Context: We are implementing a news app. For now, you can assume the news to be the same across all users, and maintains an order based on the parameters we set (according to trends, and date).
Problem: We are not sure what the best implementation for keeping track of what users read is. We want to be able to configure a way in which we can track what users read and what they didn’t.
Assumption: You can assume that the posts in the database are in a descending order, based on time.
So, the ideal scenario is that: when there are posts: A,B,C,D,E fetched from the server in the app, and the user read A,B. Now the user only gets to see C,D,E when they check for next posts. If they do previous, they see posts in the following order B-> A.
Furthermore, when P,Q is added to the database, now, the user must see next posts in the order of P->Q->C->D->E and so on.
Example: Let us assume there are 20 news in our app right now, and Gavin picks up his phone and starts reading from our app. In midst of his usage, he finds himself occupied with some other work, so quits the app after reading 5 news posts.
The challenge for us now is to figure the best way to make sure Gavin doesn’t have to re-read the 5 posts he already did.
One way we thought we could solve this problem is through use of index. We can assume uniform ordering for our posts as mentioned in the context, so we could use an index to track where Gavin was last in the order of news and show him news based on that index.
However, one problem with that approach is, we could easily have 5 new posts when Gavin picks up his phone and uses our app again. So, if we have the news based on date, technically that indexing approach means that we omit 5 unread new posts instead of the 5 read old ones.
We've also thought of maintaining three lists: Read, Unread and New so that we fetch only posts that are not in our lists. For example, in my initial example: A-B-C-D-E is in unread initially. Then, after user reads A-B, read becomes A-B. Meanwhile, when P-Q is added in the database, P-Q is added to the list of unread posts as P-Q-C-D-E.
How do you solve this problem? Any suggestions are welcome as we kind of think we're not thinking out of box when it comes to a solution for the problem. Thank you! :)