Google App Engine

URL Fetch Python API Overview

Python |Java |PHP |Go
App Engine applications can communicate with other applications or access other resources on the web by fetching URLs. An app can use the URL Fetch service to issue HTTP and HTTPS requests and receive responses. The URL Fetch service uses Google's network infrastructure for efficiency and scaling purposes.

  1. Fetching URLs in Python
  2. Making requests
  3. Secure connections and HTTPS
  4. Request headers
  5. Responses
  6. URL Fetch and the development server
  7. Quotas and limits

Fetching URLs in Python

You can use the Python standard libraries urllib, urllib2 or httplib to make HTTP requests. When running in App Engine, these libraries perform HTTP requests using App Engine's URL fetch service, which runs on Google's scalable HTTP request infrastructure.

import urllib2

url = "http://www.google.com/"
try:
  result = urllib2.urlopen(url)
  doSomethingWithResult(result)
except urllib2.URLError, e:
  handleError(e)

You can also access the URL fetch service using its Python API. In this API, the urlfetch.fetch() function performs an HTTP request.

from google.appengine.api import urlfetch

url = "http://www.google.com/"
result = urlfetch.fetch(url)
if result.status_code == 200:
  doSomethingWithResult(result.content)

The URL fetch service supports the following HTTP methods: GET, POST, HEAD, PUT, DELETE, and PATCH. The request can include HTTP headers, and body content for a POST, PUT, or PATCH request. For example, to submit data to a web form handler using the POST action using the URL fetch API:

import urllib

from google.appengine.api import urlfetch

form_fields = {
  "first_name": "Albert",
  "last_name": "Johnson",
  "email_address": "Albert.Johnson@example.com"
}
form_data = urllib.urlencode(form_fields)
result = urlfetch.fetch(url=url,
    payload=form_data,
    method=urlfetch.POST,
    headers={'Content-Type': 'application/x-www-form-urlencoded'})

Making requests

An app can fetch a URL using HTTP (normal) or HTTPS (secure). The URL specifies the scheme to use: http://... or https://...

The URL to be fetched can use any port number in the following ranges: 80-90, 440-450, 1024-65535. If the port is not mentioned in the URL, the port is implied by the scheme: http://... is port 80, https://... is port 443.

The fetch can use any of the following HTTP methods: GET (common for requesting web pages and data), POST (common for submitting web forms), PUT, HEAD, and DELETE. The fetch can include HTTP request headers and a payload (an HTTP request body).

The URL Fetch service uses an HTTP/1.1 compliant proxy to fetch the result.

To prevent an app from causing an endless recursion of requests, a request handler is not allowed to fetch its own URL. It is still possible to cause an endless recursion with other means, so exercise caution if your app can be made to fetch requests for URLs supplied by the user.

You can set a deadline for a request, the most amount of time the service will wait for a response. By default, the deadline for a fetch is 5 seconds. The maximum deadline is 60 seconds for HTTP requests and 10 minutes for task queue and cron job requests.

The URL Fetch service supports both synchronous requests and asynchronous requests. With a synchronous request, the API call to fetch a URL waits until the remote host returns a result, then returns control to the application. The app can specify the maximum amount of time to wait when it makes the call. If the maximum wait time is exceeded, the call raises an exception.

An asynchronous request to the URL Fetch service starts the request, then returns immediately with an object. The application can perform other tasks while the URL is being fetched. When the application needs the results, it calls a method on the object, which waits for the request to finish if necessary, then returns the result. The app can have up to 10 simultaneous asynchronous URL Fetch calls. If any URL Fetch requests are pending when the request handler exits, the application server waits for all remaining requests to either return or reach their deadline before returning a response to the user.

Making requests to another App Engine app

If you are making requests to another App Engine app, you should consider telling the URL Fetch service to not follow redirects when invoking it. That is, your app must set the fetch follow_redirects parameter to False when invoking fetch.

