you pass to it gets evaluated in the context of an object representing the resource, and it has setter methods for all of the things declared as Parameters for the AWS::EC2::Instance type. There is some other syntactic sugar for doing this. You can pass a symbol like :instance1 to FnRef, and it will actually check to see that you declared such a resource when you try to compile the template json. Also, if you use resource names as template variables in an FnFormat call, they get resolved to an FnRef in the giant Fn::Join that cfndsl generates.

[howech]

I guess if you have a specific way to improve on what cfndsl already does, we would love to hear about it, but for now I am marking this issue closed.

[fedecz]

Hi Chris, thanks for taking the time to write such a good reply. I didn't know about FnFormat(). I was achieving the same thing by calling FnBase64(FnJoin(...)) in order to get the Ref() values.

I didn't know about Instance() syntax either. There is no example using that (nor FnFormat() ) in the examples, or Readme. When I'm done with this, I'll make a pull request with some more examples if you don't mind.

Thanks again

Fede

[fedecz]

Where can I find the different resource types (such as Instance() ) that are supported by cfndsl ?
thanks

[fedecz]

How would you write an Instance type, when it has metadata "AWS::CloudFormation::Init " ?

[allinwonder]

In my development, I address the Metadata as one of properties of
resources, so I use Property method with parameter "
AWS::CloudFormation::Init" and some json data to specify metadata.

On Mon, Jul 14, 2014 at 11:39 AM, Federico Czerwinski <
[email protected]> wrote:

How would you write an Instance type, when it has metadata
"AWS::CloudFormation::Init " ?

—
Reply to this email directly or view it on GitHub
#57 (comment).

[fedecz]

Thanks Allinwonder, I do the same thing. My question was actually how do you specify a Cloudformatio::init parameter using the 'instance()' syntax. (not using resource() )

cheers

Fede.

[allinwonder]

Hi Fede, I think to use "AWS::Cloudformation::Init" inside "Instance()"
method is the same way as you use in other resources.

I prefer to declare resource using the "Resource()" method with resource
name and use "Type()" method inside to define the resource type.

For example

Resource("MyInstance") do
  Type("AWS::EC2::Instance")

  # add more properties ...

  MetaData("AWS::CloudFormation::Init",
    { :config => {
        :packages => {
          :yum => {
            'ruby-devel' => []
          }
        }
      }
    })
end