Why Server-to-Server Tracking Replaced Pixels — And What Breaks When You Get It Wrong

S2S (postback) tracking replaces browser-dependent pixels with direct server-to-server HTTP calls for conversion attribution. The concept is four steps: generate a click ID, store it, fire a postback on conversion, attribute the commission. The implementation is where it breaks. Five failure modes hit every production setup: click ID evaporation (the ID doesn't survive the user journey), postback delivery failure (the HTTP request never arrives), duplicate fires (the affiliate gets paid twice), parameter mismatches (the systems disagree on field names), and timing issues (sessions expire before conversion). This article covers the protocol-level mechanics, production code patterns for each failure mode, and why monitoring is the actual product — not the tracking setup itself.