Traffic from sockets is billed as outgoing bandwidth. Async IO (such as Twisted in Python) is not supported. App Engine supports the standard Python socket module API for outbound sockets only. You simply import the standard socket library using the following statement:
Libraries that import
socket, such as
nntplib, and that don't violate the
limitations and restrictions listed
below, should work without modification.
Although the Python 2 runtime supports sockets, there are certain limitations and behaviors you need to be aware of when using sockets.
You can pickle a socket descriptor and pass it between App Engine instances, such as part of a Task payload. In this scenario, you can open a socket on a frontend instance, and then pass it to a backend instance and use it there.
In SDK versions prior to 1.8.1, you could not call get/set options against sockets. (Doing so raised "Not Implemented" exceptions.) However, the Sockets API now allows this.
For supported options,
getsockopt will return a mock value and
setsockopt will be silently ignored. Errors will continue to be
raised for unsupported options.
The supported options are:
Limitations and restrictions
Socket support in App Engine has the following limitations:
- You cannot create a listen socket; you can only create outbound sockets.
- FTP is not supported.
- By default,
httplibis configured to use the urlfetch api; if you need to use
socketto get around urlfetch limits, you can do so by changing this default so
httplibuses sockets instead. For more information, see Making
- You can only use TCP or UDP; arbitrary protocols are not allowed.
- You cannot bind to specific IP addresses or ports.
- Port 25 (SMTP) is blocked; you can still use authenticated SMTP on the submission port 587.
Private, broadcast, multicast, and Google IP ranges are blocked, except those listed below:
- Google Public DNS:
- Gmail SMTPS:
smtp.gmail.comport 465 and 587
- Gmail POP3S:
- Gmail IMAPS:
- Google Public DNS:
Socket descriptors are associated with the App Engine app that created them and are non-transferable (cannot be used by other apps).
Sockets may be reclaimed after 2 minutes of inactivity; any socket operation keeps the socket alive for a further 2 minutes.
socket.gethostbyaddr()is not implemented in Python. You can still use the Python SMTP standard library (
smtplib) to open a connection:
# Open a connection to my mail server s = smtplib.SMTP('smtp.mailhostingcompany.net', 587)
Using sockets with the development server
You can run and test code using sockets on the development server, without using any special command line parameters.
Using sockets with OpenSSL
App Engine supports native Python OpenSSL for the Python 2.7 runtime.
You must configure your
app.yaml file to load the ssl library, as described in
Disabling URL Fetch from handling all outbound requests
If you import
httplib, by default it will use the
urlfetch api. To change this
httplib uses sockets instead, add the following environment variable
env_variables: GAE_USE_SOCKETS_HTTPLIB : 'anyvalue'
You can replace
anyvalue with any value including an empty string.
App Engine sample using sockets
For a sample using sockets, see the socket demo app in the Google Cloud Platform GitHub.