ksqlDB (http://ksqldb.io/ http://ksqldb.io/) does exactly this out of the box. You write SQL queries whose results you can subscribe to with a client connection

ksqlDB (http://ksqldb.io/ http://ksqldb.io/) does exactly this out of the box. You write SQL queries whose results you can subscribe to with a client connection. Today these subscriptions are somewhat expensive, but we're actively working on making them lightweight and scalable. I'm really looking forward to seeing the end result, I think it opens up whole new use cases.