Offline and stream processing of big data sets can be done with tools such as Hadoop, Spark, and Storm, but what if you need to process big data at the time a user is making a request? This presentation introduces Vespa (http://vespa.ai) – the open source big data serving engine. Vespa allows you to search, organize, and evaluate machine-learned models from e.g TensorFlow over large, evolving data sets with latencies in the tens of milliseconds. Vespa is behind the recommendation, ad targeting, and search at Yahoo where it handles billions of daily queries over billions of documents and was recently open sourced at http://vespa.ai.