How Requests are Handled

1. Requests and domains
2. Request handlers
3. Request headers
4. Responses
5. The request timer
6. SPDY
7. Logging
8. The environment
9. App caching
10. 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.

Request handlers

When App Engine receives a web request for your application, it calls the handler script that corresponds to the URL, as described in the application's app.yaml configuration file . The Python 2.7 runtime supports the WSGI standard and the CGI standard for backwards compatibility. WSGI is preferred, and some features of Python 2.7 do not work without it. The configuration of your application's script handlers determines whether a request is handled using WSGI or CGI.

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.

If you mark your application as thread-safe, concurrent requests will be enabled, which means that App Engine can dispatch multiple requests to each web server in parallel. To do so, set threadsafe: true in app.yaml. Concurrent requests are not available if any script handler uses CGI.

Requests and WSGI

The server determines which Python application object to call by comparing the URL of the request to the URL patterns in the app's configuration file. It then calls the application object using the arguments as defined in the WSGI standard. The application object performs actions appropriate to the request, then prepares a response and returns it as a list of strings.

Most applications use a framework, such as webapp2, to handle WSGI requests. webapp2 is included as part of the Python 2.7 runtime.

Requests and CGI

The server determines which Python handler script to run by comparing the URL of the request to the URL patterns in the app's configuration file. It then runs the script in a ` environment populated with the request data. As described in the CGI standard, the server puts the request data in environment variables and the standard input stream. The script performs actions appropriate to the request, then prepares a response and puts it on the standard output stream.

Most applications use a library to parse CGI requests and return CGI responses, such as the cgi module from the Python standard library, or a web framework that knows the CGI protocol (such as webapp). You can refer to the CGI documentation for details about the environment variables and the format of the input stream data.

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 handler script with a Request and waits for the script to return; all data written to the standard output stream is sent as the HTTP response.

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. The development server sets this to Development/x, where x is the version number.

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 X-AppEngine-BlobRange header will be used to replace the body with all or part of a blobstore blob's content. If Content-Type is not specified by the application, it will be set to the blob's MIME type. If a range is requested, the response status will be changed to 206 Partial Content, and a Content-Range header will be added. The X-AppEngine-BlobKey and X-AppEngine-BlobRange headers will be removed from the response. You do not normally need to set these headers yourself, as the blobstore_handlers.BlobstoreDownloadHandler class sets them. See Serving a Blob for details.

Response headers set in the application configuration

Custom HTTP Response headers can be set per URL for dynamic and static paths in your application's configuration file. See the http_headers sections in the configuration documentation for more details.

The request timer

A request handler has a limited amount of time to generate and return a response to a request, typically around 60 seconds. Once the deadline has been reached, the request handler is interrupted.

The Python runtime environment interrupts the request handler by raising a DeadlineExceededError, from the package google.appengine.runtime. If the request handler does not catch this exception, as with all uncaught exceptions, the runtime environment will return an HTTP 500 server error to the client.

The request handler can catch this error to customize the response. The runtime environment gives the request handler a little bit more time (less than a second) after raising the exception to prepare a custom response.

class TimerHandler(webapp2.RequestHandler):
    def get(self):
        from google.appengine.runtime import DeadlineExceededError

        try:
            time.sleep(70)
            self.response.write('Completed.')
        except DeadlineExceededError:
            self.response.clear()
            self.response.set_status(500)
            self.response.out.write(
                'The request did not complete in time.')

If the handler hasn't returned a response or raised an exception by the second deadline, the handler is terminated and a default error response is returned.

While a request can take as long as 60 seconds to respond, App Engine is optimized for applications with short-lived requests, typically those that take a few hundred milliseconds. An efficient app responds quickly for the majority of requests. An app that doesn't will not scale well with App Engine's infrastructure.

Refer to Dealing with DeadlineExceededErrors for common DeadlineExceededError causes and suggested workarounds.

SPDY

App Engine applications will automatically use the SPDY protocol when accessed over SSL by a browser that supports SPDY. This is a replacement for HTTP designed by Google and intended to reduce the latency of web page downloads. The use of SPDY should be entirely transparent to both applications and users (applications can be written as if normal HTTP was being used). For more information, see the SPDY project page.

Logging

The App Engine web server captures everything the handler script writes to the standard output stream for the response to the web request. It also captures everything the handler script writes to the standard error stream, and stores it as log data. Each request is assigned a request_id, a globally unique identifier based on the request's start time. Log data for your application can be viewed in the Cloud Platform Console Logs page, or downloaded using appcfg.py request_logs.

The App Engine Python runtime environment includes special support for the logging module from the Python standard library to understand logging concepts such as log levels ("debug", "info", "warning", "error", "critical").

import logging

import webapp2


class MainPage(webapp2.RequestHandler):
    def get(self):
        logging.debug('This is a debug message')
        logging.info('This is an info message')
        logging.warning('This is a warning message')
        logging.error('This is an error message')
        logging.critical('This is a critical message')

        try:
            raise ValueError('This is a sample value error.')
        except ValueError:
            logging.exception('A example exception log.')

        self.response.out.write('Logging example.')


app = webapp2.WSGIApplication([
    ('/', MainPage)
], debug=True)

The environment

The execution environment automatically sets several environment variables; you can set more in app.yaml. Of the automatically-set variables, some are special to App Engine, while others are part of the WSGI or CGI standards. Python code can access these variables using the os.environ dictionary.

