Configure Apache Pulsar Key_Shared subscription with sticky hash range assignment for ordered per-key fan-out

Steps

1. Create consumers with Key_Shared subscription type and sticky hash range mode: use ConsumerBuilder.keySharedPolicy(KeySharedPolicy.stickyHashRange().ranges(Range.of(0, 65535))) divided across consumers
2. Assign non-overlapping ranges to each consumer instance: consumer-1 gets Range.of(0, 32767), consumer-2 gets Range.of(32768, 65535) — ranges must together cover 0 to 65535 with no gaps
3. Produce messages with explicit message keys set; the broker hashes the key into the range and routes to the consumer owning that range
4. When scaling: drain the consumer for the range being reassigned (let it process all in-flight messages), then stop it and update the remaining consumers with expanded ranges before starting the new consumer
5. Monitor per-consumer message rate via the Pulsar admin API (topics stats) and confirm key routing is balanced by inspecting permitsInFlightMessages per consumer
6. For auto-split mode (less control): omit stickyHashRange and use KeySharedPolicy.autoSplitHashRange() — the broker automatically distributes hash ranges but does not guarantee stable per-key assignment during rebalance