The text discusses stateful stream processing, which allows for real-time materialized views of data to be generated. ksqlDB is an event streaming database that makes it easy to build these views using Apache Kafka and RocksDB. Materialized views are pre-aggregated versions of data that can be read-optimized, making queries faster. ksqlDB stores materialized views in two places: on its server in RocksDB for fast access and in Kafka's brokers in the changelog for durable storage and recovery. The design allows for fault tolerance and recovery from hardware failures. However, it also requires careful consideration of data partitioning and shuffling to ensure efficient performance. The text concludes that real-time materialized views are a powerful construct for figuring out what is happening right now, with strong fault tolerance and performance guarantees.