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GCP: Managing IAM Access Control Across Projects -- The Simpler Version
GCP resources are organized into projects – all resource IDs and IAM principles are grouped under a project ID. This means that by default roles assigned to a principle (e.g. a user or service account) are scoped only to project resources. This can be tricky if say your images are in one project’s storage bucket and your app is running in another
If you want to provide a service principle in one project access to resources in another , the approach is not obvious, nor is it well documented.