What is CQRS and what problem does it solve?

CQRS (Command Query Responsibility Segregation): separate the write model (Commands) and read model (Queries) into different components. Problem it solves: write model is optimized for consistency and correctness (normalized, ACID, business rules). Read model is optimized for query performance (denormalized, indexed for specific queries, possibly in Redis or Elasticsearch). Without CQRS: the same