Use Fn::ImportValue for sns and streams

[konstantinvlasenko]

This is a Feature Proposal

Description

It is nice that we have the option to specify an ARN of existing sns or DynamoDB stream. This approach is good for pets but not for cattle. We don't provide the names for our resources. Hardcoded string inside a serverless.yml doesn't work for us.

For feature proposals:

• Ability to specify a sns or stream property by using the intrinsic function Fn::ImportValue

• This PR add #3188 #3189 introduced a generic helper AwsProvider#resolveExportValue. It is in use to resolve deploymentBucket right now

• AwsProvider#resolveExportValue can be used compileSNSEvents method of class compileSNSEvents

  serverless/lib/plugins/aws/deploy/compile/events/sns/index.js

  Lines 28 to 29 in ed4a66d

  	if (typeof event.sns === 'object') {
  	['topicName', 'displayName'].forEach((property) => {

  . We just need to bring AwsProvider#resolveExportValue promise into forEach loop

  serverless/lib/plugins/aws/deploy/compile/events/sns/index.js

  Line 22 in ed4a66d

  functionObj.events.forEach(event => {

[pmuens]

Thanks for the suggestion @konstantinvlasenko 👍

Closing this issue since we're discussing a global arn parser in #3212

[manwaring]

I encountered the same limitation this past week and have been thinking about ways to work around it. Using the $(cf) variable syntax allows me to pass in the arn correctly but by evaluating and injecting the string directly I lose the benefit of CloudFormation enforcing referential integrity with that approach (e.g. I can modify/delete the referenced resource even though resources in another stack depend on it). I would much prefer to use the Fn::ImportValue syntax to prevent accidental modification/deletion from happening.

Stream events already support both Fn::GetAtt as well as Fn::ImportValue via the code block below, and something similar could easily be implemented for SNS.

const streamName = (function () {
  if (EventSourceArn['Fn::GetAtt']) {
    return EventSourceArn['Fn::GetAtt'][0];
  } else if (EventSourceArn['Fn::ImportValue']) {
    return EventSourceArn['Fn::ImportValue'];
  }
  return EventSourceArn.split('/')[1];
}());

@pmuens it looks like the global arn parser feature (proposed just over a year ago) is on hold / waiting for support. In the meantime, would you be open to a feature proposal to bring intrinsic function support to SNS events the same way that it has been implemented for stream events? It's not as universally beneficial as the universal arn parser, obviously, but it follows an existing pattern and is much simpler to implement in the here and now :)