Arroyo vs ksqlDB

Arroyo is an open-source distributed engine for processing stateful computations on data streams, with sub-second results. Users build streaming pipelines in analytical SQL, including complex queries, windows and joins. Arroyo supports checkpointing for fault tolerance and recovery, exactly-once processing, and horizontal and vertical rescaling; it says the engine can handle millions of events per second. Its connectors include Kafka, Kinesis, MQTT, MySQL, Postgres, Redis, Delta Lake, HTTP and WebSocket, among others. It reads and writes JSON, Avro, Parquet, text and binary, and includes more than 300 SQL functions. Rust scalar, aggregate and asynchronous user-defined functions can extend SQL. The web UI supports connection management, query development and testing, and pipeline monitoring; pipeline management is also available through a REST API. Single-node clusters are intended for development and testing, while production deployments use a distributed cluster with Arroyo’s scheduler or Kubernetes. It is Apache 2.0 licensed, free at free, and self-hostable.

ksqlDB has no write-up on Specifiction yet; its figures are above.