I am exploring Cassandra DB and I've come across [this video](https://academy.datastax.com/courses/ds220-data-modeling/quick-wins-challenge-6) which explains how to denormalize tables to store comments and it goes like this: 

Instead of having a comments table and users\_id column in that table that points to the user wrote the comment and another table which is a video table and in the comments table a column that is video\_id which points to the video that has been commented. 

We will have two comments table one comments\_by\_user and another one comment\_by\_video the question is how to keep these two table synchronized ?

When a user comments on a video we insert a comment in comments\_by\_video and comments\_by\_user, for a video and user respectively. However, what if the the second insert fails ? 

We will have a comment by a user on a video that however cannot be found when we select all comments for that user ?