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Currently using NGINX as a load balancer for an upstream to a sharded mongo cluster.

.conf reads:

stream {
    upstream prodapp_mongo {
        least_conn;

        server xxx.xxx.xxx.xxx:27017;
        server xxx.xxx.xxx.xxx:27017;
        server xxx.xxx.xxx.xxx:27017;
    }   

    server {
        listen 27017;
        proxy_connect_timeout 25s;
        proxy_timeout 60s;
        proxy_pass prodapp_mongo;
        error_log /var/log/nginx/mongo.error.log;
    }
}

stub_status output:

Active connections: 15751
server accepts handled requests
 15070690 15070506 129824
Reading: 0 Writing: 2 Waiting: 0

The number of active connections seems to be pegged around 15k for about 14 hours and I am concerned as to why they are not in a state of reading/writing/waiting.

Our data-dog reports show a steady increase of connections over this time period and I'm trying to further dig into "why" there are so many active connections, and what are these doing?

enter image description here

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"why" there are so many active connections

nginx decreases Active connections count on client's connection close, so it's certain that you have a lot of idle connections there. It's established, but it's currently not sending/receiving any data.

Check your DB connections from mongodb by connecting to the db and run

db.serverStatus().connections

and what are these doing?

As you don't describe your application/database client, it's probably coming from application side of things, such as connection pools from DB drivers, application's persistent connections, or even non-close()d connections.

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