Full Answer

Browser-side tracking works by loading JavaScript on the visitor's device. The script fires when a page loads, a button clicks, or a purchase completes. Every step depends on the browser cooperating — executing the script, allowing cookies, and not blocking the network request to the tracking server. Ad blockers prevent script loading for 31.5% of users. Safari caps cookies to 7 days. Consent banners cause 30–50% of visitors to decline tracking entirely.

First-party event streaming removes the browser from the critical path. When a WooCommerce customer completes a purchase, the PHP hook woocommerce_payment_complete fires on your server. The event data — order value, products, customer identifiers, attribution parameters — is captured at the application layer and sent from your server to every destination simultaneously. No JavaScript execution required. No browser cooperation needed.

The first-party subdomain is a critical detail. Events originate from a server on your own domain — data.yourstore.com — making the entire data flow first-party. Ad blockers that block third-party tracking domains do not block requests from your own subdomain. Safari's ITP restrictions that target cross-site tracking do not apply to same-site server requests.

The simultaneous multi-destination delivery is what separates event streaming from ETL. An ETL tool pulls order data from your database on a schedule. Event streaming pushes each event to GA4, Facebook, Google Ads, BigQuery, and Klaviyo in real time as it happens. Ad platforms receive conversion signals within seconds of the purchase, not hours later.