Monolithic has single database is both working for complex join queries, and also perform CRUD operations.

▪ When application goes more complex, this query and CRUD operations will become un-manageable situation.

▪ Application required some query that needs to join more than 10 table, will lock the database due to latency of query computation.

▪ Performing CRUD operations need to make complex validations and process long business logics, will cause to lock database operations.

▪ Reading and writing database has different approaches, define different strategy.

▪ «Separation of concerns» principles: separate reading database and the writing database with 2 database.

▪ Read database uses No-SQL databases with denormalized data.

▪ Write database uses Relational databases with fully normalized and supports strong data consistency. 

https://medium.com/@knoldus/read-model-and-write-model-cqrs-reactive-architecture-3463b5dee9b3