Blueprints

Blueprints are python classes that dynamically build CloudFormation templates. Where you would specify a raw Cloudformation template in a stack using the template_path key, you instead specify a Blueprint python file using the class_path key.

Traditionally Blueprints are built using troposphere, but that is not absolutely necessary.

Making your own should be easy, and you can take a lot of examples from Runway blueprints. In the end, all that is required is that the Blueprint is a subclass of runway.cfngin.blueprints.base and it have the following methods:

# Initializes the Blueprint
def __init__(self, name, context, mappings=None):

# Updates self.template to create the actual template
def create_template(self):

# Returns a tuple: (version, rendered_template)
def render_template(self):

Variables

A Blueprint can define a VARIABLES property that defines the variables it accepts from the Config Variables.

VARIABLES should be a dictionary of <variable name>: <variable definition>. The variable definition should be a dictionary which supports the following optional keys:

type:

The type for the variable value. This can either be a native python type or one of the Variable Types.

default:

The default value that should be used for the variable if none is provided in the config.

description:

A string that describes the purpose of the variable.

validator:

An optional function that can do custom validation of the variable. A validator function should take a single argument, the value being validated, and should return the value if validation is successful. If there is an issue validating the value, an exception (ValueError, TypeError, etc) should be raised by the function.

no_echo:

Only valid for variables whose type subclasses CFNType. Whether to mask the parameter value whenever anyone makes a call that describes the stack. If you set the value to true, the parameter value is masked with asterisks (*).

allowed_values:

Only valid for variables whose type subclasses CFNType. The set of values that should be allowed for the CloudFormation Parameter.

allowed_pattern:

Only valid for variables whose type subclasses CFNType. A regular expression that represents the patterns you want to allow for the CloudFormation Parameter.

max_length:

Only valid for variables whose type subclasses CFNType. The maximum length of the value for the CloudFormation Parameter.

min_length:

Only valid for variables whose type subclasses CFNType. The minimum length of the value for the CloudFormation Parameter.

max_value:

Only valid for variables whose type subclasses CFNType. The max value for the CloudFormation Parameter.

min_value:

Only valid for variables whose type subclasses CFNType. The min value for the CloudFormation Parameter.

constraint_description:

Only valid for variables whose type subclasses CFNType. A string that explains the constraint when the constraint is violated for the CloudFormation Parameter.

Variable Types

Any native python type can be specified as the type for a variable. You can also use the following custom types:

TroposphereType

The TroposphereType can be used to generate resources for use in the Blueprint directly from user-specified configuration. Which case applies depends on what type was chosen, and how it would be normally used in the Blueprint (and CloudFormation in general).

Resource Types

When type is a Resource Type, the value specified by the user in the configuration file must be a dictionary, but with two possible structures.

When many is disabled, the top-level dictionary keys correspond to parameters of the type constructor. The key-value pairs will be used directly, and one object will be created and stored in the variable.

When many is enabled, the top-level dictionary keys are resource titles, and the corresponding values are themselves dictionaries, to be used as parameters for creating each of multiple type objects. A list of those objects will be stored in the variable.

Property Types

When type is a Property Type the value specified by the user in the configuration file must be a dictionary or a list of dictionaries.

When many is disabled, the top-level dictionary keys correspond to parameters of the type constructor. The key-value pairs will be used directly, and one object will be created and stored in the variable.

When many is enabled, a list of dictionaries is expected. For each element, one corresponding call will be made to the type constructor, and all the objects produced will be stored (also as a list) in the variable.

Optional variables

In either case, when optional is enabled, the variable may have no value assigned, or be explicitly assigned a null value. When that happens the variable’s final value will be None.

