Replica set je grupa MongoDB servera koja održava iste podatke — pružajući visoku dostupnost (automatski failover) i redundanciju. Jedan čvor je primary (obrađuje upise), ostali su secondaries (repliciraju podatke primara i mogu poslužiti čitanja). To je MongoDB-ov standardan mehanizam za pouzdanost.
Struktura replica seta
PRIMARY (1) → receives all WRITES; replicates changes to secondaries
SECONDARIES (2+) → copy the primary's data (via the oplog); can serve READS
ARBITER (optional) → votes in elections but holds no data (for odd vote counts)
Write → Primary → replicated to Secondaries (via the operations log "oplog")
