How Requests are Handled

  1. Requests and domains
  2. Requests and servlets
  3. Request headers
  4. Responses
  5. The request timer
  6. SPDY
  7. Logging
  8. The environment
  9. Quotas and limits

Requests and domains

App Engine determines that an incoming request is intended for your application using the domain name of the request. A request whose domain name is http://your_app_id.appspot.com is routed to the application whose ID is your_app_id. Every application gets an appspot.com domain name for free.

appspot.com domains also support subdomains of the form subdomain-dot-your_app_id.appspot.com, where subdomain can be any string allowed in one part of a domain name (not .). Requests sent to any subdomain in this way are routed to your application.

You can set up a custom top-level domain using the Google Cloud Platform Console custom domains page. You can assign subdomains of your business's domain to various applications, such as Google Mail or Sites. You can also associate an App Engine application with a subdomain. You can set up a custom domain after you create a new project. See Using Custom Domains and SSL for more information.

Requests for these URLs all go to the version of your application that you have selected as the default version in the Cloud Platform Console. Each version of your application also has its own URL, so you can deploy and test a new version before making it the default version. The version-specific URL uses the version identifier from your app's configuration file in addition to the appspot.com domain name, in this pattern: http://version_id-dot-latest-dot-your_app_id.appspot.com You can also use subdomains with the version-specific URL: http://subdomain-dot-version_id-dot-latest-dot-your_app_id.appspot.com

The domain name used for the request is included in the request data passed to the application. If you want your app to respond differently depending on the domain name used to access it (such as to restrict access to certain domains, or redirect to an official domain), you can check the request data, such as the Host request header, for the domain from within the application code and respond accordingly.

If your app uses modules, you can address requests to a specific module and optionally a specific version of that module. For more information about module addressability, see Routing Requests to Modules.

Note: Google recommends using the HTTPS protocol to send requests to your app. Google does not issue SSL certificates for double-wildcard domains hosted at appspot.com. Therefore with HTTPS you must use the string "-dot-" instead of "." to separate subdomains, as shown in the examples below. You can use a simple "." with your own custom domain or with HTTP addresses.

Requests and servlets

When App Engine receives a web request for your application, it invokes the servlet that corresponds to the URL, as described in the application's deployment descriptor (the web.xml file in the WEB-INF/ directory). It uses the Java Servlet API, version 2.5, to provide the request data to the servlet and accept the response data .

App Engine runs multiple instances of your application, 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. The number of instances can be adjusted automatically as traffic changes.

By default, each web server processes only one request at a time. If you mark your application as thread-safe, App Engine can dispatch multiple requests to each web server in parallel. To do so, simply add a <threadsafe>true</threadsafe> element to appengine-web.xml.

The following example servlet class displays a simple message on the user's browser.

public class RequestsServlet extends HttpServlet {
  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse resp)
          throws IOException {
    resp.setContentType("text/plain");
    resp.getWriter().println("Hello, world");
  }
}

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.

The following headers are removed from the request:

  • Accept-Encoding
  • Connection
  • Keep-Alive
  • Proxy-Authorization
  • TE
  • Trailer
  • Transfer-Encoding

In addition, the header Strict-Transport-Security is removed from requests served to any domains other than appspot.com or *.appspot.com.

These headers relate to the transfer of the HTTP data between the client and server, and are transparent to the application. For example, the server may automatically send a gzipped response, depending on the value of the Accept-Encoding request header. The application itself does not need to know which content encodings the client can accept.

App Engine specific headers

As a service to the app, App Engine adds the following headers to all requests:

X-AppEngine-Country
Country from which the request originated, as an ISO 3166-1 alpha-2 country code. App Engine determines this code from the client's IP address. Note that the country information is not derived from the WHOIS database; it's possible that an IP address with country information in the WHOIS database will not have country information in the X-AppEngine-Country header. Your application should handle the special country code ZZ (unknown country).
X-AppEngine-Region
Name of region from which the request originated. This value only makes sense in the context of the country in X-AppEngine-Country. For example, if the country is "US" and the region is "ca", that "ca" means "California", not Canada. The complete list of valid region values is found in the ISO-3166-2 standard.
X-AppEngine-City
Name of the city from which the request originated. For example, a request from the city of Mountain View might have the header value mountain view. There is no canonical list of valid values for this header.
X-AppEngine-CityLatLong
Latitude and longitude of the city from which the request originated. This string might look like "37.386051,-122.083851" for a request from Mountain View.

