It seems like you are using spark MLlib's ALS model which performs matrix factorization. The result of the model are two matrices a user-features matrix and an item-features matrix.
Let's say we are going to receive a stream of data with ratings or transactions for the case of implicit, a real (100%) online update of this model will be to update both matrices for each new rating information coming by triggering a full retrain of the ALS model on the entire data again + the new rating. One is limited by the fact that running the entire ALS model is computationally expensive and the incoming stream of data could be frequent.
A single rating can not change the matrices much plus we have optimization approaches which are incremental, for example SGD. There is an interesting (still experimental) library written for the case of Explicit Ratings which does incremental updates for each batch of a DStream:
The idea of using an incremental approach such as SGD. So even if we do an update to the single new rating, only to the user feature matrix for this specific user, and only the item-feature matrix for this specific item rated, and the update is towards the gradient, we guarantee that we move towards the minimum, of course as an approximation, but still towards the minimum.
Hope this answer helps you!