How to store user actions following the example of social networks?

Question

NoSQL

Hello! Almost all social networks have a notification system with messages like: Your comment was answered, your photo was liked, your comment was liked, etc.
How is this data stored? In a separate "events" table? It seems that if someone made an action there, this action is recorded, and then other users who are allowed to see it see it? Or is it collecting data on all tables (comments, likes, answers), roughly speaking, using UNION?

Answer 1

In the social networks still use graph DBMS.
OrientDB, ArangoDB, and others

Answer 2

See the word UNION suggests that you are going to use SQL, and you will also experience the overhead of the DBMS for providing transactions and the relational model, but most of your operations are fetching data with filters and appending to the end.
It is not necessary to provide transactions in general and the relation to other tables is very doubtful. It's probably best to look in the direction of NoSQL with MapReduce support.