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InlineJS/InlinePy Cloudformation Macros

Description

The cfn-inline-1.0.yaml template defines a "meta" macro system that allows cloudformation macros to be defined within the same template in which they are used. This is done by defining macros in the Metadata section which can then be called with 'Fn::Macro' definitions within the template.

The cfn-inline-1.0.yaml template contains defintions for both the JavaScript and Python versions of the macro. Copies of the JavaScript and Python code respectively are contained in cfn-inline-1.0.js and cfn-inline-1.0.py. The code in the yaml and js/py files should be kept in sync.

The tutorial describes how to write macros starting from some very simple macros to more complex macros.

Security Caveat

Short version: You should only use the Inline* system if you trust all other users of the Inline* system within your AWS account.

Longer version: CloudFormation macros are defined using Lambda functions. For performance reasons, when the samme Lambda function is called mutliple times in close temporal proximity, the running state of the lamba function is re-used. For example, if a Node.js Lambda function defines anything in the global state variable, this state may still appear in subsequent calls of the same Lambda function. This same behavior applies to CloudFormation macro calls.

The use of Inline* magnifies the potential impact of this behavior because the deployment of a template that uses Inline* could affect subsequent deployments of templates that also use Inline*. The design of Inline* attempts to prevent unintentional effects between template deployments. However, it does not protect against intentional or malicious attacks.

Command line usage

The cfn-inline-1.0.* scripts have wrapper code that present a command line interface for being able to test the evaluation of macro code on a template. For example, this will show the result of using the Inline* macro on a template file:

./cfn-inline-1.0.js load ./tests/t1.yaml
./cfn-inline-1.0.py load ./tests/t1.yaml

CloudFormation Evaluation Process

When a CloudFormation template is deployed it goes through a number of evaluation/expansion phases that happen in the following order:

  • Template Parameters are evaluated.

  • Template macros are evaluated. Normal template macros are evaluated depth-first (inside-out). Top-level macros (Transform) are evaluated last in left-to-right order. If the InlineJS macro is specified in the top-level Transform key then it triggers the following evaluation process:

    • Code in Metadata.JSInit is evaluated.
    • InlineJS macro calls are evaluated breadth-first (outside-in)
    • InlineJS function calls are evaluated depth-first (inside-out)

    If the InlinePy macro is specified in the top-level Transform key then it triggers the following evaluation process:

    • Any python modules listed in Metadata.PyModules is imported as a global module.
    • Code in Metadata.PyInit is evaluated.
    • InlinePy macro calls are evaluated breadth-first (outside-in)
    • InlinePy function calls are evaluated depth-first (inside-out)

  • Conditionals are evaluated and resources with a false conditional are ignored in the next phase.

  • Template Resources are created in dependency order. The dependency order is both explicit (using DependsOn creates dependencies) and implicit (Ref and GetAtt intrinsics create dependencies). Intrinsic functions are resolved during this phase.

  • Template Outputs are generated

Inline Macros

Defining inline macros

Inline macros are defined in JSMacros and PyMacros sections of the template metadata. Here is an example of a trivial InlineJS macro definition that prints the event parameter and then returns an empty map/dictionary:

Metadata:
  JSMacros:
    Foo: |
        console.log(event)
        return {}

The inline macro code is called with the same top-level parameters as a regular CloudFormation macro call: event, and context. The return fragment value of the macro call will become the new value of the tree at the position where the macro is called.

Calling inline macros

A InlineJS/InlinePy macro call be made from any point in the Yaml tree under Resources or Outputs. A Macro call has the following structure:

'Fn::Macro':
  Name: MacroName
  Parameters:
    Param1: "Param1 Value"
    Param2: "Param2 Value"

Alternately, multiple macro calls can be specified to happen sequentially at the same point in the tree:

'Fn::Macro':
  - Name: MacroName1
    Parameters:
      Param1: "Param1 Value"
      Param2: "Param2 Value"
  - Name: MacroName2
    Parameters:
      Param1: "Param1 Value"
      Param2: "Param2 Value"

Inline Functions

Defining inline functions

Functions are defined in within the Metadata section in JSInit and PyInit (respectively for InlineJS and InlinePy). To make functions in JSInit available they must be added to the global exports variable (similar to how Node.js modules do function exports). Any top-level functions declared in PyInit are available to be called (similar to python modules).

Calling inline functions

Like macro calls, function calls can be made from any point in the Yaml tree under Resources or Outputs. The result of the function call is substituted into the tree at the call point. A function call has the following structure:

'Fn::Function': [FName, "FArg1", "FArg2"]

In the above call, the function named Name is called with arguments "FArg1" and "Farg2". In other words, it is equivalent to FName("FArg1", "FArg2").

Differences between inline macros and functions

  • Macro calls are evaluated breadth first (outside in until there are no more macro calls). After all macros are evaluated then functions are evaluated depth first (inside out). Function calls can make use of the result of other function calls.
  • Each macro is defined separately as a unique attribute under JSMacros or PyMacros sections in the Metadata section. Function definitions happen in the single JSInit or PyInit section under Metadata.
  • Macro definitions have implicit parameters event and context. Functions definitions are defined as normal JavaScript or Python functions with user defined parameters. Functions also have implicit access to the event and context variables (most frequently so that functions have access to global template parameters).
  • Macro calls are passed a map that contains Name and optionally Parameters (which becomes the params attribute of event). Function calls take a sequence of the function name (as a string) followed by positional parameters (only positional parameters are supported in Python functions).

Unit tests

Unit tests for the Inline* macros can be executed like this:

./cfn-inline-1.0.js test
./cfn-inline-1.0.py test

Copyright & License

This software is copyright Viasat and subject to the terms of the Mozilla Public License version 2.0 (MPL.20). A copy of the license is located at in the LICENSE file at the top of the repository or available at https://mozilla.org/MPL/2.0/.