Finally, we derive a reference architecture from the studied eight ad libraries, and we study how these libraries deviated from this architecture in the study period.

We also observe that ad library developers are constantly updating their libraries to support a wider range of Android platforms (i.e., to ensure that more devices can use the libraries without errors). We observe that fixing issues that are related to displaying video ads is the main driver for releasing new versions. To identify the main drivers for releasing a new version, we manually analyze the release notes of the eight studied ad libraries. that reduced their size have done so through: (1) creating a lighter version of the ad library, (2) removing parts of the ad library, and (3) redesigning their architecture into a more modular one. In addition, we observe that some libraries have grown exponentially in size (e.g, Facebook Audience Network), while other libraries have attempted to reduce their size as they evolved. By taking a closer look at the 8 most popular ad libraries, we find that ad libraries are continuously evolving with a median release interval of 34 days. We observe that an ad library represents 10% of the binary size of mobile apps, and that the proportion of the ad library size compared to the app size has grown by 10% over our study period. Then, we identify 63 ad libraries that are integrated into the studied popular apps. To identify popular ad libraries, we collect 35,462 updates of 1,840 top free-to-download apps in the Google Play Store. In particular, we look at their evolution in terms of size, the main drivers for releasing a new version, and their architecture. In this paper, we study the evolution of the 8 most popular ad libraries (e.g., Google AdMob and Facebook Audience Network) over a period of 33 months (from April 2016 until December 2018).

However, little is known about how such ad libraries have evolved over time.

As a result, ad libraries have become an essential part of the mobile app ecosystem. Such libraries are integrated to serve advertisements (ads) to users developers then gain revenue based on the displayed ads and the users' interactions with such ads. To gain ad revenue, app developers integrate ad libraries into their apps. In-app advertisements have become an integral part of the revenue model of mobile apps.

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