Great question—this is such a common sticking point when working with Terraform and GCP, especially since the docs don’t always call this out front-and-center. Let’s break this down clearly:

First, What Do .name and .id Actually Mean? #

For most GCP Terraform resources:

• .name is the short, human-readable name of the resource—this is usually the value you set in the name field of your resource block (or a generated name if you didn’t specify one explicitly). For your Pub/Sub topic, this would be my_topic.
• .id is the full GCP resource identifier, formatted as projects/[your-project-id]/[resource-type]/[resource-name]. For your topic, that would look like projects/your-project-id/pubsub/topics/my_topic.

The Core Rule: Match the Parameter’s Required Format #

There’s no universal list of "which resources use which"—it all depends on what the target resource’s parameter expects. Here’s how to figure it out:

1. Check the Terraform Provider Docs: For the resource you’re configuring (e.g., google_pubsub_subscription or google_cloudiot_registry), look up the specific parameter you’re setting (like topic or pubsub_topic_name). The docs will explicitly state if it expects a short name or a full resource ID.
2. Apply to Your Example:
   • For google_pubsub_subscription’s topic parameter: The docs note it accepts "the name of the topic"—but GCP and Terraform are flexible here; you can pass either .name or .id, and Terraform will resolve it correctly. Your use of .name works fine here.
   • For google_cloudiot_registry’s pubsub_topic_name parameter: The docs explicitly require "the full resource name of a Cloud Pub/Sub topic". That’s why you need to use .id here—using .name would throw an error because GCP can’t resolve the short name to a full topic path in this context.

Common Scenarios to Remember #

• Use .name when: The parameter asks for a simple, short resource name. Examples include referencing a Compute Engine instance’s name in a firewall rule, or a Cloud Storage bucket’s name in an IAM binding.
• Use .id when: The parameter requires a full resource path. Examples include linking a Cloud Function to a Pub/Sub topic (the trigger_topic parameter), or setting a logging sink destination (which needs the full resource ID of a bucket/topic).

Pro Tip #

If you’re ever unsure, you can run terraform plan and check the output—if you used the wrong attribute, Terraform will usually throw an error telling you it expected a full resource path (or vice versa). You can also inspect the state file (with terraform state show google_pubsub_topic.topic) to see exactly what values .name and .id hold for your resource.

内容的提问来源于stack exchange，提问作者Sekhar