Marketo customers have grown bigger and bigger and are demanding exceptional marketing campaign processing power, given finite compute resources. Our legacy campaign processing engine, built on top of LAMP, is not going to cut it. While Marketo has transformed some legacy systems using Big Data technologies like Spark and HBase, these technologies are not a good fit for our next generation campaign processing engine. We decided to embrace Reactive but the journey was not without challenges: - First, we needed to complete multiple prototypes to prove to the architects that Reactive system was a better fit. - Second, we had little prior knowledge in reactive programming. We put together a training plan on various topics, including Scala, Akka cluster, Reactive Programming methodologies, etc. - Third, it is important for our new campaign processing engine to co-exist with the legacy one so we can migrate the sub-components one by one, while not interrupting services for customers. In the end, it has been a success and many Marketo customers have been migrated to the new campaign processing engine and are enjoying the spectacular scalability and throughput.