(aws-cloudfront): Unexpected diffs caused by cloudfront.Function

[iliapolo]

Non deterministic auto-generated function name causing unexpected diffs.

Reproduction Steps

Using this code:

new cloudfront.Function(stack, 'Function', {
  code: cloudfront.FunctionCode.fromInline('function handler(event) { return event.request }'),
});

1. synth and extract the function name
2. synth again and extract the function name

The function name is different for every synth.

What did you expect to happen?

The function name should be the same.

What actually happened?

Function name changes per synth.

Environment

• CDK CLI Version : ALL
• Framework Version: 1.109.0
• Node.js Version: ALL
• OS : ALL
• Language (Version): ALL

Other

This happens because of:

aws-cdk/packages/@aws-cdk/aws-cloudfront/lib/function.ts

Lines 178 to 185 in 94f81c4

	private generateName(): string {
	const name = Stack.of(this).region + Names.uniqueId(this);
	if (name.length > 64) {
	return name.substring(0, 32) + name.substring(name.length - 32);
	}
	return name;
	}
	}

Where Stack.of(this).region is an unresolved token that can have a different sequence number every time the program is executed.

A workaround right now would be to provide a name explicitly and not rely on this logic.

Workaround

Provide an explicit name for cloudfront.Function. Using this.node.addr or Node.of(this).addr will return a stable fixed length unique id. Example:

const functionId = `MyFunction${this.node.addr}`;

new cloudfront.Function(stack, functionId, {
  code: cloudfront.FunctionCode.fromInline('function handler(event) { return event.request }'),
  functionName: functionId
});

---

This is 🐛 Bug Report

[hoegertn]

Shouldn't the region be !Ref AWS::Region in the template and be stable?

[eladb]

I would recommend using Node.of(this).addr which returns a stable fixed length unique id

[hoegertn]

@njlynch 's wish was to add the region information as CF is a global service. But if this is no longer needed happy to remove it. This would be a breaking change for the implementation afaik.

[eladb]

@njlynch 's wish was to add the region information as CF is a global service. But if this is no longer needed happy to remove it. This would be a breaking change for the implementation afaik.

What kind of breaking change?

[hoegertn]

If we change the generation of the name it will recreate the CF functions on the next deployment. The name is the physicalName of the function.

[eladb]

Will it cause outage or availability risk?

[hoegertn]

The ARN will change and this will break CF Distributions using it if not within the same stack. A short outage might as well be a thing as it removes one CFF and then registers the new CFF.

[eladb]

I think CFN will first create the new CFF, update the distribution and only then remove the old one (that's how resource replacement works)

[hoegertn]

I meant on the distribution. So there might be a time where no CFF is registered.

[jogold]

Where Stack.of(this).region is an unresolved token that can have a different sequence number every time the program is executed.

I don't understand how it can be different... at synth time the token is resolved no? so you get a Fn::Join:

aws-cdk/packages/@aws-cdk/aws-cloudfront/test/function.test.ts

Lines 48 to 58 in 94f81c4

	Name: {
	'Fn::Join': [
	'',
	[
	{
	Ref: 'AWS::Region',
	},
	'StackCF2CE3F783F',
	],
	],
	},

If you snapshot it, it should always return the same Name?