Validator composition for DMA validators

[rafaelmag110]

Data Management API controllers use JsonLD validators to verify if the incoming request payload adheres to certain rules. Extensions of these validators might be necessary if someone has additional verification requirements.

In case the new requirements should be composed on top of the verification rules existing in the current validator, the new validator extension has to include the existing rules too, leading to unnecessary code duplication.

Not only that, but to effectively override the existing validator, the extension providing the new validator has to register it in a different boot phase than initiate. This is not an ideal setup.

My point with this discussion is to jointly design a solution that would address these issues. The solution should also allow new extensions that intend to provide additional verification steps to an existing validator to do so without having to duplicate code, and do the registration in a non ideal phase.

[rafaelmag110]

My proposal is the following. In the JsonObjectValidatorRegistryImpl, the implementation of the register method becomes a bit more complex. When a validator is registered using an existing key, the default behavior would not be directly overwriting the existing value with the new one, but instead create a new JsonObjectValidator that is a composition of the existing and the new one.

Achieving this composition is not hard too. The class JsonObjectValidator.Builder could have an additional compose method, that creates a new builder instance by concatenating the list of validators from both validators.

[jimmarino]

That changes the behavior of how we handle registries, it is not apparent from the API signature, and it is likely to lead to unintended behavior.

Generally, when composition is needed, an additional SPI extension point should be created that allows delegates to be registered. This involves explicitly defining how the validation API can be extended, which IMO is a more maintainable solution.