ExtServices is an updatable Mainline module that contains the logic used by Android framework components such as storage cache, autofill, textclassifier, package watchdog, notifications and notification ranking that runs continually. These are algorithms for framework, making the module updatable to introduce the new features or bug fixes more frequently with monthly release than a platform quarter release cycle.
Before you start to develop on ExtServices, please read go/smart-os-extservices to get the guideline.