Where are interfaces usually placed in a project?

In Go, interfaces are usually placed in the package where they are most logically used and where they define the behavior necessary for working with the types of that package. Interfaces are often placed in the same package as the functions or methods that accept these interfaces as parameters.

This allows:

• To ensure loose coupling between packages.
• To enable other packages to implement the interface without depending on its definition.

For example, if you have a package storage that uses an interface Reader, then the Reader interface is usually declared in the storage package, and specific implementations can be located in other packages.