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In **multi-level (layered) architecture**, layers interact **strictly from top to bottom**, meaning each layer talks **only to the neighboring layer below it**, without skipping levels.

This provides a **clean structure, isolation of responsibility**, and ease of changing the code.

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### 1. Direction of interaction

```javascript
Presentation layer  ↓
Business logic layer ↓
Data layer (storage)
```

- The **presentation layer** (UI) sends requests to the business logic:
for example, "create an order", "show the list of clients".
- The **business logic** processes the rules, does the calculations, and reaches out to the data layer:
"fetch these records from the database", "save the result".
- The **data layer** performs the operations on the database (SQL queries, an ORM, and so on) and returns the data back up.

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### 2. Feedback (the response going up)

The response also moves **from bottom to top**:

```javascript
Data layer → Business logic → Presentation layer
```

- The data layer returns "raw" data (for example, records from a table).
- The business logic transforms them into business objects or ready-made results.
- The presentation layer displays the final result to the user in the needed form.

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### 3. Why this matters

This separation:

- makes the code **modular and testable** - one layer can be changed without breaking the others;
- allows **technologies to be replaced** (for example, changing the database without touching the interface);
- increases **security and resilience** - the upper layers do not have direct access to the data.

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### 4. An example in practice

A user in a web application clicks "Buy":

1. The UI passes the action to the business logic (through a controller).
2. The business logic checks the balance, applies the discount, creates the order.
3. The logic reaches out to the data layer to save the order.
4. The data layer saves the record in the database and returns an ID.
5. The business logic confirms the operation.
6. The presentation layer shows the message "The purchase was completed successfully".

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This is how the **closed loop of layer interaction** works, where each layer only knows its neighbor and is responsible for its own part of the work.

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