> ## Documentation Index
> Fetch the complete documentation index at: https://aletyx.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Rule Language

> Structure of a Drools DRL rule - declaration, when conditions (LHS), and then actions (RHS) - with bound variables and worked discount rule examples.

## Rule Structure Comparison

A Drools rule consists of three main parts:

* **Rule Declaration**: Defines the rule name and optional attributes
* **Conditions (LHS)**: The 'when' section that defines what conditions must be true for the rule to fire
* **Actions (RHS)**: The 'then' section that defines what actions to take when the rule fires

```java theme={null}
rule "Rule Name"
// Optional attributes (salience, no-loop, agenda-group, etc.)
salience 10 // (1)
no-loop true // (2)

when
// Conditions (Left-Hand Side)
$customer: Customer(age > 18, status == "active")
    $order: Order(customer == $customer, total > 100) // (3)
then
    // Actions (Right-Hand Side)
    $order.applyDiscount(10); // (4)
    update($order);
System.out.println("Discount applied to order " + $order.getId());
end
```

1. Has a priority of 10 (higher than default)
2. Will not re-trigger even if its actions modify objects (no-loop true)
3. Fires when finding an active customer over 18 with an order over \$100
4. Applies a 10% discount to the matching order

The rule above:

* Has a priority of 10 (higher than default)
* Will not re-trigger even if its actions modify objects (no-loop true)
* Fires when finding an active customer over 18 with an order over \$100
* Applies a 10% discount to the matching order

Notice how variables are bound with the \$ prefix for clarity.

## Writing Your First Rule

Let's create a simple rule for a discount system:

```java theme={null}
rule "First-time Customer Discount"
    when
        $customer: Customer(orderCount == 0)
        $order: Order(customer == $customer)
    then
        $order.applyDiscount(15);
        System.out.println("Applied 15% first-time customer discount");
        update($order);
end
```

This rule matches when:

1. We have a customer with no previous orders (first-time customer)
2. We have an order associated with this customer

When these conditions are met, it applies a 15% discount to the order.

## Additional Resources

* [Rule Attributes](https://docs.aletyx.ai/core/decisions/rule-language/rule-attributes.md)
* [Pattern Matching](https://docs.aletyx.ai/core/decisions/rule-language/pattern-matching.md)
* [Rule Execution Control](https://docs.aletyx.ai/core/decisions/rule-language/rule-execution.md)