Migration overview

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This page gives you an overview of the differences between the global external Application Load Balancer and classic Application Load Balancer, and a detailed overview of how you can migrate your existing classic Application Load Balancer resources to the global external Application Load Balancer.

To take advantage of the advanced traffic management features of the global external Application Load Balancer, we recommend that you migrate your classic Application Load Balancer resources to the global external Application Load Balancer infrastructure.

Compare classic Application Load Balancers and global external Application Load Balancers

Before you migrate resources, understand the differences between classic Application Load Balancers and global external Application Load Balancers.

Feature differences

The following features are not supported with the global external Application Load Balancer. They are only available with the classic Application Load Balancer:

• Standard Network Tier

• GKE Ingress controller

  To deploy global external Application Load Balancers for GKE clusters, use the GKE Gateway controller instead. For setup instructions, see Deploying Gateways.

Data plane differences

The following table highlights differences in the data plane between the classic Application Load Balancer and the global external Application Load Balancer. These differences affect how the load balancers respond to some common events.

Event	Classic Application Load Balancer response	Global external Application Load Balancer response
Status/error codes
All backends are unhealthy	Returns HTTP 502	Returns HTTP 503
Request uses a banned SSL cipher	Returns HTTP 502	Returns HTTP 503
Early upstream connection reset by the backend	Returns HTTP 502	Returns HTTP 503
Failed connection upgrade (for example, while upgrading to Websockets)	Returns HTTP 400	Returns HTTP 403
Header is too large	Returns HTTP 413	Returns HTTP 431
Quotas and limits
URL map configuration	There are significant differences in the URL map configuration limits between the two load balancers. For details, see the Quotas: URL maps documentation.
Header handling
Request uses a custom HTTP method with no body	Adds the Transfer Encoding: Chunked header to request sent to backend	Adds the Content-Length: 0 header to request sent to backend
Format of the X-Forwarded-For header added to requests sent to backend	Uses ', ' delimiter between IPs	Uses ',' delimiter between IPs (no space after the comma)
Header case preservation	Header case is preserved	All header keys are transformed to lowercase
Repeated headers with the same name	Allowed	Repeated headers may be combined into a single header, with the values appended in order and comma separated, as allowed by RFC 7230.
(HTTP/1.1 only) Invalid header name (for example, unsupported characters in header)	Allowed (for HTTP/1.1)	Returns HTTP 502 (for HTTP/1.1)
(HTTP/1.1 only) Repeated (but same) Content-Length header in request	Allowed (for HTTP/1.1)	Returns HTTP 502 (for HTTP/1.1)
(HTTP/1.1 only) Multiple hosts in header	When 2 or more hosts are added, and the first one is valid, the header is accepted	When 2 or more hosts are added, and any are invalid, the load balancer returns HTTP 502
(HTTP/1.1 only) Connection: Keep-Alive header	Adds the Keep-Alive header in requests sent to backend by default	Does not add this header by default
Request handling
Backwards slashes in request	URL left unchanged	Converts to forward slash
Merge duplicate slashes in request	Leaves slashes unmerged	Merges slashes
`#` in request path	Allowed	Returns HTTP 400
(HTTP/1.1 only) Illegal characters in request path (for example, `\\x7f\\x7f`)	Allowed (for HTTP/1.1)	Returns HTTP 502 (for HTTP/1.1)
Traffic distribution (URL map configuration)
Client request includes a port number	The port number is ignored even if you have configured hosts with ports in the URL map. Only the hostname is considered. For example, requests for example.com:5000 are matched to the backend service for example.com.	Both the hostname and the port number are considered. For example, requests for example.com:5000 are matched to the backend service for example.com:5000. If there isn't a match, the default backend service is used.

To learn more about the differences and supported features, see Load balancer feature comparison and Application Load Balancer overview.

Migrate from classic to global external Application Load Balancer

To migrate to the global external Application Load Balancer, you change the load balancing scheme of your load balancing resources—specifically, the backend services and forwarding rules—from EXTERNAL to EXTERNAL_MANAGED. To do this, you perform a series of migration steps where you can test parts of your network traffic with the new load balancing scheme before actually finalizing the migration. During resource migration, you control what percentage of requests is sent to either the classic Application Load Balancer infrastructure or the global external Application Load Balancer infrastructure.

The following diagram shows the load balancing schemes of your load balancing resources before and after the migration.

Migration process for classic Application Load Balancer resources (click to enlarge).

