NoSQL engine/service recommendation for geolocation data First of all, I am new in this field we call big data, so my questions may be naive. 

In order to build an application, which deals with geolocation data, which could be : latitude and longitude coordinates and Geography SQL Server column types.

I need to have the following elements made easy:


Scalability : be prepared to receive huge amount of data, adding servers to the system have to be easy
Proximity requests : in example, how much points are in a circle (at meter scale). 
Data must be accessible rapidly after being written. 


I've been looking around for existing solutions, which are "Hadoop friendly" (Hortonworks, Cloudera) and available DBMS, like Cassandra. 
I have found some interesting information, but I still think it's hard to decide, which one to choose. 
It also need drivers for NodeJS &amp; .NET (Hadoop with Cassandra seem to be OK with that). 
I've also looked around the MongoDB ecosystem, but, again, I feel that it is hard to know where to look at. By (little) experience with Mongoose, MongoDB can be disqualified by the third point because data writes are slow. But my model could certainly be improved.

Do any of you have some recent experiences, manipulating massive amount of geolocation data? I would appreciate sharing them here as well as any quality and recent literature on the subject.
One approach can be to use a no-SQL database running on top of a distributed store (like Cassandra or Hbase). Add an external index which support spatial indexing (say elastic) for fast search. This makes your solution scalable (due to distributed store) and fast enough for spatial search queries.