Apache Druid

Yes, check overlord configuration <https://druid.apache.org/docs/latest/configuration/#overlord>

Yes it is possible. You would need to have an auto scaling group for middleManager, then you can configure the auto scaling group to scale in/out based on the custom metric (queue depth etc.).

There is the relatively  undocumented `druid-ec2-extensions` that somehow made it into `core` extensions <https://github.com/apache/druid/tree/master/extensions-core/ec2-extensions>
You would need to add the extension, add some Overlord `runtime.properties` like
```druid.indexer.autoscale.strategy=ec2
druid.indexer.autoscale.strategy.type=pendingTaskBased
druid.indexer.autoscale.doAutoscale=true
druid.indexer.autoscale.provisionPeriod=PT1M
druid.indexer.autoscale.terminatePeriod=PT1M
druid.indexer.autoscale.workerIdleTimeout=PT10M
druid.indexer.autoscale.maxScalingDuration=PT20M
druid.indexer.autoscale.workerPort=8091
druid.indexer.autoscale.workerVersion=0
druid.indexer.autoscale.workerCapacityHint=4```
and then add an Overlord dynamic config like the one shown in <https://druid.apache.org/docs/latest/configuration/#overlord-dynamic-configuration>