MongoServerSelectionError: Fix for MongoDB Atlas

How to fix it

Try these in order — the most common fix is first.

1. 1. Check the IP access list

   1. Go to Security > Network Access and make sure your current public IP is listed and Active
   Did this fix it? Yes No

2. 2. Test DNS and port

   Copy steps
   1. Run dig SRV _mongodb._tcp.<cluster-host> (nslookup -q=SRV on Windows) and confirm one record per node
   2. Run nc -zv <node-hostname> 27017
   Did this fix it? Yes No

3. 3. Switch to the standard connection string

   Copy steps
   1. In the Connect dialog turn off the SRV Connection String toggle
   2. Use the mongodb:// string with all three hosts
   Did this fix it? Yes No

4. 4. Fix connection pooling

   Copy steps
   1. If you see 'too many open connections', reuse one MongoClient per process
   2. Set maxPoolSize or scale the cluster; M0 allows 500 connections
   Did this fix it? Yes No

5. 5. Resume the cluster

   1. On Database > Clusters resume a paused cluster and wait until it is healthy
   Did this fix it? Yes No

Symptoms

MongoServerSelectionError, ServerSelectionTimeoutError (PyMongo) or 'Server selection timed out after 30000 ms' when connecting or on the first query.

Why it happens

• IP not in the Atlas access list
• DNS or ISP blocking SRV lookups
• Outbound port 27017 blocked
• Cluster paused or deleted
• Wrong connection string or credentials
• Too many open connections

This guide also covers

• mongoserverselectionerror
• mongoserverselectionerror atlas
• server selection timed out after 30000 ms
• pymongo serverselectiontimeouterror atlas

The same problem in other apps

All not loading, error messages & connection guides →

• Supabase Max Client Connections Reached (Supabase)
• Firebase auth/invalid-api-key Error (Firebase)

Other MongoDB Atlas errors

• MongoDB Atlas Connection Timed Out (IP Not Whitelisted)
• MongoDB Atlas Authentication Failed (bad auth)
• MongoDB querySrv ENOTFOUND / ECONNREFUSED Error
• MongoDB Atlas Free Tier (M0) Limits and Throttling
• MongoDB Atlas Cluster Paused (Resume or Auto-Pause)
• MongoDB Atlas Unexpected Bill: Why It's So High
• MongoDB Atlas Too Many Connections Error
• MongoDB Atlas AWS VPC Peering Not Connecting