Wednesday, August 27, 2008

Freebase & Google App Engine - Write Queries

If you would like to use Freebase with your application that is hosted with Google App Engine, then you may have run into some difficulties trying to get write queries to work!

Now that I have been able to get writes to work, I am going to write down what it is that I did so that other people don't run into the same problems that I did!

Basically, to get writes to work, I created a proxy between my app engine page and freebase. In other words, I had a form that allows you to modify an entity in freebase and upon submit, the mql write statement was created and sent to the app engine. The app engine then forwarded this request to Freebase and then waited for the response. The response was then forwarded back to the to the client page.

To get this to work, you need to first obtain the cookies that are set when you authenticate with Freebase. To do this, we have a simple python subroutine:

# domain is either or
# user is the username
# pass is the password
def login(domain, user, pass):
result = urlfetch.fetch(
+ domain
+ '/api/account/login?username='
+ user
+ '&password='
+ pass
+ '&rememberme=true',

return result.headers['set-cookie']

This subroutine basically calls the login service for Freebase and returns any cookies that their server says to save.

The next thing that you need to do is to forward your write mql to Freebase. This can be done in a similar manner:

# domain is either or
# user is the username performing the write
# pass is the password for the user performing the write
# mql is the write query wrapped in the query envelope
def write_to_freebase(domain, user, pass, mql):
form_fields = {
form_data = urllib.urlencode(form_fields)
cookies = login(domain, user, pass)
result = urlfetch.fetch(
url='http://' + domain+ '/api/service/mqlwrite',
'Content-Type': 'application/x-www-form-urlencoded',
'Cookie' : cookies
return result.content;

This method simply calls the mqlwrite service and forwards the JSON object to you.

The imports required are:

from google.appengine.api import urlfetch
import urllib

That is all that is required in order to get writes to work with google app engine!

Please let me know if I missed anything so that I can update this post!

Thanks and good luck!