How to decide whether to create a new instance of the database-related library for each incoming request in the controller?

The decision to create a new instance of a library for database operations for each incoming request depends on several factors:

• Thread safety: If the library is not thread-safe, it is better to create a new instance for each request to avoid conflicts.
• Resource intensity: If creating an instance is costly, consider using an object pool or a singleton with caution.
• Request lifecycle: In web applications, the "per-request" pattern is often used, where a separate database context (e.g., EntityManager in JPA) is created for each request.
• Transaction management: A new instance allows isolating transactions and avoiding unwanted side effects.

In Java applications using Spring, it is common to create a bean with a "request" scope or use @Transactional to manage the lifecycle and create a new EntityManager instance per request. If the library supports pooling, it can be used for optimization.

Therefore, it is preferable to create a new instance for each request if it is not too costly and ensures data safety and isolation.