The Essential Guide to Migrating Your GCP Classic Load Balancer to the Global External Load Balancer

The Global External Application Load Balancer is the latest evolution of Google Cloud’s Layer 7 load balancing service. It provides a modern control plane with enhanced traffic management, security, and global reach. If your applications are still running behind a Classic Application Load Balancer, it’s time to upgrade to take advantage of these new capabilities. This guide provides a step-by-step walkthrough of the migration process using the gcloud command-line tool, following a safe, staged approach to ensure zero downtime.

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Understanding the Migration Strategy

The migration process is a managed operation handled by GCP. It’s designed to be non-disruptive, allowing you to gracefully shift traffic from the old infrastructure to the new. The core strategy involves a two-part phased migration:

1. Backend Service Migration: This prepares your backend resources (like Managed Instance Groups or NEGs) to serve traffic from the new load balancer infrastructure.
2. Forwarding Rule Migration: This updates the frontend, which handles incoming traffic, to use the new control plane.

For each of these steps, you’ll progress through four distinct states: PREPARE, TEST_BY_PERCENTAGE, TEST_ALL_TRAFFIC, and MIGRATE. This staged approach gives you full control and allows for testing at each stage.

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Step 1: Upgrade the Backend Service

This phase prepares your backend infrastructure to be compatible with the new load balancer. The gcloud beta compute backend-services update command is used for this entire process.

a. Prepare the Backend

This command places the backend service in a state where it can start accepting traffic from the new load balancer without changing the current traffic flow.

gcloud beta compute backend-services update [BACKEND_SERVICE_NAME] \
--external-managed-migration-state=PREPARE \
--global

b. Test with a Percentage of Traffic

Once the backend is prepared, you can begin sending a small, controlled amount of traffic to the new infrastructure. This lets you monitor performance and logs to ensure there are no issues.

gcloud beta compute backend-services update [BACKEND_SERVICE_NAME] \
--external-managed-migration-state=TEST_BY_PERCENTAGE \
--external-managed-migration-testing-percentage=10 \
--global

You can increase the --external-managed-migration-testing-percentage as you gain confidence.

c. Test with All Traffic

After successful testing with a percentage of traffic, you can route all traffic through the new load balancer’s infrastructure. The backend service is now fully migrated, but the forwarding rule is not yet updated.

gcloud beta compute backend-services update [BACKEND_SERVICE_NAME] \
--external-managed-migration-state=TEST_ALL_TRAFFIC \
--global

d. Complete Backend Migration

The final step for the backend service is to permanently set its load balancing scheme to the new EXTERNAL_MANAGED type.

gcloud beta compute backend-services update [BACKEND_SERVICE_NAME] \
--external-managed-migration-state=MIGRATE \
--global

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Step 2: Upgrade the Forwarding Rule

With the backend successfully migrated, the next step is to update the frontend forwarding rule to use the new infrastructure. This is also a staged process, mirroring the steps for the backend. Use the gcloud beta compute forwarding-rules update command for this.

a. Prepare the Forwarding Rule

gcloud beta compute forwarding-rules update [FORWARDING_RULE_NAME] \
--external-managed-migration-state=PREPARE \
--global

b. Test with a Percentage of Traffic

gcloud beta compute forwarding-rules update [FORWARDING_RULE_NAME] \
--external-managed-migration-state=TEST_BY_PERCENTAGE \
--external-managed-migration-testing-percentage=10 \
--global

c. Test with All Traffic

gcloud beta compute forwarding-rules update [FORWARDING_RULE_NAME] \
--external-managed-migration-state=TEST_ALL_TRAFFIC \
--global

d. Complete Forwarding Rule Migration

This final command updates the forwarding rule’s load balancing scheme to the EXTERNAL_MANAGED type, completing the full migration.

gcloud beta compute forwarding-rules update [FORWARDING_RULE_NAME] \
--external-managed-migration-state=MIGRATE \
--global

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The Importance of Rollback ↩️

GCP’s managed migration process includes a built-in rollback capability, which is crucial for safety. If you encounter any issues during the migration, you can revert the state.

• To roll back to the previous state, you would use the same update command but change the --external-managed-migration-state flag to the desired prior state. For example, to revert from TEST_ALL_TRAFFIC back to TEST_BY_PERCENTAGE:

  gcloud beta compute forwarding-rules update [FORWARDING_RULE_NAME] \
  --external-managed-migration-state=TEST_BY_PERCENTAGE \
  --external-managed-migration-testing-percentage=50 \
  --global
  

• For a full rollback to the classic load balancer, you would change the load balancing scheme of the forwarding rule back to EXTERNAL. The rollback functionality is available for a limited time (90 days) after the migration is completed.

By following this controlled, staged process, you can upgrade your load balancer with confidence, unlocking a new level of performance and features while minimizing any risk to your application’s availability.