Adds a MongoDB broker

* adds mongo broker tests
* adds mongo brokers docs

* broker info is now cached when it needs to connect to a server, to reduce traffic.

django_q/brokers/mongo.py

@@ -0,0 +1,64 @@
+from datetime import timedelta
+from time import sleep
+from bson import ObjectId
+
+from django.utils import timezone
+
+from pymongo import MongoClient
+
+from django_q.brokers import Broker
+from django_q.conf import Conf
+
+
+def _timeout():
+    return timezone.now() - timedelta(seconds=Conf.RETRY)
+
+
+class Mongo(Broker):
+    def __init__(self, list_key=Conf.PREFIX):
+        super(Mongo, self).__init__(list_key)
+        self.collection = self.connection[Conf.MONGO_DB][list_key]
+
+    @staticmethod
+    def get_connection(list_key=Conf.PREFIX):
+        return MongoClient(**Conf.MONGO)
+
+    def queue_size(self):
+        return self.collection.count({'lock': {'$lte': _timeout()}})
+
+    def lock_size(self):
+        return self.collection.count({'lock': {'$gt': _timeout()}})
+
+    def purge_queue(self):
+        return self.delete_queue()
+
+    def ping(self):
+        return self.info is not None
+
+    def info(self):
+        if not self._info:
+            self._info = 'MongoDB {}'.format(self.connection.server_info()['version'])
+        return self._info
+
+    def fail(self, task_id):
+        self.delete(task_id)
+
+    def enqueue(self, task):
+        inserted_id = self.collection.insert_one({'payload': task, 'lock': _timeout()}).inserted_id
+        return str(inserted_id)
+
+    def dequeue(self):
+        task = self.collection.find_one_and_update({'lock': {'$lte': _timeout()}}, {'$set': {'lock': timezone.now()}})
+        if task:
+            return [(str(task['_id']), task['payload'])]
+        # empty queue, spare the cpu
+        sleep(0.2)
+
+    def delete_queue(self):
+        return self.collection.drop()
+
+    def delete(self, task_id):
+        self.collection.delete_one({'_id': ObjectId(task_id)})
+
+    def acknowledge(self, task_id):
+        return self.delete(task_id)