Writing a Subresource

In this guide we’re going to write a CloudWanderer subresource definition for lambda layer versions.

A CloudWanderer subresource is a specific type of Boto3 resource.

  • Boto3 subresources are any AWS resource which depends on another resource for its existence (e.g. subnets depend on VPCs and so subnets would be a subresource).

  • CloudWanderer subresources are resources which depend on their parent for their identity. A resource is a subresource in CloudWanderer terms if you must supply the id of its parent in order to fetch it (e.g. IAM Role inline policies).

In our case lambda layer versions are subresources because you cannot fetch metadata about them from the AWS API without supplying the name of the layer of which they are a version.

Note

This guide assumes you have read the Writing a Custom Resource guide as a pre-requisite. As it has a lot more detail on some of the steps outlined here. Look to that page for further clarification if need be.

Getting the test data

$ aws lambda list-layer-versions --layer-name test-layer
{
    "LayerVersions": [
        {
            "LayerVersionArn": "arn:aws:lambda:eu-west-1:123456789012:layer:test-layer:1",
            "Version": 1,
            "Description": "This is a test layer!",
            "CreatedDate": "2020-10-17T13:18:00.303+0000",
            "CompatibleRuntimes": [
                "nodejs10.x"
            ]
        }
    ]
}

Create the tests

Subresources are always discovered alongside their parents, so our tests for lambda layer versions will fit nicely alongside the tests for lambda layers.

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class TestLambdaLayers(NoMotoMock, unittest.TestCase):
    ...

    layer_version_payload = {
        "LayerVersionArn": "arn:aws:lambda:eu-west-1:123456789012:layer:test-layer:1",
        "Version": 1,
        "Description": "This is a test layer!",
        "CreatedDate": "2020-10-17T13:18:00.303+0000",
        "CompatibleRuntimes": ["nodejs10.x"],
    }

    mock = {
        "lambda": {
            "list_layers.return_value": {
                "Layers": [layer_payload],
            },
            "list_layer_versions.return_value": {"LayerVersions": [layer_version_payload]},
            "get_layer_version.return_value": layer_version_payload,
        }
    }

    single_resource_scenarios = [
        SingleResourceScenario(
            urn=URN.from_string("urn:aws:123456789012:eu-west-1:lambda:layer:test-layer"),
            expected_results=UnsupportedResourceTypeError,
        ),
        SingleResourceScenario(
            urn=URN.from_string("urn:aws:123456789012:eu-west-1:lambda:layer_version:test-layer/1"),
            expected_results=[layer_version_payload],
        ),
    ]

    multiple_resource_scenarios = [
        MultipleResourceScenario(
            arguments=CloudWandererCalls(regions=["eu-west-1"], service_names=["lambda"], resource_types=["layer"]),
            expected_results=[layer_payload, layer_version_payload],
        )
    ]

We’ve added our test payload as a class variable, referenced it in the mock on line 17, as well as the mock on line 18, and expected it as a result in lines 27-29 and 36. We have added it to single_resource_scenarios and get_layer_version.return_value because while lambda layers cannot be discovered individually, lambda layer versions can!

Populate the definition

Visit Botocore’s specification data on GitHub and open the latest service-2.json for your service.

We’re here following much the same process as in Writing a Custom Resource. We’re following our ListLayerVersions through its return of ListLayerVersionsResponse to its shape of LayerVersionsList to its member of LayerVersionsListItem. Along the way we’ve identified the

  • Collection Request Operation (ListLayerVersions)

  • Resource Shape (LayerVersionsListItem)

  • Identifiers (LayerName and Version)

Subresource Identifiers

Subresources always have two identifiers, one is the identifier of their parent, and the other is their identifier. In CloudWanderer’s definition this is what makes them a subresource, that they do not have an independent identity without their parent.

[
    {
        "target": "LayerName",
        "source": "identifier",
        "name": "LayerName"
    },
    {
        "target": "Version",
        "source": "response",
        "path": "LayerVersions[].Version"
    }
]

Subresource Request Operation

Resource request operations were pretty simple as they had no arguments. Subresources on the other hand need to supply the identity of their parent as an argument to whatever API method they’re calling.

{
    "operation": "ListLayerVersions",
    "params": [
        {
            "target": "LayerName",
            "source": "identifier",
            "name": "LayerName"
        }
    ]
}

This will take the LayerName from the parent resource, and submit it as a parameter called LayerName to the ListLayerVersions API method.

Bringing it all together

In the resources definiton we’ve added the highlighted lines.

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{
    "resources": {
        "Layer": {
            "identifiers": [
                {
                    "name": "LayerName",
                    "memberName": "LayerName"
                }
            ],
            "shape": "LayersListItem",
            "hasMany": {
                "LayerVersions": {
                    "request": {
                        "operation": "ListLayerVersions",
                        "params": [
                            {
                                "target": "LayerName",
                                "source": "identifier",
                                "name": "LayerName"
                            }
                        ]
                    },
                    "resource": {
                        "type": "LayerVersion",
                        "identifiers": [
                            {
                                "target": "LayerName",
                                "source": "identifier",
                                "name": "LayerName"
                            },
                            {
                                "target": "Version",
                                "source": "response",
                                "path": "LayerVersions[].Version"
                            }
                        ],
                        "path": "LayerVersions[]"
                    }
                }
            }
        },
        "LayerVersion": {
            "identifiers": [
                {
                    "name": "LayerName",
                    "memberName": "LayerName"
                },
                {
                    "name": "Version",
                    "memberName": "Version"
                }
            ],
            "shape": "LayerVersionsListItem"
        }
    }
}

You’ll notice we’ve added the collection specification inside the Layer resource instead of inside the service as we did in Writing a Custom Resource, this is what allows us to reference the LayerName of the parent resource when we call ListLayerVersions in our collection API call.

Writing the Service Map

The service map is CloudWanderer’s store for resource type metadata that does not fit into the Boto3 specification. It broadly follows the structure of Boto3’s to try and keep things simple and consistent. For our new Layer resource we just need to ensure that the following exists in service_mappings/lambda.json

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{
    "service": {
        "globalService": false,
        "regionalResources": true
    },
    "resources": {
        "Layer": {
            "type": "baseResource"
        },
        "LayerVersion": {
            "type": "subresource",
            "parentResourceType": "Layer"
        }
    }
}

We added the LayerVersion key to resources to indicate that we’ve added a subresource whose parent resource type is Layer. This allows CloudWanderer to determine the proper relationship between these resources and properly generate URNs.

  • globalService indicates whether the service has a Global API or not (that is, whether all API requests go to a single region, e.g. us-east-1 for IAM).

  • regionalResources indicates whether the service has regional resources or not (most do, but some, like IAM do not).

  • type indicates what type of resource this is, either baseResource, subresource or secondaryAttribute.

Running the tests

Now you’ve put all the pieces together you need to run the tests. You can see the full test code on github. As well as the full resource specification (alongside Lambda Function).

To run the tests:

# Install the pre-reqs
$ pip install -r requirements-test.txt -r requirements.txt
# Install the package in interactive mode
$ pip install -e .
# Run the tests
$ pytest tests/integration/custom_resources/lambda/test_layers.py
=== 2 passed in 2.28s ==

Submit a PR!

Congratulations! You have successfully created a new resource! If you submit a Pull Request to https://github.com/CloudWanderer-io/CloudWanderer/ with your new resource we will get it merged in and released for everyone to use as quickly as we possibly can! If you find you’re not getting the attention you deserve for whatever reason, contact us on twitter.