Client-side tracking means the user’s browser sends data to ad platforms and analytics tools. Server-side tracking means a server you control does it. Everything else about the debate is detail, but the details decide your data quality, so here they are.
How client-side works, and where it leaks
A tag in the page fires on each event and calls each vendor’s endpoint from the browser. Simple to deploy, and for years it was fine. Now Safari caps script-set cookies at seven days (sometimes 24 hours), Firefox partitions storage, ad blockers strip vendor scripts entirely, and slow third-party tags degrade the page for everyone. Each leak is a few percent. Together they are a materially wrong picture of your marketing.
How server-side works
The browser sends one stream to a first-party endpoint on your own domain. From there, a server distributes events to every destination: Meta, Google, Snap, TikTok, your analytics. The browser does less work, ad blockers see a first-party request they have no reason to block, and cookie lifetime rules for first-party context are friendlier.
Server-side also changes what you can send. A browser only knows the session in front of it. A server can join the order to the CRM record, attach lifetime value, apply consent rules centrally, and hash identifiers before anything leaves your infrastructure.
What does not change
Consent obligations. Server-side is a delivery mechanism; it does not launder data you had no right to send. Your CMP still governs, and a proper setup enforces consent server-side too, which is actually stricter than tag-by-tag enforcement.
The pixel usually stays as well. Platforms recommend both paths, deduplicated by a shared event ID. The pixel carries real-time browser context; the server carries completeness.
The build question
You can build this with server-side GTM: host a container, write and maintain each destination tag, own the dedup logic and the monitoring. Teams with one or two destinations and engineering slack do it well. Past that, maintenance is the product, and it is what platforms like Signals sell: partner-verified destinations kept current as Meta, Google, Snap, and TikTok change their specs, which all four did in the last year.