One mongod is enough to learn queries, indexes and aggregation. It is not enough to run anything real: one copy of the data, no failover, no authentication, no warning before memory runs out. Getting MongoDB Running covered installing and starting a server, so none of that is repeated here. The commands below ran against a real single-node replica set named rsops on MongoDB 8.3.11 1,815 — enough to show elections, oplog entries, write concern errors, dumps, users and roles. The two places where it is not, a genuine failover and a sharded cluster, are called out where they appear.