HTTP(S) Load Balancing Concepts

This document introduces the concepts you need to understand to configure HTTP or HTTPS Load Balancing.


An HTTP(S) load balancer is composed of several components. The following diagram illustrates the architecture of a complete HTTP(S) load balancer:

Cross-region load balancing diagram (click to enlarge)
Cross-region load balancing diagram (click to enlarge)

The following sections describe how each component works together to make up each type of load balancer. For a detailed description of each component, see Components below.

HTTP Load Balancing

A complete HTTP load balancer is structured as follows:

  1. A global forwarding rule directs incoming requests to a target HTTP proxy.
  2. The target HTTP proxy checks each request against a URL map to determine the appropriate backend service for the request.
  3. The backend service directs each request to an appropriate backend based on serving capacity, zone, and instance health of its attached backends. The health of each backend instance is verified using an HTTP health check, an HTTPS health check, or an HTTP/2 health check. If the backend service is configured to use an HTTPS or HTTP/2 health check, the request will be encrypted on its way to the backend instance.
  4. Sessions between the load balancer and the instance can use the HTTP, HTTPS, or HTTP/2 protocol. If you use HTTPS or HTTP/2, each instance in the backend services must have an SSL certificate.

HTTPS Load Balancing

An HTTPS load balancer has the same basic structure as an HTTP load balancer (described above), but differs in the following ways:

Client communications with the load balancer

  • Clients can communicate with the load balancer using the HTTP 1.1 or HTTP/2 protocol.
  • When HTTPS is used, modern clients default to HTTP/2. This is controlled on the client, not on the HTTPS load balancer.
  • You cannot disable HTTP/2 by making a configuration change on the load balancer itself. However, you can configure some clients to use HTTP 1.1 instead of HTTP/2. For example, with curl, use the --http1.1 parameter.


The following are components of HTTP(S) load balancers.

Global forwarding rules and addresses

Global forwarding rules route traffic by IP address, port, and protocol to a load balancing configuration consisting of a target proxy, URL map, and one or more backend services.

Each global forwarding rule provides a single global IP address that can be used in DNS records for your application. No DNS-based load balancing is required. You can either specify the IP address to be used or let Google Cloud Load Balancing assign one for you.

Target proxies

Target proxies terminate HTTP(S) connections from clients. One or more global forwarding rules direct traffic to the target proxy, and the target proxy consults the URL map to determine how to route traffic to backends.

The proxies set HTTP request/response headers as follows:

  • Via: 1.1 google (requests and responses)
  • X-Forwarded-Proto: [http | https] (requests only)
  • X-Forwarded-For: <unverified IP(s)>, <immediate client IP>, <global forwarding rule external IP>, <proxies running in GCP> (requests only)
    A comma-separated list of IP addresses appended by the intermediaries the request traveled through. If you are running proxies inside GCP that append data to the X-forwarded-For header, then your software must take into account the existence and number of those proxies. Only the <immediate client IP> and <global forwarding rule external IP> entries are provided by the load balancer. All other entries in the list are passed along without verification. The <immediate client IP> entry is the client that connected directly to the load balancer. The <global forwarding rule external IP> entry is the external IP address of the load balancer's forwarding rule. If there are more entries than that, then the first entry in the list is the address of the original client. Other entries before the <immediate client IP> entry represent other proxies that forwarded the request along to the load balancer.
  • X-Cloud-Trace-Context: <trace-id>/<span-id>;<trace-options> (requests only)
    Parameters for Stackdriver Trace.

You can create custom request headers if the default headers do not meet your needs. For more information on this feature, see User-defined request headers.

URL maps

URL maps define matching patterns for URL-based routing of requests to the appropriate backend services. A default service is defined to handle any requests that do not match a specified host rule or path matching rule. In some situations, such as the cross-region load balancing example, you might not define any URL rules and rely only on the default service. For content-based routing of traffic, the URL map allows you to divide your traffic by examining the URL components to send requests to different sets of backends.

