MongoDB replica sets provide redundancy and automatic failover — keeping multiple synchronized copies of your data across servers, with automatic promotion of a new primary if the current one fails.
What a Replica Set Provides
A group of MongoDB servers maintaining identical data sets — one primary handles writes, secondaries replicate from it and can serve reads; if the primary becomes unavailable, an automatic election promotes a secondary to primary.
Prerequisites
- At least 3 servers running MongoDB (odd numbers are recommended for proper election quorum)
- MongoDB already installed on each (see How to Install MongoDB on Ubuntu & Debian)
- Network connectivity between all servers
Step 1 — Configure Each Server for Replica Set Mode
sudo nano /etc/mongod.confreplication:
replSetName: "rs0"
net:
bindIp: 0.0.0.0Step 2 — Restart MongoDB on Each Server
sudo systemctl restart mongodStep 3 — Initiate the Replica Set (From One Server)
mongoshrs.initiate({
_id: "rs0",
members: [
{ _id: 0, host: "server1:27017" },
{ _id: 1, host: "server2:27017" },
{ _id: 2, host: "server3:27017" }
]
})Step 4 — Verify Replica Set Status
rs.status()Confirm all members show as expected, with one PRIMARY and the others as SECONDARY.
Step 5 — Restrict Network Access Appropriately
Configure your firewall (see How to Configure UFW Firewall on a Linux VPS) to allow MongoDB's port (27017) only between the replica set members themselves and authorized application servers, never openly to the internet.
Connecting Your Application to the Replica Set
mongodb://server1:27017,server2:27017,server3:27017/mydb?replicaSet=rs0The MongoDB driver automatically discovers the current primary and handles failover, routing writes correctly even after an election changes which server is primary.
Read Preference Configuration
readPreference=secondaryPreferredConfigure whether reads should go to the primary, secondaries, or a mix — useful for distributing read load, though be aware secondary reads may reflect slightly stale data given replication lag.
Testing Automatic Failover
Deliberately stop the current primary (in a test environment) and verify a secondary is automatically elected primary within a reasonable time — confirm your application correctly reconnects to the new primary without manual intervention.
Using an Arbiter (Cost-Saving Option for Odd Member Count)
rs.addArb("server4:27017")An arbiter participates in elections (helping maintain quorum) without storing actual data — useful if you want an odd number of voting members without the cost of a full additional data-storing replica.
Common Errors
Replica set members can't see each other — verify network connectivity and firewall rules allow port 27017 between all member servers, and that hostnames used in the configuration actually resolve correctly from each server.
Continue Reading
- How to Install MongoDB on Ubuntu & Debian
- How to Set Up a Read Replica for Scaling Database Reads
- Database Security Checklist: Protecting MySQL, PostgreSQL & MongoDB
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