IInserter.cs

public interface IInserter<TSource> where TSource : ExternalSourceModel
{
  void Insert(TSource source);
}

IUpdater.cs

public interface IUpdater<TSource> where TSource : ExternalSourceModel
{
  void Update(TSource source);
}

IValidator.cs

public interface IValidator<TSource> where TSource : ExternalSourceModel
{
  bool Validate(TSource source, out string validationMessage);
}