This setting does two things:

  1. Makes your calls faster
  2. Adds an X-Appengine-Inbound-Appid header containing your app ID to the request, so the responding app can determine the source of incoming requests. For more information, see Asserting identity to other App Engine apps.

Secure connections and HTTPS

An app can fetch a URL with the HTTPS method to connect to secure servers. Request and response data are transmitted over the network in encrypted form.

In the Python API, the proxy by default does not validate the host it is contacting. The proxy server cannot detect "man in the middle" attacks between App Engine and the remote host when using HTTPS. However, you can add an optional validate_certificate argument to the fetch() method to enable host validation. The urllib module currently provides no methods to validate hosts, but will default to host validation in the near future.

Request headers

An app can set HTTP headers for the outgoing request.

When sending an HTTP POST request, if a Content-Type header is not set explicitly, the header is set to x-www-form-urlencoded. This is the content type used by web forms.

For security reasons, the following headers cannot be modified by the application:

  • Content-Length
  • Host
  • Vary
  • Via
  • X-Appengine-Inbound-Appid
  • X-Forwarded-For
  • X-ProxyUser-IP

These headers are set to accurate values by App Engine, as appropriate. For example, App Engine calculates the Content-Length header from the request data and adds it to the request prior to sending.

Headers identifying request source

The following headers indicate the app ID of the requesting app:

  • User-Agent. This header can be modified but App Engine will append an identifier string to allow servers to identify App Engine requests. The appended string has the format "AppEngine-Google; (+http://code.google.com/appengine; appid: APPID)", where APPID is your app's identifier.
  • X-Appengine-Inbound-Appid. This header cannot be modified, and is added automatically if the request is sent via the URL Fetch service when the follow redirects parameter is set to False.

Responses

The URL Fetch service returns all response data, including the response code, header and body.

By default, if the URL Fetch service receives a response with a redirect code, the service will follow the redirect. The service will follow up to 5 redirect responses, then return the final resource. You can use the API to tell the URL Fetch service to not follow redirects and just return a redirect response to the application.

By default, if the incoming response exceeds the maximum response size limit, the URL fetch service raises an exception. (See below for the amount of this limit.) You can tell the API to truncate the response instead of raising an exception. Note that if you use the urllib, urllib2 or httplib libraries to fetch URLs, the response will always be truncated instead of raising an exception.

URL Fetch and the development server

When your application is running in the development server on your computer, calls to the URL Fetch service are handled locally. The development server fetches URLs by contacting remote hosts directly from your computer, using whatever network configuration your computer is using to access the Internet.

When testing the features of your app that fetch URLs, be sure that your computer can access the remote hosts.

If your app is using the Google Secure Data Connector to access URLs on your intranet, be sure to test your app while connected to your intranet behind the firewall. Unlike App Engine, the development server does not use the SDC Agent to resolve intranet URLs. Only Google Apps and App Engine can authenticate with your SDC Agent.

Quotas and limits

Each URL Fetch request counts toward the URL Fetch API Calls quota.

Data sent in an HTTP or HTTPS request using the URL Fetch service counts toward the following quotas:

  • Outgoing Bandwidth (billable)
  • URL Fetch Data Sent

In addition to these quotas, data sent in an HTTPS request also counts toward the following quota:

  • Secure Outgoing Bandwidth (billable)

Data received in response to an HTTP or HTTPS request using the URL Fetch service counts toward the following quotas:

  • Incoming Bandwidth (billable)
  • URL Fetch Data Received

In addition to these quotas, data received in response to an HTTPS request also counts toward the following quota:

  • Secure Incoming Bandwidth (billable)

For more information on quotas, see Quotas, and the "Quota Details" section of the Admin Console.

In addition to quotas, the following limits apply to the use of the URL Fetch service:

Limit Amount
request size 10 megabytes
response size 32 megabytes
maximum deadline (request handler) 60 seconds
maximum deadline (task queue and cron job handler) 10 minutes

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