App Engine services may add additional request headers:

The Task Queue service adds additional headers to requests from that provide details about the task in the request, and the queue it is associated with.

Requests from the Cron Service will also contain a HTTP header:

X-AppEngine-Cron: true

See Securing URLs for cron for more details.

Requests coming from other App Engine applications will include a header identifying the app making the request:

X-Appengine-Inbound-Appid

See the App Identity documentation for more details.

Responses

App Engine calls the servlet with a request object and a response object, then waits for the servlet to populate the response object and return. When the servlet returns, the data on the response object is sent to the user.

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

Response size limits

Dynamic responses are limited to 32MB. If a script handler generates a response larger than this limit, the server sends back an empty response with a 500 Internal Server Error status code. This limitation does not apply to responses that serve data from the Blobstore or Google Cloud Storage .

Streaming Responses

App Engine does not support streaming responses where data is sent in incremental chunks to the client while a request is being processed. All data from your code is collected as described above and sent as a single HTTP response.

Response compression

If the client sends HTTP headers with the original request indicating that the client can accept compressed (gzipped) content, App Engine compresses the handler response data automatically and attaches the appropriate response headers. It uses both the Accept-Encoding and User-Agent request headers to determine if the client can reliably receive compressed responses.

Custom clients can indicate that they are able to receive compressed responses by specifying both Accept-Encoding and User-Agent headers with a value of gzip. The Content-Type of the response is also used to determine whether compression is appropriate; in general, text-based content types are compressed, whereas binary content types are not.

When responses are compressed automatically by App Engine, the Content-Encoding header is added to the response.

Headers removed

The following headers are ignored and removed from the response:

  • Connection
  • Content-Encoding*
  • Content-Length
  • Date
  • Keep-Alive
  • Proxy-Authenticate
  • Server
  • Trailer
  • Transfer-Encoding
  • Upgrade

* May be re-added if the response is compressed by App Engine.

In addition, the header Strict-Transport-Security is removed from responses served from any domains other than *.appspot.com.

Headers with non-ASCII characters in either the name or value are also removed.

Headers added or replaced

The following headers are added or replaced in the response:

Cache-Control, Expires and Vary

These headers specify caching policy to intermediate web proxies (such as Internet Service Providers) and browsers. If your script sets these headers, they will usually be unmodified, unless the response has a Set-Cookie header, or is generated for a user who is signed in using an administrator account. Static handlers will set these headers as directed by the configuration file. If you do not specify a Cache-Control, the server may set it to private, and add a Vary: Accept-Encoding header.

If you have a Set-Cookie response header, the Cache-Control header will be set to private (if it is not already more restrictive) and the Expires header will be set to the current date (if it is not already in the past). Generally, this will allow browsers to cache the response, but not intermediate proxy servers. This is for security reasons, since if the response was cached publicly, another user could subsequently request the same resource, and retrieve the first user's cookie.

Content-Encoding
Depending upon the request headers and response Content-Type, the server may automatically compress the response body, as described above. In this case, it adds a Content-Encoding: gzip header to indicate that the body is compressed. See the section on response compression for more detail.
Content-Length or Transfer-Encoding
The server always ignores the Content-Length header returned by the application. It will either set Content-Length to the length of the body (after compression, if compression is applied), or delete Content-Length, and use chunked transfer encoding (adding a Transfer-Encoding: chunked header).
Content-Type

If not specified by the application, the server will set a default Content-Type: text/html header.

Date
Set to the current date and time.
Server
Set to Google Frontend.

If you access your site while signed in using an administrator account, App Engine includes per-request statistics in the response headers:

X-AppEngine-Estimated-CPM-US-Dollars
An estimate of what 1,000 requests similar to this request would cost in US dollars.
X-AppEngine-Resource-Usage
The resources used by the request, including server-side time as a number of milliseconds.

Responses with resource usage statistics will be made uncacheable.

If the X-AppEngine-BlobKey header is in the application's response, it and the optional