In the previous diagram, note the following:

• Before resources are migrated, all requests use the classic Application Load Balancer infrastructure.
• While the resources are migrated, some requests are sent to the global external Application Load Balancer infrastructure and the remaining requests are sent to the classic Application Load Balancer infrastructure.
• After resources are migrated, all requests use the global external Application Load Balancer infrastructure.

To help ensure a smooth transition, migrate the following classic Application Load Balancer resources in the specified order:

1. Migrate all backend services attached to the load balancer's forwarding rules.

2. Migrate all backend buckets attached to the load balancer's forwarding rules. You do this at the forwarding rule level because backend buckets don't have load balancing schemes.

3. Migrate the load balancer's forwarding rules.

   You can only migrate a forwarding rule after all the backend services and backend buckets attached to the forwarding rule have already been migrated.

Migration states

You migrate resources by setting them to different states before changing their load balancing scheme to EXTERNAL_MANAGED.

1. PREPARE: set a resource to this state to prepare it for migration.
2. TEST_BY_PERCENTAGE: to test a prepared resource, set the resource to this state to send a percentage of the network traffic. This stage is optional.
3. TEST_ALL_TRAFFIC: set a resource to this state to send all network traffic to the resource through the global external Application Load Balancer infrastructure instead of the classic Application Load Balancer infrastructure.

Note: After you change a resource's state from PREPARE to TEST_BY_PERCENTAGE or TEST_ALL_TRAFFIC, wait for some time (approximately six minutes) for the resource to be ready to accept traffic. The resource must be in the TEST_ALL_TRAFFIC state before you change the load balancing scheme.

Migration process

To help ensure no downtime, you migrate resources in a specific order from the classic Application Load Balancer infrastructure to the global external Application Load Balancer infrastructure, and then change the load balancing scheme from EXTERNAL to EXTERNAL_MANAGED to finalize the migration.

1. Migrate the load balancer's backend services.

   Repeat the following steps for each backend service.

   1. Prepare the backend service for migration.

      Before any traffic can be sent through the global external Application Load Balancer infrastructure, set the state of the backend service to PREPARE. This prepares the backend service to handle global external Application Load Balancer infrastructure network traffic. A backend service takes some time (approximately six minutes) to be ready to send traffic through the global external Application Load Balancer infrastructure.

   2. Optional: Test the prepared backend service.

      After the backend service is in the PREPARE state, set its state to TEST_BY_PERCENTAGE and set a percentage of the classic Application Load Balancer network traffic to the global external Application Load Balancer infrastructure.

      Though this stage is optional, we recommend that you test the traffic before migrating a backend service. Start with a small percentage value and monitor logs of the resources. If the backend service behaves as expected, gradually increase the percentage to 100%.

      In the TEST_BY_PERCENTAGE state, you can't use the additional capabilities of the global external Application Load Balancer infrastructure.

   3. Send all classic Application Load Balancer network traffic to the prepared backend service.

      After the testing of the backend service is successful, set its state to TEST_ALL_TRAFFIC and send all classic Application Load Balancer network traffic to the prepared backend service. A backend service takes some time (approximately six minutes) to be ready to handle the network traffic.

      In the TEST_ALL_TRAFFIC state, you can't use the additional capabilities of the global external Application Load Balancer infrastructure.

   4. Change the load balancing scheme of migrated backend service to EXTERNAL_MANAGED.

      After testing your prepared backend service on the global external Application Load Balancer infrastructure, change its load balancing scheme to EXTERNAL_MANAGED. A backend service takes some time (approximately six minutes) to be fully migrated. After the load balancing scheme of the backend service changes to EXTERNAL_MANAGED, you can use the advanced features of the global external Application Load Balancer infrastructure.

2. Migrate the load balancer's backend buckets. You do this at the forwarding rule level because backend buckets don't have load balancing schemes.

   Repeat the following steps for each bucket.

   1. Prepare the backend bucket for migration.

      Before any traffic can be sent through the global external Application Load Balancer infrastructure, set the state of the backend bucket to PREPARE, and wait for some time (approximately six minutes).

   2. Optional: Test the prepared backend service.

      After the backend bucket is in the PREPARE state, set its state to TEST_BY_PERCENTAGE and set a percentage of the classic Application Load Balancer network traffic to the global external Application Load Balancer infrastructure.

   3. Send all classic Application Load Balancer network traffic to the prepared backend bucket.

      Set the state of the backend bucket to TEST_ALL_TRAFFIC and send all classic Application Load Balancer network traffic to it. A backend bucket takes some time (approximately six minutes) to be ready to handle the network traffic.

      In the TEST_ALL_TRAFFIC state, you can't use the additional capabilities of the global external Application Load Balancer infrastructure.

