Best way to implement validation rules

Hi, 

I'm developing a system for large scope project. I'm very confuse with validation in the CodeIgniter guide/documentation  it is hard to understand.

What is the best strategy to store validation rule?
Use Validation service or Controller or Models?

If you referring to data-entity validation during db INSERT/UPDATE/DELETE operations,
the setValidationRule method is mentioned in the docs (https://codeigniter.com/user_guide/model...dationRule) as taking place in the model.

Please keep us updated on how your project goes!

Incidentally, how did you select CodeIgniter4?

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(Using CodeIgniter since 1.x)