8000 Add support for Rules by nielslaukens · Pull Request #1412 · cloudtools/troposphere · GitHub
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Merged
merged 6 commits into from
Jun 30, 2019
Merged

Add support for Rules #1412

merged 6 commits into from
Jun 30, 2019

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nielslaukens
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Rules are a feature of AWS Service Catalogue 1 but is undocumented
for pure CloudFormation. However, it does work in vanilla CloudFormation
as well 2.

This adds minimal support for Rules in templates. It provides a bare
dict, and exports it to JSON/YAML.

@nielslaukens
Add support for Rules
c38883f 
Rules are a feature of AWS Service Catalogue [1] but is undocumented
for pure CloudFormation. However, it does work in vanilla CloudFormation
as well [2].

This adds minimal support for Rules in templates. It provides a bare
dict, and exports it to JSON/YAML.

[1]: https://docs.aws.amazon.com/servicecatalog/latest/adminguide/reference-template_constraint_rules.html
[2]: https://www.cloudar.be/awsblog/undocumented-feature-using-template-constraint-rules-in-cloudformation/
@nielslaukens
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I intend to add additional classes & methods to enhance the usability of Rules; but I wanted to check first what your policy for "undocumented" features is?

@markpeek
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markpeek commented Jun 3, 2019

Interesting. If it works I'm not opposed to it. I'm going to check with some of my AWS contacts about it first though.

@rkz
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rkz commented Jun 28, 2019

I confirm the Rules statement does work in CloudFormation (used it). Following up on @nielslaukens' proposal, I guess the usage would be:

template = Template()

# Prepare a rule to lock the region in which this template can run
check_region_rule = {
    "CheckRegion": {
        "Assertions": [
            {
                "Assert": Equals(Ref("AWS::Region"), "eu-west-1"),
                "AssertDescription": "This template is only available in eu-west-1"
            }
        ]
    }
}

# Add it to the template
template.rules.append(check_region_rule)

Correct?

That does not seem consistent with the way of doing things in Template for two reasons:

  1. Developer needs to hack into the .rules property using append(). Would make sense to have a Template.add_rule() method instead.
  2. The format of the rule itself could be enforced by creating Rule and Assertion classes, instead of using plain dicts.

Happy to implement an improvement for issue 1, but I think issue 2 could be addressed later depending on the roadmap.

What do you think @markpeek ? Happy to implement that if you're ready to merge soon after.

@nielslaukens
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# Add it to the template
template.rules.append(check_region_rule)

template.rules is a dict, so you would need to call .update() instead of .append(). But, yes, this is current usage.

That does not seem consistent with the way of doing things in Template for two reasons:

  1. Developer needs to hack into the .rules property using append(). Would make sense to have a Template.add_rule() method instead.

Yes, you are correct. As I mentioned in my original PR: "This adds minimal support for Rules in templates. It provides a bare dict, and exports it to JSON/YAML." This needs more work to provide better usability. I wanted to find out if this would be acceptable before putting in the extra effort.

@markpeek
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I checked with a contact on the CloudFormation team. Rules are supported for Service Catalog and not CloudFormation. Having heard that, I will accept this PR (when it's good to go) to implement it for CloudFormation but obviously can't make promises about AWS continuing to allow its use.

I would suggest people wanting this as a supported feature to make it known to AWS through their forums, support channels, and/or your sales reps.

@rkz
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rkz commented Jun 28, 2019

Thanks for checking with AWS. I also verified on my side, uploading the following template to CloudFormation:

{
    "Description": "Testing the 'Rules' feature at the template level",
    "Parameters": {
        "Parameter1": {
            "Type": "String"
        },
        "Parameter2": {
            "Type": "String"
        }
    },
    "Resources": {
        "MyInstance": {
            "Properties": {
                "ImageId": "ami-01b282b0f06ba5fd2",
                "InstanceType": "t3.nano"
            },
            "Type": "AWS::EC2::Instance"
        }
    },
    "Rules": {
        "DemoRule": {
            "Assertions": [
                {
                    "Assert": {
                        "Fn::Equals": [
                            {
                                "Ref": "Parameter1"
                            },
                            {
                                "Ref": "Parameter2"
                            }
                        ]
                    },
                    "AssertDescription": "Parameters must match"
                }
            ]
        }
    }
}

It does bring validators to the UI, checking that Parameter1 and Parameter2 are equal. The Rules feature for CloudFormation is working but not documented.

I'll prepare the PR with docstrings mentioning the feature is undocumented.

@rkz rkz mentioned this pull request Jun 28, 2019
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@rkz
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rkz commented Jun 28, 2019

Final PR ready: #1446

Thanks @nielslaukens and @markpeek for your reactivity!

@markpeek markpeek merged commit dd06320 into cloudtools:master Jun 30, 2019
@markpeek
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Thanks @nielslaukens and @rkz!

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3 participants
@nielslaukens @markpeek @rkz
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