Interface on the producer side
Deciding where to implement an interface is a pretty common mistake. An interface defined in the same package as the concrete implementation is referred to as the Producer Side. The Consumer Side is considered to be an interface defined in some external package where it is being used.
Mistake
Creating an interface on the producer side. Interfaces are satisfied implicitly in Go and because of this we should be trying to discover the abstraction, not create it. Creating an interface on the producer side means the producer is creating the abstraction for all it’s consumers.
Fix
The client/consumer should be where the interface is created and should also be the decider if an interface is needed at all.