There are requirement for application to follow 3 layers architecture to separate database, application and front-end UI. How to do it in the platform? What I imagine: 1. Separate espace for data model, actions and screens 2. Publish espace to different servers. For screens, we are not supposed to run get data in Preparation? All required data will be pass in as input? How to do refresh for only one dropdown? Kindly share some lights.

HI Leo,
This will help you to understand how OS application architecture should be designed..Also there are seprate articals about having multiple front end and zones to deploy diffrent espaces.

https://success.outsystems.com/Support/Enterprise_Customers/Maintenance_and_Operations/Designing_the_architecture_of_your_OutSystems_applications

Regards
-PJ-

You always need to get data in your screen's Preparation.

If you have a requirement that your front-end server doesn't connect directly to the database (I've had clients who required this - mostly banks), I would suggest you write your business layer as REST services and only use those REST services as data sources.

sorry my question is not specific enough. considering a example: 
i have espace A deploied to database server, it has one entiry EMPLOYEE
i have espace B deployed to application server, it has actions to create employee
i have espace C deoployed to frontend server, it has UI for user to create new employee

scenario 1: if in espace C, i referrence espace A and give rights to create EMPLOYEE record, is this considered as two tiers?
scenario 2: if in espace C, i referrence espace A, and get data in preparation and show in web pages, it this consider as two tiers?

if both are not, does it means i have to create actions/web services in espace B for espace c to consume, then is it 3 tiers?