Check the current number of connections to MongoDb

connect to the admin database and run db.serverStatus():

> var status = db.serverStatus()
> status.connections
   {"current" : 21, "available" : 15979}
> 

You can directly get by querying

db.serverStatus().connections

To understand what does MongoDb's db.serverStatus().connections response mean, read the documentation here.

connections

"connections" : {
   "current" : <num>,
   "available" : <num>,
   "totalCreated" : NumberLong(<num>)
},

connections A document that reports on the status of the connections. Use these values to assess the current load and capacity requirements of the server.

connections.current The number of incoming connections from clients to the database server . This number includes the current shell session. Consider the value of connections.available to add more context to this datum.

The value will include all incoming connections including any shell connections or connections from other servers, such as replica set members or mongos instances.

connections.available The number of unused incoming connections available. Consider this value in combination with the value of connections.current to understand the connection load on the database, and the UNIX ulimit Settings document for more information about system thresholds on available connections.

connections.totalCreated Count of all incoming connections created to the server. This number includes connections that have since closed.

db.serverStatus() gives no of connections opend and avail but not shows the connections from which client. For more info you can use this command sudo lsof | grep mongod | grep TCP. I need it when i did replication and primary node have many client connection greater than secondary.

$ sudo lsof | grep mongod | grep TCP
mongod    5733             Al    6u     IPv4 0x08761278       0t0       TCP *:28017 (LISTEN)
mongod    5733             Al    7u     IPv4 0x07c7eb98       0t0       TCP *:27017 (LISTEN)
mongod    5733             Al    9u     IPv4 0x08761688       0t0       TCP 192.168.1.103:27017->192.168.1.103:64752 (ESTABLISHED)
mongod    5733             Al   12u     IPv4 0x08761a98       0t0       TCP 192.168.1.103:27017->192.168.1.103:64754 (ESTABLISHED)
mongod    5733             Al   13u     IPv4 0x095fa748       0t0       TCP 192.168.1.103:27017->192.168.1.103:64770 (ESTABLISHED)
mongod    5733             Al   14u     IPv4 0x095f86c8       0t0       TCP 192.168.1.103:27017->192.168.1.103:64775 (ESTABLISHED)
mongod    5733             Al   17u     IPv4 0x08764748       0t0       TCP 192.168.1.103:27017->192.168.1.103:64777 (ESTABLISHED)

This shows that I currently have five connections open to the MongoDB port (27017) on my computer. In my case I'm connecting to MongoDB from a Scalatra server, and I'm using the MongoDB Casbah driver, but you'll see the same lsof TCP connections regardless of the client used (as long as they're connecting using TCP/IP).

Connection Count by ClientIP, with Total

We use this to view the number of connections by IPAddress with a total connection count. This was really helpful in debugging an issue... just get there before hit max connections!

For Mongo Shell:

db.currentOp(true).inprog.reduce((accumulator, connection) => { ipaddress = connection.client ? connection.client.split(":")[0] : "Internal"; accumulator[ipaddress] = (accumulator[ipaddress] || 0) + 1; accumulator["TOTAL_CONNECTION_COUNT"]++; return accumulator; }, { TOTAL_CONNECTION_COUNT: 0 })

Formatted:

db.currentOp(true).inprog.reduce(
  (accumulator, connection) => {
    ipaddress = connection.client ? connection.client.split(":")[0] : "Internal";
    accumulator[ipaddress] = (accumulator[ipaddress] || 0) + 1;
    accumulator["TOTAL_CONNECTION_COUNT"]++;
    return accumulator;
  },
  { TOTAL_CONNECTION_COUNT: 0 }
)

Example return:

{
    "TOTAL_CONNECTION_COUNT" : 331,
    "192.168.253.72" : 8,
    "192.168.254.42" : 17,
    "127.0.0.1" : 3,
    "192.168.248.66" : 2,
    "11.178.12.244" : 2,
    "Internal" : 41,
    "3.100.12.33" : 86,
    "11.148.23.34" : 168,
    "81.127.34.11" : 1,
    "84.147.25.17" : 3
}

(the 192.x.x.x addresses at Atlas internal monitoring)

"Internal" are internal processes that don't have an external client. You can view a list of these with this:

db.currentOp(true).inprog.filter(connection => !connection.client).map(connection => connection.desc);