Troubleshooting KSQL – Part 1: Why Isn't My KSQL Query Returning Data? | Confluent

This post provides a foundational guide to troubleshooting ksqlDB queries that are not returning data. It breaks down common issues into five categories: no data in the source topic, no new data arriving, ksqlDB consuming from a later offset, data not matching predicates, and deserialization errors. For each issue, it offers diagnostic steps using ksqlDB commands (DESCRIBE EXTENDED, LIST PROPERTIES, SET) and external tools like kafkacat. It emphasizes the importance of verifying source topics, understanding offset management (setting 'auto.offset.reset' to 'earliest'), and systematically debugging predicates and data formats.