Example

Below is an annotated example:

from runway.cfngin.blueprints.base import Blueprint
from runway.cfngin.blueprints.variables.types import TroposphereType
from troposphere import s3, sns

class Buckets(Blueprint):

    VARIABLES = {
        # Specify that Buckets will be a list of s3.Bucket types.
        # This means the config should a dictionary of dictionaries
        # which will be converted into troposphere buckets.
        "Buckets": {
            "type": TroposphereType(s3.Bucket, many=True),
            "description": "S3 Buckets to create.",
        },
        # Specify that only a single bucket can be passed.
        "SingleBucket": {
            "type": TroposphereType(s3.Bucket),
            "description": "A single S3 bucket",
        },
        # Specify that Subscriptions will be a list of sns.Subscription types.
        # Note: sns.Subscription is the property type, not the standalone
        # sns.SubscriptionResource.
        "Subscriptions": {
            "type": TroposphereType(sns.Subscription, many=True),
            "description": "Multiple SNS subscription designations"
        },
        # Specify that only a single subscription can be passed, and that it
        # is made optional.
        "SingleOptionalSubscription": {
            "type": TroposphereType(sns.Subscription, optional=True),
            "description": "A single, optional SNS subscription designation"
        }
    }

    def create_template(self):
        t = self.template
        variables = self.get_variables()

        # The Troposphere s3 buckets have already been created when we
        # access variables["Buckets"], we just need to add them as
        # resources to the template.
        [t.add_resource(bucket) for bucket in variables["Buckets"]]

        # Add the single bucket to the template. You can use
        # `Ref(single_bucket)` to pass CloudFormation references to the
        # bucket just as you would with any other Troposphere type.
        # single_bucket = variables["SingleBucket"]
        t.add_resource(single_bucket)

        subscriptions = variables["Subscriptions"]
        optional_subscription = variables["SingleOptionalSubscription"]
        # Handle it in some special way...
        if optional_subscription is not None:
            subscriptions.append(optional_subscription)

        t.add_resource(sns.Topic(
            TopicName="one-test",
            Subscriptions=))

        t.add_resource(sns.Topic(
            TopicName="another-test",
            Subscriptions=subscriptions))

A sample config for the above:

stacks:
  - name: buckets
    class_path: path.to.above.Buckets
    variables:
      Buckets:
        # resource name (title) that will be added to CloudFormation.
        FirstBucket:
          # name of the s3 bucket
          BucketName: my-first-bucket
        SecondBucket:
          BucketName: my-second-bucket
      SingleBucket:
        # resource name (title) that will be added to CloudFormation.
        MySingleBucket:
          BucketName: my-single-bucket
      Subscriptions:
        - Endpoint: one-lambda
          Protocol: lambda
        - Endpoint: another-lambda
          Protocol: lambda
      # The following could be omitted entirely
      SingleOptionalSubscription:
        Endpoint: a-third-lambda
        Protocol: lambda

CFNType

The CFNType can be used to signal that a variable should be submitted to CloudFormation as a Parameter instead of only available to the Blueprint when rendering. This is useful if you want to leverage AWS- Specific Parameter types (e.g. List<AWS::EC2::Image::Id>) or Systems Manager Parameter Store values (e.g. AWS::SSM::Parameter::Value<String>). See runway.cfngin.blueprints.variables.types for available subclasses of the CFNType.

Example

Below is an annotated example:

from runway.cfngin.blueprints.base import Blueprint
from runway.cfngin.blueprints.variables.types import (
    CFNString,
    EC2AvailabilityZoneNameList,
)


class SampleBlueprint(Blueprint):

    VARIABLES = {
        "String": {
            "type": str,
            "description": "Simple string variable",
        },
        "List": {
            "type": list,
            "description": "Simple list variable",
        },
        "CloudFormationString": {
            "type": CFNString,
            "description": "A variable which will create a CloudFormation Parameter of type String",
        },
        "CloudFormationSpecificType": {
            "type": EC2AvailabilityZoneNameList,
            "description": "A variable which will create a CloudFormation Parameter of type List<AWS::EC2::AvailabilityZone::Name>"
        },
    }

    def create_template(self):
        t = self.template

        # `get_variables` returns a dictionary of <variable name>: <variablevalue>.
        # For the subclasses of `CFNType`, the values are
        # instances of `CFNParameter` which have a `ref` helper property
        # which will return a troposphere `Ref` to the parameter name.
        variables = self.get_variables()

        t.add_output(Output("StringOutput", variables["String"]))

