claude-code-mastery-project-starter-kit: Skill for Claude Code

MongoDB Replica Sets in Production

How to run a replica set, not how to query it (that's mongodb-rules). These are the operational decisions Claude tends to get wrong.

Topology: odd voting members, real hostnames, never localhost

Use an odd number of voting members, three, so a majority can still elect a primary when one is lost. A two-member set has no majority if either dies, it goes read-only. Avoid arbiters unless you truly must: an arbiter holds no data, so a three-node primary-secondary-arbiter set that loses its one data secondary can no longer satisfy w:"majority". Address every member by a hostname that all other members and all clients can resolve and reach; localhost in the config is a classic break, the other members can't reach it. Initialize once, from a single member, after all members are up, and wait for the election before creating users.

rs.initiate({ _id: "rs0", members: [
  { _id: 0, host: "mongo1.internal:27017", priority: 2 },
  { _id: 1, host: "mongo2.internal:27017", priority: 1 },
  { _id: 2, host: "mongo3.internal:27017", priority: 1 }
]})

Connect to the set, not to one node

The connection string must list the seed members and name the set, so the driver can find the current primary and follow failover. Pointing the app at a single host throws away the high availability the replica set exists to provide.

mongodb://mongo1,mongo2,mongo3/db?replicaSet=rs0

Durability and reads

Writes always go to the primary. w:"majority" means a majority of members acknowledged before the app gets the OK, that is durability, use it for data you can't lose; w:1 (primary only) is fine for the regenerable. Add wtimeoutMS so a degraded set doesn't block writes forever. Reads default to the primary and are strongly consistent. Secondaries are eventually consistent, they lag, so only route reads to them (secondaryPreferred, secondary, or nearest) when stale data is acceptable, analytics and reporting, not a read-after-write the user just made.