voterlator/main.py

   1 #!/usr/bin/python2.4
   2 #
   3 # Copyright (C) 2011 Google Inc.
   4 #
   5 # Licensed under the Apache License, Version 2.0 (the "License");
   6 # you may not use this file except in compliance with the License.
   7 # You may obtain a copy of the License at
   8 #
   9 #      http://www.apache.org/licenses/LICENSE-2.0
  10 #
  11 # Unless required by applicable law or agreed to in writing, software
  12 # distributed under the License is distributed on an "AS IS" BASIS,
  13 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14 # See the License for the specific language governing permissions and
  15 # limitations under the License.
  16
  17 """An very simple app using pull queues to tally votes."""
  18
  19 __author__ = 'nverne@google.com (Nicholas Verne)'
  20
  21 import os
  22 import time
  23
  24 from google.appengine.api import memcache
  25 from google.appengine.api import taskqueue
  26 from google.appengine.ext import db
  27 from google.appengine.ext import webapp
  28 from google.appengine.ext.webapp import template
  29 from google.appengine.ext.webapp.util import run_wsgi_app
  30
  31 LANGUAGES = ['Java', 'Go', 'C++', 'Perl', 'Python']
  32
  33 class Tally(db.Model):
  34   """Simple counting model."""
  35   count = db.IntegerProperty(default=0, required=True)
  36
  37   @classmethod
  38   def increment_by(cls, key_name, count):
  39     """Increases a Tally's count. Should be run in a transaction."""
  40     tally = cls.get_by_key_name(key_name)
  41     if tally is None:
  42       tally = cls(key_name=key_name)
  43     tally.count += count
  44     tally.put()
  45
  46
  47 class VoteHandler(webapp.RequestHandler):
  48   """Handles adding of vote tasks."""
  49
  50   def get(self):
  51     """Displays the voting form."""
  52     self.render_template('index.html', {'tallies': self.get_tallies(),
  53                                         'LANGUAGES': LANGUAGES})
  54
  55   def post(self):
  56     """Adds tasks to votes queue if ugliest is valid."""
  57     ugliest = self.request.get('ugliest')
  58
  59     if ugliest and ugliest in LANGUAGES:
  60       q = taskqueue.Queue('votes')
  61       q.add(taskqueue.Task(payload=ugliest, method='PULL'))
  62     self.redirect('/')
  63
  64   def get_tallies(self):
  65     """Fetches tallies from memcache if possible, otherwise from datastore."""
  66     tallies = memcache.get('tallies')
  67     if tallies is None:
  68       tallies = Tally.all().fetch(len(LANGUAGES))
  69       memcache.set('tallies', tallies, time=5)
  70     return tallies
  71
  72   def render_template(self, name, template_args):
  73     """Renders a named django template."""
  74     path = os.path.join(os.path.dirname(__file__), 'templates', name)
  75     self.response.out.write(template.render(path, template_args))
  76
  77
  78 class TallyHandler(webapp.RequestHandler):
  79   """Pulls tasks from the vote queue."""
  80
  81   def store_tallies(self, tallies):
  82     """Updates the tallies in datastore."""
  83     for key_name, count in tallies.iteritems():
  84       db.run_in_transaction(Tally.increment_by, key_name, count)
  85
  86   def post(self):
  87     """Leases vote tasks, accumulates tallies and stores them."""
  88     q = taskqueue.Queue('votes')
  89     # Keep leasing tasks in a loop. When the task fails due to
  90     # deadline, it should be retried.
  91     while True:
  92       tasks = q.lease_tasks(300, 1000)
  93       if not tasks:
  94         # Let the retry parameters of the queue cause this
  95         # task to be rescheduled.
  96         self.error(500)
  97         return
  98
  99       tallies = {}
 100       # accumulate tallies in memory
 101       for t in tasks:
 102         tallies[t.payload] = tallies.get(t.payload, 0) + 1
 103
 104       self.store_tallies(tallies)
 105       q.delete_tasks(tasks)
 106
 107 class StartHandler(webapp.RequestHandler):
 108   """Starts some tally tasks."""
 109   def get(self):
 110     workers = int(self.request.get('workers', 2))
 111     q = taskqueue.Queue('tally')
 112     q.purge()
 113     time.sleep(2)
 114
 115     if workers > 0:
 116       q.add([taskqueue.Task(url='/tally',
 117                             countdown=x) for x in xrange(workers)])
 118     self.redirect('/')
 119
 120 application = webapp.WSGIApplication([
 121     ('/', VoteHandler),
 122     ('/tally', TallyHandler),
 123     ('/start', StartHandler),
 124     ], debug=True)
 125
 126
 127 def main():
 128   run_wsgi_app(application)
 129
 130 if __name__ == '__main__':
 131   main()