ZooKeeper

Coordination for Solr. Never selected on its own; Solr pulls it in, and it goes where Solr goes — on the same machine, whether that is your web server or a search machine of its own.

Solr runs in cloud mode, and cloud mode keeps its configuration — the collections, their schemas, the login — in ZooKeeper rather than on Solr's own disk. It runs as a single node, keeps its data on disk so a restart loses nothing, and clears out its old snapshots once a day.

Your application never talks to it. Solr finds it at zookeeper:2181, and nothing else needs to. Its memory is sized from the machine it runs on.

#Versions

Pin the version, not the build: name 3.9 and the platform matches it to the current build, so a security rebuild reaches you without anybody editing a repository.

The version you name is the one your environment runs from the next deploy. This service keeps data, and not every version can read another's — before you change it, read Changing a service's version.

#What you can change

Nothing on this one. Everything it takes decides where it connects, what it is, or whether it starts — and an application able to set those could break its own environment in ways nothing here would catch.