@aws-c2a/engine

AWS CDK Change Analyzer (C2A) - Engine

@aws-c2a/engine is a package that the toolkit consumes to analyze two CloudFormation templates, extract their differences and produce a report of changes, customizable with a rules language.

Table of Contents

1. Platform Mapping
2. Model Diffing
3. Aggregations
4. User Configuration

Platform Mapping

The platform-mapping directory holds parsers that transform an artifact into an InfraModel - in this case, CloudFormation templates.

CloudFormation Parser

The CloudFormation parser takes any CloudFormation template and generates an InfraModel

The type of CloudFormation entity (e.g. Resource, Parameter, Output) gets mapped to the type of Component. In the case of CloudFormation resources, in particular, their type gets mapped to the Component's subtype (i.e. an AWS Lambda Function resource generates a Component with type "Resource" and subtype "AWS::Lambda::Function").

The CloudFormation parser builds instances of CFEntity's subclasses, which have the responsibility of properly building the respective Components, Property Values, and outgoing Dependency Relationships.

The CFRef class extracts references to entities in an entity's declaration, from the used intrinsic functions and resources' DependsOn field.

The following image is an example of the created relationships:

• References in intrinsic functions and in DependsOn fields are transformed into Dependency Relationships
• Structural Relationships connect resources to their stack

AWS CDK Parser

Parsing CDK-generated CloudFormation templates begins by using the CloudFormation parser and adding a Component for each CDK Construct (extracted from the CloudFormation resources metadata). Afterwards, the stack Component and its Structural Relationships are removed and the CDK Construct Components are connected to the corresponding CloudFormation resource Components, as seen here:

Model Diffing

The process of diffing InfraModels is contained in the model-diffing directory.

In the context of AWS CDK/CloudFormation, this is where we extract the operations (changes) that occurred between the old CloudFormation template and the new one.

The basic diff is created in model-diffing/diff-creator.ts. It groups components of the same type and subtype and matches them based on their name and similarity. This similarity is calculated by comparing the properties of each component, in model-diffing/property-diff.ts.

Since detecting property operations and determining their similarity require the same underlying logic, they are both done simultaneously in model-diffing/property-diff.ts. A few notes on how this property diffing currently works:

• When calculating similarity, there is currently no distinction between arrays and sets, so property array order is not considered. In other words, moving elements in an array as no effect on similarity. However, Move operations are still created if an element at index 0 is matched with an element at index 1, for example.
• A weight is associated with a given similarity value, which is the number of primitive values of the structure it applies to. Consider the following:

      {
          a: {b: "string", c: "string},
          d: "string"
      }
  

  Let's consider the string value of key "d" has been changed and the similarity between the new and old value is 0.5. However, the value of key "a" will have similarity 1 because it has not been changed. We can calculate the similarity of the full properties by doing a weighted average between both similarities. "a" will have a weight of 4 (because it holds 4 unchanged values with similarity 1, two keys and two values) and "d" will have a weight of 1 (because it has only 1 primitive value). The similarity for this example is 1*(4/5)+0.5*(1/5)=0.9.

Change Propagation

change-propagator.ts is responsible for taking the observed changes and propagating them. This means:

• Modified properties with componentUpdateType of "REPLACEMENT" or "POSSIBLE_REPLACEMENT" generate an operation (change) of type Replace for their component.
• Renamed Components have an new Replace operation.
• Replace operations in Components with incoming Dependency Relationships generate an Update Operation to the source property of such relationships, indicating that a referenced value may have changed.

Aggregations

Aggregations are structures that group Operations (changes) in a tree-like structure. based on their characteristics, according to a given structure. These are used to collapse changes when presenting them in an interface. Take the following example:

These are resulting aggregations that narrow down operations by:

• type and subtype of the affected Component
• type of the operation
• whether it affects a full component or just a property and, in case of the latter, the property path.

The characteristics that should be grouped at each level, and how, are described in aggregations/component-operation/module-tree.ts. Aggregation modules define how to split a group of operations and a module tree is a configuration of these modules that is used to generate the aggregations.

User Configuration

Users can write rules classify the risk of each change and if it should be automatically approved or rejected. These rules are based on a custom grammar in JSON syntax. Take the following example of a rule:

{
    "description": "Allow all insert operations",
    "let": {
        "insertChange": { "change": {"type": "INSERT" } }
    },
    "effect": {
        "target": "insertChange",
        "risk": "low",
        "action": "approve"
    }
}

This is a very simple rule that sets automatic approval and low risk for all operations of type "INSERT". It is broken down below:

• the "let" field associates objects with identifiers. In this case, there is only one identifier ("insertChange"). An identifier takes the value of all objects that match the query on the right. In this example, the query is matching all "change" objects of type "INSERT". So "insertChange" represents all insertions that occured.
• the "effect" field applies consequences to a given change, identified as the "target". In this case, the "target" is "insertChange", which corresponds to all insert operations. The risk and automatic approval behavior for these changes are specified in the fields "risk" and "action" respectively.

Below is a more complex rule:

{
    "description": "CLOUDFRONT",
    "let": {"cf": { "Resource": "AWS::CloudFront::Distribution" } },
    "then": [{
            "description": "Cloudfront Distributions origin changes are risky",
            "let": {
                "change": { "change": {}, "where": "change appliesTo cf.Properties.DistributionConfig.Origins" }
            },
            "effect": {
                "risk": "high"
            }
        }, {
            "description": "Cloudfront Distributions origin protocol security can increase",
            "let": {
                "change": { "change": {}, "where": [
                    "change appliesTo cf.Properties.DistributionConfig.Origins.*.OriginProtocolPolicy",
                    "change.old == 'http-only'",
                    "change.new == 'https-only'"
                    ]
                }
            },
            "effect": {
                "risk": "low",
                "action": "approve"
            }
    }]
}

In this rule, the "then" field is also used, which allows applying sub-rules that have access to the identifiers declared in their parent.

Component and Property objects allow accessing their inner properties by using the dot (".") notation. For example component.someArray.*.property will correspond to all values of key "property" in elements of array "someArray" inside "component".

You can also notice that queries can have a "when" field, specifying further conditions, such as: checking if a change applies to a given object (Component or Property) with the operator "applies to"; comparing old and new values of changes to properties with the ".old" accessor and "==" operator.

This rules language maps finds the objects in the graph generated from the InfraModelDiff and traverses its edges when relating objects, such as when navigating properties or checking whether a change applies to an object.