Correlating mongodb connections to application threads

We are currently in the process of making sure all of our applications that connect to mongodb Atlas are using connection pooling. There are several benifits to managing connnections effectively and efficiently, namely that connections are recycled efficiently reducing resource overhead.

The mongodb cost of a connection is 1MB so it can quickly add up and eat into valuable RAM that could otherwise be use for cache activity.

In a managed service world, everything is based around limitations and tiering based on some core components such as CPU, IOPS, disk space and from a database perspective; connections.

The number of connections allowed on a mongodb cluster correlates directly to the instance size. For example;

Instance Size Connection Limit
M10 350
M20 700
M30 2000

To see all the connection limits see

Connections limits mattered less to a DBA in an on prem world, as you would set the ulimit settings for open files and processes/threads to 64000 as per the mongodb recommendations ( However, it becomes extremely critical when you have an M10 that only allows 350 connections of which around 5-10% are taken up by mongo system processes.

Analysing mongodb logs for connections

I use a little app called mtools developed by Thomas Rückstieß who works at mongodb. It is a collection of helper scripts to parse, filter, and visualize MongoDB log files (mongodmongos).

You can pick it up here –

The setup is straightforward and you can quickly start seeing how many connections are being opened and closed grouped by IPs.

mloginfo mongod.log --connections
     source: core-prod-vms-scaled.log
     host: unknown
     start: 2018 Dec 31 10:56:08.404
     end: 2018 Dec 31 13:25:40.320
date format: iso8601-local
     length: 2714
     binary: unknown
     version: >= 3.0 (iso8601 format, level, component)
     storage: unknown

     total opened: 155
     total closed: 143
     no unique IPs: 4
     socket exceptions: 0

35.X.X.1    opened: 55        closed: 55
35.X.X.2    opened: 49        closed: 49
35.X.X.3    opened: 39        closed: 39
35.X.X.4    opened: 12        closed: 0

Correlating open connections against an app server

If we take the example output above and use the 35.X.X.4 IP – we can see that it has sent 12 incoming connections to mongo. The best way i’ve found to see established connections on an app server is to use netstat.

netstat -anp | grep ESTABLISHED | grep ":27017" | grep " 172." | awk '{print $5}' | sort | uniq -c | sort -n
      12 172.X.X.1:27017
      12 172.X.X.2:27017
      12 172.X.X.3:27017

The above is telling us that there are 12 threads connected to 3 different IPs. When looking into the IP’s, they reference the 3 nodes on a mongo replica set which tells us that each connection on mongo is actually 3 threads on an app server (or however many nodes there are in the replica set).


Setting the maxPoolSize  property on the mongo driver will help control how many threads an app server is allowed to open against a mongodb node. Be wary that the maxPoolSize default varies in different drivers – for example, in python its 100, but in node.js its 5.

Knowing the maxPoolSize for applications that have databases on the same cluster can then allow you to accurately calculate what the max connections could potentially be for a cluster. This could then help make more informed decisions about whether to scale or upsize a mongodb cluster or split applications out.

YOu can get more info about connection pool options here –

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