What does the "business logic layer" do?

The business logic layer (or application logic layer, business logic layer / application layer) is the central part of the system, where the rules, meaning, and behavior of the application are implemented.

It is responsible for what exactly happens to the data and according to which rules.

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#Main tasks of the business logic layer:

1. Implementing business rules Defines what is and is not allowed:

• an order can only be placed if the item is in stock,
• the discount applies when the amount is over 5000,
• the user must be authenticated before purchasing.

2. Processing data Gets data from storage (through the data layer), transforms it, combines it, computes results, and prepares it for display.
3. Managing application processes Defines the sequence of steps, for example "placing an order -> payment -> notifying the customer".
4. The connecting link between layers Receives requests from the presentation layer and reaches out to the data layer, without exposing implementation details.

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#Business logic layer examples:

• In an online store: calculating the order's total, checking that an item is in stock, applying discounts.
• In banking: checking limits, calculating interest, processing transactions.
• In a CRM: automatically assigning a manager to a client, creating tasks.

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#The main rule:

The business logic layer is the "brain" of the system. It does not display data (that is the UI's job) and does not store it (that is the database's job), it decides what to do with it and by which rules.