Server-to-Server (S2S) Integration

A postback (also called a server-to-server callback or S2S callback) is a server-side communication mechanism used to send conversion data from one system to another without relying on client-side tracking. In mobile attribution, postbacks are the primary method by which Mobile Measurement Partners (MMPs) communicate install and event data back to ad networks. When a user installs an app and the MMP attributes that install to a specific campaign, the MMP fires a postback to the relevant ad network containing the attribution details (campaign ID, ad group, creative, conversion timestamp, etc.). This enables the ad network to optimize its algorithms and allows the advertiser to measure performance. In the SKAdNetwork framework, Apple sends postbacks directly from the device to the ad network, bypassing the MMP entirely for initial attribution. Postbacks are also used in web-to-app flows to communicate web conversion events to advertising platforms — for example, when a user completes a subscription purchase on a web checkout page, a server-side postback can be sent to Meta or Google to feed their optimization algorithms with conversion signals.

A Mobile Measurement Partner (MMP) is a third-party analytics platform that provides independent attribution, analytics, and measurement services for mobile app marketing campaigns. MMPs track which ad clicks and impressions lead to app installs and post-install events, providing a unified view of marketing performance across multiple ad networks and channels. Major MMPs in the mobile ecosystem include AppsFlyer, Adjust, Branch, Singular, and Kochava. MMPs play a neutral intermediary role — since they are independent from both the advertiser and the ad networks, their attribution data is considered unbiased. Core MMP capabilities include last-touch attribution, multi-touch attribution, fraud detection, deep linking, audience segmentation, SKAdNetwork management, and cohort-based analytics. In the post-ATT landscape, MMPs have evolved to offer probabilistic modeling, predictive analytics, and aggregated measurement solutions that work within Apple's privacy constraints. For subscription apps, MMPs provide critical insights into which channels and campaigns deliver subscribers with the highest LTV, enabling marketers to shift budget toward the most profitable traffic sources.

Mobile attribution is the process of connecting app installs and in-app actions to specific marketing campaigns, ads, or channels that drove them. It enables marketers to understand which advertising efforts deliver results, optimize ad spend across channels, and make data-driven decisions about user acquisition strategies.

Third-party billing refers to the use of external payment processors — such as Stripe, Paddle, Braintree, or Adyen — to handle subscription and purchase transactions outside of Apple's App Store or Google Play's native billing systems. Third-party billing enables app developers to process payments on their own terms, typically through web-based checkout flows, and often at significantly lower processing fees (2–5% compared to 15–30% platform commissions). This approach has become increasingly viable as regulatory pressure and legal rulings in multiple jurisdictions have forced app store platforms to allow or accommodate alternative payment options. For developers implementing web-to-app subscription models, third-party billing is the technical backbone — it handles payment processing, subscription management, invoicing, tax calculation, PCI compliance, and often provides webhook-based events for real-time subscription status updates. The trade-off is added complexity: developers must build and maintain the web checkout experience, handle subscription lifecycle management outside the native app store frameworks, and ensure a seamless user experience as subscribers transition between the web payment flow and the app itself.

Web-to-App (Web2App) refers to the strategy and infrastructure that enables mobile app developers to acquire users and process subscription purchases through web-based flows before transitioning the user into the native mobile app experience. In a Web2App model, a user typically discovers the app through a paid advertisement or organic search, lands on a web page where they learn about the product's value proposition, completes a subscription purchase through a web-based checkout (processed by a third-party payment provider like Stripe), and is then directed to download and activate the app with their subscription already provisioned. The Web2App model has gained significant adoption among subscription apps for several reasons: it allows developers to avoid the 15–30% App Store and Google Play commissions by processing payments outside the native billing systems; it provides richer attribution data since web-based tracking isn't subject to the same ATT restrictions that limit in-app attribution on iOS; it offers a larger canvas for communicating value and presenting offers compared to in-app paywalls; and it enables more flexible pricing, offer, and discount strategies that aren't constrained by app store billing rules. The primary technical challenges of Web2App include maintaining a seamless user experience across the web-to-app transition, reliably syncing subscription entitlements between the web billing system and the app backend, handling edge cases (user doesn't download the app, switches devices, etc.), and solving post-iOS 14.5 attribution challenges that affect the ability to connect web conversion events to the advertising campaigns that drove them.