Older versions of Apache Druid supported a standalone 'Realtime' process to query and index 'stream pull' modes of real-time ingestion. These processes would periodically build segments for the data they had collected over some span of time and then set up hand-off to Historical servers.
This processes could be invoked by
org.apache.druid.cli.Main server realtime
This model of stream pull ingestion was deprecated for a number of both operational and architectural reasons, and removed completely in Druid 0.16.0. Operationally, realtime nodes were difficult to configure, deploy, and scale because each node required an unique configuration. The design of the stream pull ingestion system for realtime nodes also suffered from limitations which made it not possible to achieve exactly once ingestion.
druid-rocketmq were also
removed at this time, since they were built to operate on the realtime nodes.