SSL certificates

If you are using HTTPS Load Balancing, you must install one or more SSL certificates on the target HTTPS proxy. You can have up to ten (10) SSL certificates installed. They are used by target HTTPS proxies to authenticate communications between the load balancer and the client. These can be Google-managed or self-managed SSL certificates.

For more information on installing SSL certificates for an HTTPS load balancer, see SSL Certificates.

If you are using HTTPS or HTTP/2 from the load balancer to the backends, you must install SSL certificates on each VM instance. To install SSL certificates on a VM instance, use the instructions in your application documentation. These certificates can be self-signed.

SSL policies

SSL policies give you the ability to control the features of SSL that your HTTPS load balancer negotiates with HTTPS clients.

By default, HTTPS Load Balancing uses a set of SSL features that provides good security and wide compatibility. Some applications require more control over which SSL versions and ciphers are used for their HTTPS or SSL connections. You can define SSL policies that control the features of SSL that your load balancer negotiates and associate an SSL policy with your target HTTPS proxy.

Backend services

Backend services direct incoming traffic to one or more attached backends. Each backend is composed of an instance group and additional serving capacity metadata. Backend serving capacity can be based on CPU or requests per second (RPS).

Each backend service also specifies which health checks will be performed against the available instances.

HTTP(S) Load Balancing supports Cloud Load Balancing Autoscaler, which allows users to perform autoscaling on the instance groups in a backend service. For more information, see Scaling Based on HTTP Load Balancing serving capacity.

You can enable connection draining on backend services to ensure minimal interruption to your users when an instance that is serving traffic is terminated, removed manually, or removed by an autoscaler. To learn more about connection draining, read the Enabling Connection Draining documentation.

Protocol to the backends

When you configure a backend service for the HTTP(S) load balancer, you set the protocol that the backend service uses to communicate with the backends. You can choose HTTP, HTTPS, or HTTP/2. The load balancer uses only the protocol that you specify. The load balancer does not fall back one of the other protocols if it is unable to negotiate a connection to the backend with the specified protocol.

Use an HTTP/2 health check if you are using HTTP/2 to the backend VMs.

Using gRPC with your Google Cloud Platform applications

gRPC is an open-source framework for remote procedure calls. It is based on the HTTP/2 standard. Use cases for gRPC include the following:

  • Low latency, highly scalable, distributed systems
  • Developing mobile clients that communicate with a cloud server
  • Designing new protocols that must be accurate, efficient, and language independent
  • Layered design to enable extension, authentication, and logging

To use gRPC with your Google Cloud Platform applications, you must proxy requests end-to-end over HTTP/2. To do this with an HTTP(S) load balancer:

  1. Configure an HTTPS load balancer.
  2. Enable HTTP/2 as the protocol from the load balancer to the backends.

The load balancer negotiates HTTP/2 with clients as part of the SSL handshake by using the ALPN TLS extension.

The load balancer may still negotiate HTTPS with some clients or accept insecure HTTP requests on an HTTP(S) load balancer that is configured to use HTTP/2 between the load balancer and the backend instances. Those HTTP or HTTPS requests are transformed by the load balancer to proxy the requests over HTTP/2 to the backend instances.

For information on troubleshooting problems with HTTP/2, see Troubleshooting issues with HTTP/2 to the backends. For information on HTTP/2 limitations, see HTTP/2 limitations.

Backend buckets

Backend buckets direct incoming traffic to Google Cloud Storage buckets. See Adding a Cloud Storage bucket to content-based load balancing for an example of adding buckets to an existing load balancer setup.

Firewall rules

You must create a firewall rule that allows traffic from and to reach your instances. These are IP address ranges that the load balancer uses to connect to backend instances. This rule allows traffic from both the load balancer and the health checker. The rule must allow traffic on the port your global forwarding rule has been configured to use, and your health checker should be configured to use the same port. If your health checker uses a different port, then you must create another firewall rule for that port.

