Deploy a replica set, even for small applications

A standalone mongod has no automatic failover, so one crashed machine means downtime, and it cannot run multi-document transactions, which need a replica set (or a sharded cluster).

A three-member replica set gives you:

• automatic failover when the primary fails
• data on more than one machine
• transactions and change streams
• the option to read from secondaries for analytics

Use three data-bearing members where you can. Spread them across failure zones, and connect with the replica set connection string (or mongodb+srv://) so drivers discover members and fail over on their own.