Apache Flink
Stream-first compute engine — low latency, stateful processing, exactly-once semantics, event-time watermarks as a first-class concept.
Born as Stratosphere at TU Berlin (2009), donated to the ASF in 2014. Differs from Spark Streaming in being true stream (Spark is batch-first with micro-batch streaming). Checkpointing via the Chandy–Lamport algorithm gives exactly-once. SQL API + Table API + DataStream API.
Powers Alibaba (real-time e-commerce), Uber (matchmaking), Netflix (real-time alerts), Stripe (fraud detection). Confluent acquired the team behind Flink in 2024. The standard for stream processing when Kafka alone isn't enough.