Apache Kafka is used as the primary message bus for propagating events and logs across Uber. In particular, it pairs with Apache Pinot, a real-time distributed OLAP datastore, to deliver real-time insights seconds after the messages produced to Kafka. One challenge we faced was to update existing data in Pinot with the changelog in Kafka, and deliver an accurate view in the real-time analytical results. For example, the financial dashboard can report gross booking with the corrected Ride fares. And restaurant owners can analyze the UberEats orders with their latest delivery status. Implementing upserts in an immutable real-time OLAP store like Pinot is nontrivial. We need to make architectural changes in how data is distributed via Kafka amongst the server nodes, how it's indexed and queried in a distributed fashion. In this talk I will discuss how we leveraged Kafka's partition-by-key feature to this end and how we added this ability in Pinot without any performance degradation.