How to fix NotWritablePrimary: not primary in MongoDB

Check your MongoDB connection string to ensure it connects to the replica set name, not individual nodes. Use the correct format:

mongodb://host1:27017,host2:27017,host3:27017/?replicaSet=rs0

Avoid connecting directly to a single node:

// Bad - connects to secondary
mongodb://secondary-node:27017

Good - connects to replica set and automatically finds primary:

// Good - connects to replica set
mongodb://primary:27017,secondary1:27017,secondary2:27017/?replicaSet=myReplicaSet

Connect to any replica set member using the MongoDB shell and check which node is the primary:

mongosh
rs.status()

Look for the member with "stateStr" : "PRIMARY" in the output. Verify all members show a valid state (PRIMARY, SECONDARY, ARBITER). Any members in STARTUP, RECOVERING, or UNKNOWN states indicate potential issues.

If your connection string is hardcoded to a specific node, update it to use the replica set name instead:

// Node.js example
const uri = "mongodb://primary:27017,secondary1:27017,secondary2:27017/?replicaSet=mySet";
const client = new MongoClient(uri, { retryWrites: true });

# Python example
from pymongo import MongoClient
uri = "mongodb://primary:27017,secondary1:27017,secondary2:27017/?replicaSet=mySet"
client = MongoClient(uri)

Ensure your application is not explicitly setting read preference to "secondary" or "secondaryPreferred" for write operations:

// Node.js - use primary read preference for writes
const db = client.db("mydb", { readPreference: "primary" });
const result = await db.collection("users").insertOne({ name: "John" });

# Python - correct read preference
from pymongo import ReadPreference
client = MongoClient(uri, readPreference=ReadPreference.PRIMARY)

If you just experienced a primary failure, wait 10-30 seconds for MongoDB to automatically elect a new primary. During this election window, write operations will fail. Most MongoDB drivers automatically retry failed operations once the new primary is elected.

You can monitor the election progress:

rs.status()

Wait until one member shows "stateStr" : "PRIMARY".

Verify all replica set members can communicate with each other. Test connectivity:

# From each node, test connectivity to other nodes
telnet host2 27017
telnet host3 27017

If any connections fail, check firewall rules, security groups, and network configuration. All replica set members must have bidirectional connectivity on the MongoDB port (default 27017).

For a replica set to elect a primary, a majority of voting members must be online:

- 3-member set: need 2 members available
- 5-member set: need 3 members available
- 2-member set + arbiter: need primary + arbiter

If you have multiple downed members, bring them back online or remove them from the replica set configuration:

rs.remove("hostname:27017")