Definition
Server-to-server (S2S) tracking, also called postback or server-side tracking, records conversions through a direct communication between the advertiser's server and the affiliate's tracking server, rather than through code running in the user's browser. When a conversion happens, the advertiser's server sends an HTTP request (the postback) to the tracker's endpoint, carrying the click ID and event details, and the tracker attributes the conversion without ever relying on a cookie or a pixel firing in the browser.
S2S is the more robust of the two tracking approaches because it does not depend on the browser environment. Cookie and pixel tracking break when third-party cookies are blocked, tracking-prevention features intervene, an ad blocker suppresses the pixel, or the user converts on a different device.
S2S is immune to all of these because the record is passed between servers using an identifier that was planted in the click URL.
In context
For iGaming affiliate programmes, S2S postback is the standard mechanism for the conversion record that governs payment, precisely because gambling traffic runs across many browsers and devices where cookie tracking is unreliable. A typical setup: the affiliate passes a click ID into the operator's tracked link; the operator stores it against the registration; on a qualifying deposit the operator's server posts back to the affiliate's tracker with the click ID, the event type and, often, a value; the tracker credits the source and the affiliate reconciles against the operator's report.
S2S is more reliable but also more work to set up and harder to debug, because there is no visible browser artefact to inspect — a failure is a request that did not arrive or arrived malformed. Implementation is verified end to end before launch (test conversion, confirm postback received with correct parameters), and postback logs are kept for reconciliation.
Many programmes run S2S for the authoritative payment record alongside a pixel for the ad platform's own audience-building and modelled optimisation, since each serves a different purpose.
Worked example
An affiliate programme moves from cookie-based to S2S postback attribution after Safari and Firefox users — a third of the traffic — stopped being tracked. Post-migration, the affiliate's conversion count matches the operator's within normal reconciliation tolerance across all browsers, and payment disputes drop sharply.
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