At what stage and what exactly is recommended to check first during application development according to DDD principles?

According to the principles of Domain-Driven Design (DDD), the primary focus during application development should be on understanding and validating the domain. This occurs during the analysis and design phase of the domain model.

First and foremost, you should:

• Identify the main business entities and their behaviors.
• Verify the correctness of business rules and constraints.
• Ensure that the model reflects real processes and terms understandable to domain experts.

For example, if you are developing a system for a bank, you should first verify how account operations should work, what restrictions exist on transfers, etc., before moving on to technical implementation.

Thus, the primary validation is to validate and refine the domain model and business logic to avoid errors early on and to ensure the correct architecture of the application.