Zalando, a major European online fashion retailer, has been utilizing Apache Kafka's Streams API since its initial release in 2016 to build real-time applications on AWS, despite initial challenges with stability and resource management. The Kafka Streams API, a Java library within Apache Kafka, facilitates real-time data processing with features like stateless and stateful operations, elastic scalability, and fault tolerance, which are crucial for mission-critical applications. Zalando faced issues such as stream consumer group rebalancing, AWS I/O credit depletion, and memory management challenges, which were gradually resolved through software updates and infrastructure adjustments, including the allocation of larger disk spaces and optimized monitoring solutions. The company developed tools like Remora for consumer lag monitoring and leveraged Dropwizard and Prometheus for comprehensive system metrics, ultimately achieving enhanced productivity and ease of scaling with the Kafka Streams API's user-friendly programming model. Future installments of Zalando's series will explore disaster recovery strategies involving Kafka and Zookeeper backups to Amazon S3.