        # variables["List"] is a native list
        for index, value in enumerate(variables["List"]):
            t.add_output(Output("ListOutput:{}".format(index), value))


        # `CFNParameter` values (which wrap variables with a `type`
        # that is a `CFNType` subclass) can be converted to troposphere
        # `Ref` objects with the `ref` property
        t.add_output(Output("CloudFormationStringOutput",
                            variables["CloudFormationString"].ref))
        t.add_output(Output("CloudFormationSpecificTypeOutput",
                            variables["CloudFormationSpecificType"].ref))

Utilizing Stack name within your Blueprint

Sometimes your Blueprint might want to utilize the already existing stack name within your Blueprint. CFNgin provides access to both the fully qualified stack name matching what’s shown in the CloudFormation console, in addition to the stacks short name you have set in your YAML config.

Referencing Fully Qualified Stack name

The fully qualified name is a combination of the CFNgin namespace + the short name (what you set as name in your YAML config file). If your CFNgin namespace is CFNginIsCool and the stacks short name is myAwesomeEC2Instance, the fully qualified name would be:

CFNginIsCool-myAwesomeEC2Instance

To use this in your Blueprint, you can get the name from context using self.context.get_fqn(self.name).

Referencing the Stack short name

The Stack short name is the name you specified for the stack within your YAML config. It does not include the namespace. If your CFNgin namespace is CFNginIsCool and the stacks short name is myAwesomeEC2Instance, the short name would be:

myAwesomeEC2Instance

To use this in your Blueprint, you can get the name from self.name: self.name

Example

Below is an annotated example creating a security group:

# we are importing Ref to allow for CFN References in the EC2 resource.  Tags
# will be used to set the Name tag
from troposphere import Ref, ec2, Tags
from runway.cfngin.blueprints.base import Blueprint
# CFNString is imported to allow for stand alone stack use
from runway.cfngin.blueprints.variables.types import CFNString

class SampleBlueprint(Blueprint):

  # VpcId set here to allow for Blueprint to be reused
  VARIABLES = {
  "VpcId": {
      "type": CFNString,
      "description": "The VPC to create the Security group in",
      }
  }


  def create_template(self):
      template = self.template
      # Assigning the variables to a variable
      variables = self.get_variables()
      # now adding a SecurityGroup resource named `SecurityGroup` to the CFN template
      template.add_resource(
        ec2.SecurityGroup(
          "SecurityGroup",
          # Referencing the VpcId set as the variable
          VpcId=variables['VpcId'].ref,
          # Setting the group description as the fully qualified name
          GroupDescription=self.context.get_fqn(self.name),
          # setting the Name tag to be the stack short name
          Tags=Tags(
            Name=self.name
            )
          )
        )

Testing Blueprints

When writing your own Blueprints its useful to write tests for them in order to make sure they behave the way you expect they would, especially if there is any complex logic inside.

To this end, a sub-class of the unittest.TestCase class has been provided: runway.cfngin.blueprints.testutil.BlueprintTestCase. You use it like the regular TestCase class, but it comes with an addition assertion: assertRenderedBlueprint. This assertion takes a Blueprint object and renders it, then compares it to an expected output, usually in tests/fixtures/blueprints.

Yaml (CFNgin) format tests

In order to wrap the BlueprintTestCase tests in a format similar to CFNgin’s stack format, the YamlDirTestGenerator class is provided. When subclassed in a directory, it will search for yaml files in that directory with certain structure and execute a test case for it. As an example:

---
namespace: test
stacks:
  - name: test_stack
    class_path: cfngin_blueprints.s3.Buckets
    variables:
      var1: val1

When run from tests, this will create a template fixture file called test_stack.json containing the output from the cfngin_blueprints.s3.Buckets template.