Aws-cdk: s3-deployment: blue/green

[eladb]

See #936 for context. There may be use cases where users would want to deploy each update to a new subfolder in the destination bucket to support "blue/green" deployments (old users can still access the old contents and new users are directed to the new content).

This can be achieved by allocating a unique ID during deployment and returning it as resource attribute so it can be used downstream (e.g. by CloudFront).

[eladb]

Still releavnt

[gavindoughtie-aon]

was about to implement this myself, so yes. Please do it for me.

[eladb]

We can combine something like S3File combined with toJsonString with this to allow web apps to find AWS resources defined in the same CDK apps and referenced through attributes.

[ericzbeard]

This feature will help with the difficulties around configuring a single page web app. If your stack has resources like an API Gateway or Cognito User Pools, you have to use a custom resource to create a config file with environment-specific variables that is copied into the site bucket after deployment. This can lead to downtime since there is a small window between the deployment of the static resources and the addition of configuration file. Doing a blue green deployment would solve this part of the problem.