When is the CQRS pattern used, and how will the database work when services are logically separated into read and write — will it be one or different?

The CQRS (Command Query Responsibility Segregation) pattern is used when there is a need to separate operations that modify state (commands) from data reading operations (queries) to improve scalability, performance, and architecture simplicity.

When services are logically separated into read and write, databases can be organized differently:

• Single database: both read and write operations work with the same database but through different services. This is simpler but may limit scalability and performance.

• Different databases: the write service updates the main database, while the read service uses a separate database optimized for queries (e.g., a replica or specialized storage). Data is synchronized asynchronously between them (via events, queues, etc.).

The choice depends on system requirements. Using different databases allows optimization for specific tasks but complicates data consistency.

Example:

• The write service updates transactional data in the main database.
• The read service reads from a replica or a denormalized storage updated through events.

Thus, CQRS is often accompanied by database separation to enhance efficiency and scalability.