When / then

One of the primary constructs in the DropStream rule syntax is when/then for checking a condition. This construct is represented as an IF-THEN construct in DropStream's GUI Editor and works like a programming language if-then statement that evaluates whether an expression is true, and if it is, does something.

For example, consider the following rule:

when
      order_item.sku is "My-Order"
then
      add 1.0 item to order_items in order
        set item.sku to "You-got-a-gift"
      end
end

When an order is processed by DropStream, the rules engine will search for a particular SKU called “My-Order”. If such an item is found, i.e., the condition after when evaluates to true, the then block will be executed, and a new order item with an SKU "You-got-a-gift" will be added to the order.

A when/then construct ends with an end keyword.

Note: add statements also include an end, as illustrated by the following code snippet:

add X item to order_items in order
  set item.sku to “string”
end

Note: A common mistake that can occur while adding an item, is using order_item.sku or order_items.sku in the set command, instead of item.sku. Here, item in the add statement is a variable that is declared in the add command, so it should be used in the same form after set, which is "within the scope" of add.

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