GCP Professional Cloud Architect Practice Question

Your team delivers a Go microservice that uses Google Cloud client libraries for BigQuery and Pub/Sub. The same container image must run in three locations:

Developers' laptops during local integration tests
A Cloud Build continuous-integration job
A production Cloud Run service in the europe-west1 region
The service code must obtain credentials automatically in every environment without any code changes or long-lived key files baked into the image. What should you do?

Store an OAuth 2.0 client ID and secret in Secret Manager; have the container retrieve them at startup and perform a three-legged OAuth web flow to obtain access tokens.
Use Application Default Credentials everywhere: instruct developers to run "gcloud auth application-default login", configure Cloud Build to run with its default service account, and assign a dedicated IAM service account to the Cloud Run service. Rely on the client libraries to obtain tokens from each environment's metadata server or well-known file.
Require developers and Cloud Build to pass a user access token as a command-line flag at runtime, and configure the Cloud Run service with the run.invoker IAM role only.
Embed a JSON key for a single service account inside the container and set the GOOGLE_APPLICATION_CREDENTIALS environment variable to that path in all three environments.

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Answer Description

Application Default Credentials (ADC) discover usable credentials in a fixed order.

On a developer's workstation, running "gcloud auth application-default login" places user credentials in the well-known file location. The client library then picks them up automatically.
Cloud Build automatically runs each build step under the Cloud Build service account. Because a metadata server is exposed inside the build step, ADC retrieves a short-lived access token for that service account without needing any key file.
Cloud Run lets you specify a service account for the revision. When the container starts, the Cloud Run metadata server delivers tokens for that account, and ADC picks them up.
This approach meets the requirement with no code changes and no embedded keys. The other options either embed long-lived secrets in the image or rely on OAuth client credentials that are not intended for server-to-server workloads, creating additional operational and security burden.

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