React to Changes with Apex Triggers

Here’s an example of a simple rule that sends a notification when an item is low on stock.

trigger LowOnStock on Products__ChangeEvent (after insert) {
    // Get the change event’s product ID for added and updated products
    Set<Id> productIds = new Set<Id>();
    for (Products__ChangeEvent event: Trigger.new) {
        if (event.ChangeEventHeader.getChangeType() != 'DELETE') {
            String productId = event.ChangeEventHeader.getRecordIds()[0];
            productIds.add(productId);
        }
    }
    if (productIds.size() > 0) {
        ProductNotifications.notifyOnLowOnStockProductUpdates(productIds);
    }
}

The product notifications class queries the external Product objects for the given changed product IDs and LowOnStock threshold value. Looking up the current Product records on the external system for qualifying products is a callout and is scheduled as an Apex future.

public class ProductNotifications {    
    // Notify subscribers on product updates that are low on stock.
    // This method is run asynchronously as it is performing a callout to the external
    // system to get the latest product details
    @future (callout=true)
    public static void notifyOnLowOnStockProductUpdates(Set<Id> productIds) {
        // Look up the current stock threshold to filter the products with
        Integer productStockThreshold = ...;
        List<Products__x> lowOnStockProducts =
            getProductsLowOnStock(productIds, productStockThreshold);
        // Notify subscribers
        notifySubscribers(lowOnStockProducts,
            ‘These items are running out of stock. Act soon!’);
    }
    
    // Get the external Product objects for each Product ID
    public static List<Products__x>
    getProductsLowOnStock(Set<Id> productIds, Integer productStockThreshold) {
        return [SELECT Id, ExternalId, Name__c, ProductId__c, Stock__c, UnitPrice__c  
            FROM Products__x
            WHERE Stock__c < :productStockThreshold  AND Stock__c > 0 AND 
                Id IN :productIds];
    }
    
    // Notify subscribers by sending an email or SMS
    public static void notifySubscribers(String message, List<Products__x> products) {
        ...
    }
}