mongodb://mongo1.local,mongo3.local/?replicaSet=demo ⏎
    &readPreference=secondary&readPreferenceTags=dc:europe
Servers (after removing "far away" ones):


mongo3.local - secondary -  4ms - dc:europe,server:big
mongo4.local - secondary - 17ms - dc:europe,server:big
And from those a random one per (read) query is picked.