MongoDB Atlas clusters have a connection limit per instance size. When the limit is exceeded, no new connections can be opened to the cluster.
Where do Atlas connections come from?
Heartbeats and replication between the nodes (20-25 connections)
Agent connections (backup, monitoring, and automation agents)
Driver connections
Application connections
Note: Some existing connections will automatically exist when you first deploy your Atlas cluster for the monitoring and administrative tasks. This is expected behavior and ensures your cluster(s) are healthy.
How can I see the connections to my cluster?
For all MongoDB Atlas clusters:
The Atlas UI displays the total active connections to each cluster (for the previous 6 hours) and the connection limit.
- Replica sets show the number of active connections to the primary.
- Sharded clusters show the sum of all active connections to each primary in the cluster.
The METRICS tab on your Atlas cluster displays the active connections for the chosen date/time range;
For M10+ Instances:
The Real-Time Performance Panel displays the number of current connections to the machine hosting MongoDB and the number of inbound and outbound bytes as reported by mongostat.
Connect to your cluster via the command line tools (see Connect with Mongostat)
Download your MongoDB logs to examine when connections are opened and closed (click “...” on your cluster and selecting “Download Logs”).
Run the following script (via the mongo shell) to generate a basic list of all active client connections (not available for free tier (M0), M2 and M5 Instances):
db.currentOp(true).inprog.forEach( function(op) { if(op.active = true && op.client != undefined) { var miniOp = { client : op.client, description : op.desc }; printjson(miniOp);} } );
See also the article Closing Connections in MongoDB Atlas Clusters for an explanation of how to close connections to a MongoDB Atlas cluster.