Flink

Flink processes streaming data at scale: real-time event processing, continuous ETL, streaming analytics, all with exactly-once processing guarantees. Picture a factory assembly line for data: events flow in, get transformed, aggregated, and routed, all with exactly-once guarantees so nothing gets lost or double-counted.

Apache 2.0. Flink handles both stream processing (real-time) and batch processing (historical) through the same API. It manages state across billions of events, handles late-arriving data with watermarks, and checkpoints automatically for fault tolerance.

Fully free. No paid tier from Apache. Confluent and AWS offer managed Flink services ($0.11-0.18/hr per compute unit on AWS), but the open source version is complete.

The catch: Flink is not simple. Setting up a production Flink cluster requires serious ops knowledge: YARN or Kubernetes deployment, tuning checkpointing intervals, managing state backends (RocksDB), monitoring backpressure. This is enterprise infrastructure. A solo developer processing a few thousand events per second should look at simpler tools first.