I am working on one project that is similar to JIO cinema or Netflix. I have designed MySQL database for storing information of channels, VOD(Video on demand such as Movie or series episodes), seasons , series and user. It may also include information related to packages and subscription.

Now my requirements are to track user activities, including

1. Tracking which user is currently watching which VOD or which Channel.
2. Determining how many users are watching a particular channel.
3. Identifying the most-watched VOD.
4. Calculating the amount of time user spend on platform. etc.

To achieve this I need to store user activity and viewing logs, I am currently considering following options and have some concerns.

1. Storing user activity logs in MongoDB,

then the possible schema will be

{
userId : (refers to the MySQL database user table)
channelId : (refers to the MySQL database channel table)
startTime : 
endTime : 
}

I am not sure is it good to store such dependent data in different database , and what consequences we may face in future.

2. Storing user activity logs in MySQL
   If I store user-activity logs in MySQL it self then after some time as users will increases it will grow exponentially (userActivityLogs table to be specific) which will ultimately slow down performance.

3. Should I move existing database structure to MongoDB and store payment related information which required consistency in MySQL ? If so how should I approach this ?

I researched various blogs that explains how Netflix System works and found that they use a Cassandra database for storing Viewing activity information which is NoSQL database

Here is a reference links
Netflix backend System design
, Netflix’s Viewing Data
, Slideshow

Can someone explain in an abstract way(if possible some using some simple example) how I can store such logs or suggest any other approaches I can use ?