How to find number of MongoDB connections - Andrey Zhukov's Tech Blog

From the MongoDB side the current connections can be found with db.currentOp() command. Then they can be grouped by client ip, counted and sorted.

var ips = db.currentOp(true).inprog.filter(
        d => d.client
    ).map(
        d => d.client.split(':')[0]
    ).reduce(
        (ips, ip) => {
            if(!ips[ip]) {
                ips[ip] = 0;
            }
            ips[ip]++;
            return ips;
        }, {}
    );
Object.keys(ips).map(
        key => {
            return {"ip": key, "num": ips[key]};
        }
    ).sort(
        (a, b) => b.num - a.num
    );

The result will be like this:

[
    {
        "ip" : "11.22.33.444",
        "num" : 77.0
    },
    {
        "ip" : "11.22.33.445",
        "num" : 63.0
    },
    {
        "ip" : "11.22.33.344",
        "num" : 57.0
    }
]

Then if there are several Docker containers on client host, the connections can be found by netstat command in each of them. Suppose there are several MongoDB replicas with ips starting on 44.55... and 77.88..., the command to count all connections to the replicas is:

netstat -tn | grep -e 44.55 -e 77.88 | wc -l