This page describes how App Engine applications use the URL Fetch service to issue HTTP and HTTPS requests and receive responses. To see code samples demonstrating how to issue HTTP and HTTPS requests from your App Engine application, see Issuing HTTP(S) Requests.
App Engine uses the URL Fetch service to issue outbound requests.
In Python, you can use the
urllib2 libraries to make HTTP requests;
in an App Engine application, each library will perform these requests
by using the URL Fetch service. You can also use the
An application can fetch a URL using either HTTP or HTTPS. The URL Fetch service infers the protocol that should be used by looking at the protocol in the target URL.
The URL to be fetched can use any port number in the following ranges:
If the port is not mentioned in the URL, the port is implied by the
protocol. HTTP requests occur on port
80, and HTTPS requests occur
Requests issued through the URL Fetch service can use any of the following HTTP methods:
A request can include HTTP headers and, for
requests, a payload.
The URL Fetch service uses an HTTP/1.1 compliant proxy to fetch the result.
To prevent an application 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 application can be made to fetch requests for URLs supplied by the user.
Your application 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
This is the content type used by web forms.
For security reasons, the following headers cannot be modified by the application:
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.
The following headers indicate the application 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
APPIDis 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
You can set a deadline, or timeout, for a request. By default, the timeout for a request is 5 seconds.
The URL Fetch service supports both synchronous requests and asynchronous requests:
- Synchronous requests: The fetch call waits until the remote host returns a result, and then returns control to the application. If the maximum wait time for the fetch call is exceeded, the call raises an exception.
- Asynchronous requests: 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. 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.
Secure connections and HTTPS
Your application can fetch a URL securely by using HTTPS to connect to secure servers. Request and response data are transmitted over the network in encrypted form.
In the Python API, the URL Fetch proxy does not validate the host it is
contacting by default. You can add an optional
argument to the
method to enable host validation.
The URL Fetch service returns all response data, including the response code, headers, 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 five redirect responses, then return the final resource. You can instruct the URL Fetch service to not follow redirects and instead return a redirect response to the application.
URL Fetch on the App Engine development server
When your application is running on 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 application that fetch URLs, make sure that your computer can access the remote hosts.
Quotas and limits
In addition, the following limits apply to the use of the URL Fetch service:
|Request size||10 megabytes|
|Request header size||16 KB (Note that this limits the maximum length of the URL that can be specified in the header)|
|Response size||32 megabytes|
Run code samples and get guidance on how to issue requests from your application in Issuing HTTP(S) Requests.