Make Rule classes final

INFO

This document represents an architecture decision record (ADR) and has been mirrored from the ADR section in our Shopware 6 repository. You can find the original version here

Context

The existing rule system is flexible but complex, making it difficult to evolve and maintain. Allowing unrestricted extensions of rule classes slows down improvements and increases the complexity of the system.

See RFC: https://github.com/shopware/shopware/discussions/5785

Decision

We will mark existing rule classes as final, limiting their extension by third parties. Developers should create new rule classes instead of extending existing ones.

Nearly all rule classes will be marked as final, with a few exceptions:

LineItemOfTypeRule
LineItemProductStatesRule
PromotionCodeOfTypeRule
ZipCodeRule
BillingZipCodeRule
ShippingZipCodeRule

These classes will remain extendable for now, because they rely on configuration which is reasonably expected to be extended by third-party developers.

Consequences

• Faster evolution of the rule system
• Clearer extension mechanisms for developers
• Potential migration efforts for third-party developers currently extending rule classes