I have implemented a Login module in Kinvey. I am able to write data in the data store. I want to check if the user logging in is already present in the data base. How can I write a business logic for this? How can I call this code in business logic from the client?

I am doing a simple web login, trying to get a grip on kinvey, I fail to understand the business logic part, before-after-save :( , How do I pass values to my business logic

updated post, your help is much needed, I cant figure this out, any example mobdro