Note that firewall rules block and allow traffic at the instance level, not at the edges of the network. They cannot prevent traffic from reaching the load balancer itself.

If you need to determine external IP addresses at a particular time, use the instructions in the Google Compute Engine FAQ.

Load distribution algorithm

HTTP(S) Load Balancing provides two methods of determining instance load. Within the backend service resource, the balancingMode property selects between the requests per second (RPS) and CPU utilization modes. Both modes allow a maximum value to be specified; the HTTP(S) load balancer tries to ensure that load remains under the limit, but short bursts above the limit can occur during failover or load spike events.

Incoming requests are sent to the region closest to the user, provided that region has available capacity. If more than one zone is configured with backends in a region, the traffic is distributed across the instance groups in each zone according to each group's capacity. Within the zone, the requests are spread evenly over the instances using a round-robin algorithm. You can override round-robin distribution by configuring session affinity. However, note that session affinity works best if you also set balancing mode to requests per second (RPS).

Session affinity

Session affinity sends all request from the same client to the same virtual machine instance as long as the instance stays healthy and has capacity.

GCP HTTP(S) Load Balancing offers two types of session affinity:

WebSocket proxy support

HTTP(S) Load Balancing has native support for the WebSocket protocol. Backends that use WebSocket to communicate with clients can use the HTTP(S) load balancer as a front end, for scale and availability. The load balancer does not need any additional configuration to proxy WebSocket connections.

The WebSocket protocol, which is defined in RFC 6455, provides a full-duplex communication channel between clients and servers. The channel is initiated from an HTTP(S) request.

When HTTP(S) Load Balancing recognizes a WebSocket Upgrade request from an HTTP(S) client and the request is followed by a successful Upgrade response from the backend instance, the load balancer proxies bidirectional traffic for the duration of the current connection. If the backend does not return a successful Upgrade response, the load balancer closes the connection.

The timeout for a WebSocket connection depends on the configurable response timeout of the load balancer, which is 30 seconds by default. This timeout is applied to WebSocket connections regardless of whether they are in use or not. For more information about the response timeout and how to configure it, refer to Timeouts and retries.

If you have configured either client IP or generated cookie session affinity for your HTTP(S) load balancer, all WebSocket connections from a client are sent to the same backend instance, provided the instance continues to pass health checks and has capacity.

QUIC protocol support for HTTPS Load Balancing

HTTPS Load Balancing supports the QUIC protocol in connections between the load balancer and the clients. QUIC is a transport layer protocol that provides congestion control similar to TCP and security equivalent to SSL/TLS for HTTP/2, with improved performance. QUIC allows faster client connection initiation, eliminates head-of-line blocking in multiplexed streams, and supports connection migration when a client's IP address changes.

QUIC affects connections between clients and the load balancer, not connections between the load balancer and backends.

The target proxy's QUIC override setting allows you to enable one of the following:

  • When possible, negotiate QUIC for a load balancer OR
  • Always disable QUIC for a load balancer.

If you specify no QUIC override, you allow Google to manage when QUIC is used. Google does not enable QUIC with no override specified. For information on enabling and disabling QUIC support, see Target Proxies.

How QUIC is negotiated

When you enable QUIC, the load balancer can advertise its QUIC capability to clients, allowing clients that support QUIC to attempt to establish QUIC connections with the HTTPS load balancer. Properly implemented clients always fall back to HTTPS or HTTP/2 when they cannot establish a QUIC connection. Because of this fallback, enabling or disabling QUIC in the load balancer does not disrupt the load balancer’s ability to connect to clients.

