api.py

""" API access to other Dojo apps

Here we have wrappers for accessing data in other Hacker Dojo applications.
They will take care of local caching and failure scenarios.

One common failure scenario is that App Engine instances can take a while to
respond as they spin up after a period of inactivity, which commonly times out
the App Engine API for making HTTP requests. The core request caching
machanism here attempts to deal with this by keeping a failover copy of the
last good response that it will use when a request cache expires and the new
request fails to respond in a timely fashion. It will keep using this while
trying every time until it gets a good response.

"""
from google.appengine.api import memcache
from google.appengine.api import urlfetch
from django.utils import simplejson

DEADLINE = 7

def _request(url, cache_ttl=3600, force=False):
    request_cache_key = 'request:%s' % url
    failure_cache_key = 'failure:%s' % url
    resp = memcache.get(request_cache_key)
    if force or not resp:
        try:
            resp = simplejson.loads(urlfetch.fetch(url, deadline=DEADLINE,
                                    follow_redirects=False).content)
            memcache.set(request_cache_key, resp, cache_ttl)
            memcache.set(failure_cache_key, resp, cache_ttl*10)
        except (ValueError, urlfetch.DownloadError), e:
            # Not valid JSON or request timeout
            resp = memcache.get(failure_cache_key)
            if not resp:
                resp = []
    return resp


def domain(path, force=False):
    base_url = 'http://hd-domain-hrd.appspot.com'
    return _request(base_url + path, force=force)