The business logic layer is meant to handle business objects and the interaction between them. Decouple it from the service and database layers—it should know as little about the database access or user interaction as possible, yet exchange information with them as needed using a level of abstraction, such as interfaces or base classes. The business logic should focus on transforming and calculating data, leaving other tasks to other layers.

Minimize the complexity of the business logic itself by separating concerns into different areas. Keep the processing, workflow, and business entities separated and loosely coupled. The separation will make the implementation easier to follow, whereas loose coupling will allow modification with a relatively low cost. Then, make sure you avoid the duplication of functionalities in different areas by reusing common parts of business logic.

Identify the consumers of the business layer so that the data can be exposed in the desired way. This will prevent the additional effort of converting data from one format to another. Having consumers in mind, make sure you have prepared not only the functional logic, but also various auxiliary aspects, such as security requirements, validations, exception management, and concurrency—keep them consistent and manage them centrally if possible.

Do not forget about unexpected situations and audits—use logs to store the history of critical changes or errors, yet without business-sensitive data. Ensure that errors in the logging process itself do not affect the normal functionality of the system—keep it as a separate logical unit. Make sure that the logged information is sufficient to track the root causes of any problems.