When you have QUIC enabled in your HTTPS load balancer, some circumstances can cause your client to fall back to HTTPS or HTTP/2 instead of negotiating QUIC. These include:

  • When a client supports versions of QUIC that are not compatible with the QUIC versions supported by the HTTPS load balancer
  • When the load balancer detects that UDP traffic is blocked or rate limited in a way that would prevent QUIC from working
  • If QUIC is temporarily disabled for HTTPS load balancers in response to bugs, vulnerabilities, or other concerns.

When a connection falls back to HTTPS or HTTP/2 because of these circumstances, we do not count this as a failure of the load balancer.

Ensure that the above described behaviors are acceptable for your workloads before you enable QUIC.


Your HTTP(S) Load Balancing service can be configured and updated through the following interfaces:

  • The gcloud command-line tool: a command-line tool included in the Cloud SDK. The HTTP(S) Load Balancing documentation calls on this tool frequently to accomplish tasks. For a complete overview of the tool, see the gcloud Tool Guide. You can find commands related to load balancing in the gcloud compute command group.

    You can also get detailed help for any gcloud command by using the --help flag:

    gcloud compute http-health-checks create --help
  • The Google Cloud Platform Console: Load balancing tasks can be accomplished through the Google Cloud Platform Console.

  • The REST API: All load balancing tasks can be accomplished using the Google Cloud Load Balancing API. The API reference docs describe the resources and methods available to you.

TLS support

By default, an HTTPS target proxy accepts only TLS 1.0, 1.1, and 1.2 when terminating client SSL requests. You can use SSL policies to change this default behavior and control how the load balancer negotiates SSL with clients.

When the load balancer uses HTTPS as a backend service protocol, it can negotiate TLS 1.0, 1.1, or 1.2 to the backend.

Timeouts and retries

HTTP(S) Load Balancing has two distinct types of timeouts:

  • A configurable response timeout, which represents the amount of time the load balancer will wait for your backend to return a complete response. It is not an idle (keepalive) timeout. This timeout is configurable by modifying the timeout setting for your backend service. The default value is 30 seconds. Consider increasing this timeout under these circumstances:

    • If you expect a backend to take longer to return HTTP responses, or
    • If the connection is upgraded to a WebSocket.
  • A TCP session timeout, whose value is fixed at 10 minutes (600 seconds). This session timeout is sometimes called a keepalive or idle timeout, and its value is not configurable by modifying your backend service. You must configure the web server software used by your backends so that its keepalive timeout is longer than 600 seconds to prevent connections from being closed prematurely by the backend. This timeout does not apply to WebSockets.

This table illustrates changes necessary to modify keepalive timeouts for common web server software:

Web Server Software Parameter Default Setting Recommended Setting
Apache KeepAliveTimeout KeepAliveTimeout 5 KeepAliveTimeout 620
nginx keepalive_timeout keepalive_timeout 75s; keepalive_timeout 620s;

HTTP(S) Load Balancing retries failed GET requests in certain circumstances, such as when the response timeout is exhausted. It does not retry failed POST requests. Retries are limited to two attempts. Retried requests only generate one log entry for the final response. Refer to Logging for more information.

Illegal request handling

The HTTP(S) load balancer blocks client requests from reaching the backend for a number of reasons: some strictly for HTTP/1.1 compliance and others to avoid unexpected data being passed to the backends.

The load balancer blocks the following for HTTP/1.1 compliance:

  • It cannot parse the first line of the request.
  • A header is missing the : delimiter.
  • Headers or the first line contain invalid characters.
  • The content length is not a valid number, or there are multiple content length headers.
  • There are multiple transfer encoding keys, or there are unrecognized transfer encoding values.
  • There's a non-chunked body and no content length specified.
  • Body chunks are unparseable. This is the only case where some data will make it to the backend. The load balancer will close the connections to client and backend when it receives an unparseable chunk.

The load balancer also blocks the request if any of the following are true:

  • The combination of request URL and headers is longer than about 15KB.
  • The request method does not allow a body, but the request has one.
  • The request contains an upgrade header.
  • The HTTP version is unknown.

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