We've got two absolutely independent systems, which are running in one datacenter. Now we are going to use ELK stack for both of them (logs collection and system monitoring). Each component (Logstash, Elasticsearch and Kibana) will be running in a Docker container.
It seems logical to have an ELK stack per system (because our systems have nothing common), but at the moment we are limited in resources and there is just one VM for monitoring both of them.
Knowing that Elasticsearch is pretty memory hungry, should we share one elasticsearch instance between two systems? If so, is that a good idea to still have separate logstash and kibana instances?