The following environment variables are specific to App Engine:

• CURRENT_VERSION_ID: The major and minor version of the currently running application, as "X.Y". The major version number ("X") is specified in the app's app.yaml file. The minor version number ("Y") is set automatically when each version of the app is uploaded to App Engine. On the development web server, the minor version is always "1".
• AUTH_DOMAIN: The domain used for authenticating users with the Users API. Apps hosted on appspot.com have an AUTH_DOMAIN of gmail.com, and accept any Google account. Apps hosted on a custom domain have an AUTH_DOMAIN equal to the custom domain.
• INSTANCE_ID: Contains the instance ID of the frontend instance handling a request. The ID is a hex string (for example, 00c61b117c7f7fd0ce9e1325a04b8f0df30deaaf). A logged-in admin can use the id in a url: http://[INSTANCE_ID].myApp.appspot.com/. The request will be routed to that specific frontend instance. If the instance cannot handle the request it returns an immediate 503.

The following environment variables are part of the WSGI and CGI standards, with special behavior in App Engine:

• SERVER_SOFTWARE: In the development web server, this value is "Development/X.Y" where "X.Y" is the version of the runtime. When running on App Engine, this value is "Google App Engine/X.Y.Z".

Additional environment variables are set according to the WSGI or CGI standard. For more information on these variables, see the WSGI standard or the CGI standard, as appropriate.

You can also set environment variables in the app.yaml file:

env_variables:
  DJANGO_SETTINGS_MODULE: 'myapp.settings'

The following webapp2 request handler displays every environment variable visible to the application in the browser:

class PrintEnvironmentHandler(webapp2.RequestHandler):
    def get(self):
        self.response.headers['Content-Type'] = 'text/plain'
        for key, value in os.environ.iteritems():
            self.response.out.write(
                "{} = {}\n".format(key, value))

Request IDs

At the time of the request, you can save the request ID, which is unique to that request. The request ID can be used later to look up the logs for that request, using the fetch() function's request_ids parameter.

The following sample code shows how to get the request ID in the context of a request:

class RequestIdHandler(webapp2.RequestHandler):
    def get(self):
        self.response.headers['Content-Type'] = 'text/plain'
        request_id = os.environ.get('REQUEST_LOG_ID')
        self.response.write(
            'REQUEST_LOG_ID={}'.format(request_id))

App caching

The Python runtime environment caches imported modules between requests on a single web server, similar to how a standalone Python application loads a module only once even if the module is imported by multiple files. Since WSGI handlers are modules, they are cached between requests. CGI handler scripts are only cached if they provide a main() routine; otherwise, the CGI handler script is loaded for every request. See Requests for more information about the differences between WSGI and CGI.

App caching provides a significant benefit in response time. We recommend that all CGI handler scripts use a main() routine, as described below.

Imports are cached

For efficiency, the web server keeps imported modules in memory and does not re-load or re-evaluate them on subsequent requests to the same application on the same server. Most modules do not initialize any global data or have other side effects when they are imported, so caching them does not change the behavior of the application.

If your application imports a module that depends on the module being evaluated for every request, the application must accommodate this caching behavior.

CGI handler scripts can also be cached

You can tell App Engine to cache the CGI handler script itself, in addition to imported modules. If the handler script defines a function named main(), then the script and its global environment will be cached like an imported module. The first request for the script on a given web server evaluates the script normally. For subsequent requests, App Engine calls the main() function in the cached environment.

To cache a handler script, App Engine must be able to call main() with no arguments. If the handler script does not define a main() function, or the main() function requires arguments (that don't have defaults), then App Engine loads and evaluates the entire script for every request.

Keeping the parsed Python code in memory saves time and allows for faster responses. Caching the global environment has other potential uses as well:

• Compiled regular expressions. All regular expressions are parsed and stored in a compiled form. You can store compiled regular expressions in global variables, then use app caching to re-use the compiled objects between requests.

• GqlQuery objects. The GQL query string is parsed when the GqlQuery object is created. Re-using a GqlQuery object with parameter binding and the bind() method is faster than re-constructing the object each time. You can store a GqlQuery object with parameter binding for the values in a global variable, then re-use it by binding new parameter values for each request.

• Configuration and data files. If your application loads and parses configuration data from a file, it can retain the parsed data in memory to avoid having to re-load the file with each request.

The handler script should call main() when imported. App Engine expects that importing the script calls main(), so App Engine does not call it when loading the request handler for the first time on a server.

App caching with main() provides a significant improvement in your CGI handler's response time. We recommend it for all applications that use CGI.

Quotas and limits

Google 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 (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 Cloud Platform 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. See the Quotas page for more information.

In addition to system-wide safety limits, the following limits apply specifically to the use of request handlers: