If you are using the Enterprise Routing API you could do the following:
1) Make a series of routing requests to HERE from your point A to your point B using the manueverattribute=none&routeattributes=none
parameters in the request which suppresses unwanted information. Store the routeId
in your database:
2) When a user queries your sparse database from from your point A to your point B , retrieve the routeId
from your database and make a query to HERE using the getroute
endpoint passing in the id from previous request as shown:
This way you can get away with storing the minimal amount of information yourself - i.e. a matrix of all possible start points against all possible end points, and use the RESTful routing API to do all the intermediate calculations.