Apache Flink

Solution

Processing time windows close on wall clock. Late Kafka events fall in the wrong window. Event time plus watermarks is the honest window. If you cannot explain watermarks, you are not ready to ship the job.

Solution

Checkpoints are automatic recovery. Savepoints are operator-triggered, versioned points for deploy and resize. You do not “take a checkpoint” for a migration; you take a savepoint.

Solution

A transactional or idempotent sink and a business key. Checkpoint barriers without a sink contract are at-least-once in practice. Dual-write without MERGE is a duplicate table.

Solution

Flink for low-latency keyed state and true event-time. Spark for micro-batch ETL that happens to read Kafka. If the SLA is warehouse minutes, you may not need either — Dynamic Tables / DLT might be enough.

Solution

Name the key, salt or pre-aggregate, or isolate the skew. Adding task managers without fixing the key just moves the hotspot. Same answer as Spark skew.