How Requests are Handled

This document describes how your App Engine application receives requests and sends responses. For more details, see the Request Headers reference.

If your application uses services, you can address requests to a specific service or a specific version of that service. For more information about service addressability, see How Requests are Routed.

Handling requests

Your application is responsible for starting a webserver and handling requests. You can use any web framework that is available for your development language.

App Engine runs multiple instances of your application, and each instance has its own web server for handling requests. Any request can be routed to any instance, so consecutive requests from the same user are not necessarily sent to the same instance. An instance can handle multiple requests concurrently. The number of instances can be adjusted automatically as traffic changes. You can also change the number of concurrent requests an instance can handle by setting the max_concurrent_requests element in your app.yaml file.

The following example contains the PHP code to create a new Silex Application object, start a server, and respond to two GET requests:

require_once __DIR__ . '/../vendor/autoload.php';

$app = new Silex\Application();

$app->get('/', function () {
    return 'Hello World';

$app->get('/goodbye', function () {
    return 'Goodbye World';

// @codeCoverageIgnoreStart
if (PHP_SAPI != 'cli') {
// @codeCoverageIgnoreEnd

return $app;

This example uses Silex, but you can use other PHP web frameworks.

Quotas and limits

App Engine automatically allocates resources to your application as traffic increases. However, this is bound by the following restrictions:

  • App Engine reserves automatic scaling capacity for applications with low latency, where the application responds to requests in less than one second. Applications with very high latency, such as over one second per request for many requests, and high throughput require Silver, Gold, or Platinum support. Customers with this level of support can request higher throughput limits by contacting their support representative.

  • Applications that are heavily CPU-bound may also incur some additional latency in order to efficiently share resources with other applications on the same servers. Requests for static files are exempt from these latency limits.

Each incoming request to the application counts toward the Requests limit. Data sent in response to a request counts toward the Outgoing Bandwidth (billable) limit.

Both HTTP and HTTPS (secure) requests count toward the Requests, Incoming Bandwidth (billable), and Outgoing Bandwidth (billable) limits. The GCP Console Quota Details page also reports Secure Requests, Secure Incoming Bandwidth, and Secure Outgoing Bandwidth as separate values for informational purposes. Only HTTPS requests count toward these values. For more information, see the Quotas page.

The following limits apply specifically to the use of request handlers:

Request limits

  • A maximum of ~15KB in request headers is allowed.
  • The total size of the request is limited to ~32MB.
  • All HTTP/2 requests will be translated into HTTP/1.1 requests when forwarded to the application server.
  • SSL connections are terminated at the load balancer. Traffic from the load balancer is sent to the instance over an encrypted channel, and then forwarded to the application server over HTTP. The X-Forwarded-Proto header lets you understand if the origin request was HTTP or HTTPS.

Response limits

  • Responses are buffered by 64k blocks.
  • The response size is unlimited.
  • The response time limit is one hour.

Unsupported HTTP requests

The following features are not supported by App Engine flexible environment:

  • HTTP/2 traffic to the backend service.
  • HTTP requests that directly access instances.

Request headers

An incoming HTTP request includes the HTTP headers sent by the client. For security purposes, some headers are sanitized or amended by intermediate proxies before they reach the application.

For more information, see the Request headers reference.

Request responses

App Engine calls the script with the $_REQUEST array populated, buffers any output from the script, and when the script completes execution, sends the buffered output to the end user.

There are limits that apply to the response you generate, and the response may be modified before it is returned to the client.

Disabling buffering

By default, all responses from App Engine are buffered in 64k-blocks. In some cases, it might make sense to disable buffering and directly stream bytes to the client. This is generally preferred when using hanging GETs or Server Sent Events (SSEs). To disable buffering, you can set the X-Accel-Buffering response header to no.

X-Accel-Buffering: no

Forcing HTTPS connections

For security reasons, all applications should encourage clients to connect over https. You can use the Strict-Transport-Security header to instruct the browser to prefer https over http for a given page or an entire domain, for example:

Strict-Transport-Security: max-age=31536000; includeSubDomains

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