This page describes how to issue HTTP and HTTPS requests and receive responses using the URL Fetch service.
Requests
Request protocols
An application can fetch a URL using either HTTP or HTTPS. The protocol that should be used is inferred by looking at the protocol in the target URL.
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
protocol. HTTP requests occur on port 80
, and HTTPS requests occur
on port 443
.
Request methods
If you issue requests through the URL Fetch service, you can use any of the following HTTP methods:
GET
POST
PUT
HEAD
DELETE
PATCH
A request can include HTTP headers and, for POST
, PUT
, and PATCH
requests, a payload.
Request proxying
Note that 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.
Request headers
Your application can set HTTP headers for the outgoing request.
For example, 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.
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)"
, whereAPPID
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.
Request timeouts
You can set a deadline, or timeout, for a request. By default, the timeout for a request is 10 seconds.
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.
Responses
If you use the URL Fetch API, note that 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.
Using URL Fetch on the development server
When your application is running on the App Engine 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 for URL Fetch
For information about URL Fetch service quotas, see Quotas. To see the current quota usage of your application, go to the Quota Details page in the Google Cloud console.
In addition, the following limits apply to the use of the URL Fetch service:
Limit | Amount |
---|---|
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 |