Build only what is needed now

How to do it

One implementation needs no interface, unless it sits at a boundary such as a database or an outside service. Add the abstraction when the second case arrives. Delete settings that nobody changes.

Example

# Bad: a factory and an interface for the only exporter there will ever be
class ExporterFactory:
    def create(self) -> "IExporter": return CsvExporter()

# Good: call the one thing directly
def export_orders(orders): return to_csv(orders)

// Bad
class ExporterFactory { create(): Exporter { return new CsvExporter(); } }

// Good
const exportOrders = (orders: Order[]) => toCsv(orders);

How to check

In review, every interface has two implementations or sits at a boundary. Every option has a caller that sets it.