This is a Preview release of the Socket API. As a result, the API is subject to change and the service itself is currently not covered by any SLA or deprecation policy. These characteristics will be evaluated as the API and service moves towards General Availability, but developers should take this into consideration when using the Preview release of Socket API.
App Engine supports the standard Python socket module API for outbound sockets only. You simply import the standard socket library using the following statement:import socket
Libraries that import
socket
, such as
poplib
or
nntplib
, and that don't violate the
limitations and restrictions
listed
below, should work without modification.
App Engine supports sockets without requiring any special App Engine libraries or add any special App Engine code. However, there are certain limitations and behaviors you need to be aware of when using sockets.
Notice that 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,
calls to
getsockopt
will return a mock value and
calls to
setsockopt
will be silently ignored. Errors will continue to be
raised for unsupported options.
The currently supported options are:
-
SO_KEEPALIVE
-
SO_DEBUG
-
TCP_NODELAY
-
SO_LINGER
-
SO_OOBINLINE
-
SO_SNDBUF
-
SO_RCVBUF
-
SO_REUSEADDR
Limitations and restrictions
App Engine supports sockets without requiring you to import any special App Engine libraries or add any special App Engine code. However, there are certain limitations and behaviors you need to be aware of when using sockets:
- Sockets are available only for paid apps.
- You cannot create a listen socket; you can only create outbound sockets.
-
By default,
httplib
is configured to use the urlfetch api ; if you need to usesocket
to get around urlfetch limits, you can do so by changing this default sohttplib
uses sockets instead. For more information, see Makinghttplib
use sockets . - 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 (except those
whitelisted below), are blocked:
-
Google Public DNS:
8.8.8.8
,8.8.4.4
,2001:4860:4860::8888
,2001:4860:4860::8844
port 53 -
Gmail SMTPS:
smtp.gmail.com
port 465 and 587 -
Gmail POP3S:
pop.gmail.com
port 995 -
Gmail IMAPS:
imap.gmail.com
port 993
-
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.
-
Currently,
socket.gethostbyaddr()
is not implemented in Python. If you are trying to send mail using the Python SMTP standard library (smtplib
), calls tosmtplib.SMTP
will raiseNotImplementedError()
if your code doesn't supply alocal_hostname
. To work around this, simply supplylocal_hostname
as follows:# Open a connection to my mail server s = smtplib.SMTP('smtp.mailhostingcompany.net', 587, "mymailhostname")
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
OpenSSL Support
.
Making
httplib
use sockets
If you import
httplib
, by default it will use the
urlfetch api
. To change this so that
httplib
uses sockets instead, you simply add the environment variable to your
app.yaml
file for your project.
env_variables:
GAE_USE_SOCKETS_HTTPLIB : 'anyvalue'
The value can be any value (including an empty string). If the variable is not
supplied,
httplib
will continue to use urlfetch.
App Engine sample using sockets
For a sample using sockets, see the socket demo app in the Google Cloud Platform GitHub.