As is shown below, a typical 3-tiers software application includes a presentation layer, business layer, and data layer…
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Presentation layer
The presentation layer receives a client request and responds to the client with structured data.
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Business layer
The business layer controls the business logic, while the request is in accordance with logic rules will fetch data from the data layer
Also if the request is to fetch some hot data, it will be passed and quickly to get data from the non-persistent data layer.
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Data layer
The data layer restores some configuration info, user info, business info, and log info.
The most expensive part of an application is the data. The application fight for fetching data from the business world, then based on the data to make the business decision for the next step.