As @imslavko said, when using Meteor with MongoDB (which I believe is the only production ready DB driver) it observes the oplog [1] for changes. You can use the polling observer too though.
You can find out more about the LiveUpdate core project of Meteor on their site [2] - it basically says the implementation of Live Updates for each db driver is independent to what the db is capable of. Specific mention of RethinkDB and Firebase is made as DBs that are built with making realtime data something that you get for relatively little work.
You can find out more about the LiveUpdate core project of Meteor on their site [2] - it basically says the implementation of Live Updates for each db driver is independent to what the db is capable of. Specific mention of RethinkDB and Firebase is made as DBs that are built with making realtime data something that you get for relatively little work.
[1] https://github.com/meteor/meteor/blob/devel/packages/mongo/o...
[2] https://www.meteor.com/livequery