3. Migrate the forwarding rules.

   For each forwarding rule, change the load balancing scheme of the forwarding rule to EXTERNAL_MANAGED, and wait for some time (approximately six minutes). After the load balancing scheme of the forwarding rule changes to EXTERNAL_MANAGED, you can use the advanced features of the global external Application Load Balancer infrastructure.

For a detailed step-by-step process, see Migrate resources from classic Application Load Balancer.

The following diagram shows the classic Application Load Balancer resources in the different states of migration.

Migration states of classic Application Load Balancer resources (click to enlarge).

After migrating a resource, if you want to roll it back to classic Application Load Balancer, change its load balancing scheme within 90 days of its migration. You can't roll back a resource after the 90-day window has passed.

Using session affinity

Consider the following caveats when migrating classic Application Load Balancer backend services with session affinity:

• Setting a value for TEST_BY_PERCENTAGE redirects some traffic targeting the classic Application Load Balancer to the global external Application Load Balancer. This breaks the client IP affinity. Changing the migration percentage (for example, increasing it by 10%) breaks the session affinity for the same percentage of client IP addresses (10% in this example), until the affinity is re-established on the next request.

• Setting a value for TEST_BY_PERCENTAGE redirects that percentage of traffic without a session cookie to the global external Application Load Balancer. It also redirects all traffic with a session cookie to the load balancer fleet that generated the cookie.

• Setting a value for TEST_BY_PERCENTAGE to 0%, or leaving it unset, or setting the backend service to the PREPARE state, invalidates all existing cookies that are directed to the global external Application Load Balancer.

• Changing the load balancing scheme of the backend service to EXTERNAL_MANAGED invalidates all cookies generated by the classic Application Load Balancer fleet. This lets you roll back traffic if there's an issue with your application using the global external Application Load Balancer. For example, if a client presents a cookie from the classic Application Load Balancer fleet, but the backend service scheme is EXTERNAL_MANAGED, the client's cookie is not honored. The global external Application Load Balancer ignores the cookie and creates a new one.

Roll back migrated resources

After migrating a resource, if you want to roll it back from the global external Application Load Balancer infrastructure to the classic Application Load Balancer infrastructure, you can do so within 90 days of changing its load balancing scheme.

Rolling back a backend service to the EXTERNAL scheme requires rolling back the forwarding rule. Rolling back a forwarding rule to the EXTERNAL scheme doesn't require rolling back the backend services.

To roll back resources, follow these steps:

1. Remove any new advanced traffic management features of global external Application Load Balancer configured on the resource.

2. Roll back the forwarding rules.

   Change the load balancing scheme of forwarding rules from EXTERNAL_MANAGED to EXTERNAL.

3. Roll back the backend buckets.

   1. Set the migration state of backend buckets to TEST_ALL_TRAFFIC, and wait for some time (approximately six minutes).
   2. Optional: To decrease the traffic, set the migration state of backend buckets to TEST_BY_PERCENTAGE and set a percentage of the traffic.
   3. Set the migration state of backend buckets to PREPARE.
   4. Revert backend buckets to their pre-migration states.

4. Roll back the backend services.

   1. Set the migration state of backend services to TEST_ALL_TRAFFIC, and wait for some time (approximately six minutes).
   2. Optional: To decrease the traffic, set the migration state of backend services to TEST_BY_PERCENTAGE and set a percentage of the traffic.
   3. Set the migration state of backend services to PREPARE.
   4. Revert backend services to their pre-migration states.

For a detailed step-by-step process, see Roll back migrated resources to classic Application Load Balancer.

Track your migration process

While migrating your resources, you can check their load balancing schemes by viewing the following:

• The logging and monitoring dashboards of the global external Application Load Balancer. For more information, see Global external Application Load Balancer logging and monitoring.

• The values of the following HTTP request and response headers:

  • X-External-Managed-Migration-Scheme-Override: this request header routes the request based on its value. If the header value is EXTERNAL, it directs the request to the classic Application Load Balancer infrastructure. If the value is EXTERNAL_MANAGED, it routes the request through the global external Application Load Balancer infrastructure.

    Use this header to direct a request to a particular load balancer fleet.

  • X-External-Managed-Migration-Selected-Scheme: this request and response header informs the backend and the client about the load balancing scheme used to route the request. The header is returned to the client and passed to the customer's backend.

    If the request is routed through the classic Application Load Balancer infrastructure, its value is EXTERNAL. If the request is routed through the global external Application Load Balancer infrastructure, its value is EXTERNAL_MANAGED.

What's next

• Migrate resources from classic Application Load Balancer
• Roll back migrated resources to classic Application Load Balancer