Blog

Engineering notes from a visual Kafka Streams builder: client-library traps, testing stories, and the reasoning behind design decisions. RSS.

• Debezium change events in Kafka Streams: flatten, route, and survive the delete 22 Sept 2026

  A Debezium change event is an envelope: before, after, op, source, and a timestamp or three. The first three things everyone does with it in Kafka Streams are the same — flatten after, route by op, and survive the delete, whose after is null and which is followed by a tombstone. Each is one expression, and each can be proved against a captured change event before anything is deployed.

• A Kafka Streams topology starts in a sandbox, not on your cluster 19 Sept 2026

  The expensive part of a Kafka Streams topology is finding out whether the logic is right, and the place to find out is a stack of your own that you are free to break: one command, its own broker, your schemas, a sample of your records. Design it there, prove it against real records, and carry it to the cluster that matters as a file.

• Monitoring Kafka Streams applications: the state a cluster view cannot show 14 Sept 2026

  Monitoring Kafka Streams from the cluster shows groups, offsets and lag. What the application is doing — restoring a store, lagging on a standby, losing threads — is in the process, not on the cluster. Why we built a console around the stream, and the State tab that shows what the process knows.

• The compiler was never what you wanted 3 Sept 2026

  Reading an Avro topic from Kafka Streams in Java is seven steps, and five of them run again every time you need a field somebody added. What that buys is a compiler — and a compiler answers a narrower question than the one you actually have.

• SpEL in Kafka Streams: from evaluator to editor 31 Aug 2026

  Spring's SpEL tokenizer is package-private. What it takes to build an editor on an evaluator: highlighting, completions, and errors a user can act on.

• Kafka Streams topologies you can draw and run 19 Aug 2026

  A Kafka Streams diagram shows the shape but not the logic. What changes when you draw the topology with the expressions inside, checked on real records.

• KafkaStreams.state() says REBALANCING. Every thread is dead. 2 Aug 2026

  A Kafka Streams client whose processing threads have all died keeps reporting REBALANCING — forever. Why it happens, how to detect it, and what any tool showing raw state() should do instead.