At Criteo, we work at the cutting edge of commerce marketing, using Machine Learning and Artificial Intelligence to help our customers grow their businesses through hyper-relevant advertising. We run tens of thousands of servers, host containers that continuously move across data centers, and scale services through our managed APIs, with HAProxy playing a critical role across our fast-paced, event-driven infrastructure. This presentation will describe our journey to achieve load balancing served via a user-centric API in such a large and complex environment. We will share tricks and design considerations that helped us to go from a user intent expressed through an API to a scalable service running globally.