Hard to offer suggestions without knowing rough size of data - depending on how much money you're willing to cough up, even 1 TB is in the range of "can fit in the memory" territory.
Having said that, Spark is really great for running iterative algorithms and will definitely fit with what you have described. I suggest staying away from building it on your own using riak/redis (atleast until you have ruled out spark), as you will run into lots of operational issues like handling failures, resource allocation, retries etc.
Having said that, Spark is really great for running iterative algorithms and will definitely fit with what you have described. I suggest staying away from building it on your own using riak/redis (atleast until you have ruled out spark), as you will run into lots of operational issues like handling failures, resource allocation, retries etc.