Why does MongoDB (Version 3.2) creates two connections instead of one?

[Mayank Jain]

Hi,

I have noticed that after I create a connection, and do say an insert mongodb creates another connection.

I have seen this behaviour with mongodb python and clojure (which uses java) library both.

Here's an example of how to reproduce it:

from pymongo import MongoClient


## Run `db.serverStatus().connections`via mongo shell to check the current number of connections
client = MongoClient()


## Run the `db.serverStatus().connections` command again, it should increment by 1
client.test_database.posts.insert_one({"author": "Mike"}).inserted_id
## Now if you run `db.serverStatus().connections` command again, it again increments by 1

##But if you repeat insert (or any other command besides createing a new connection) it wont create another connection

I am using:

Python        - 3.5.1
Mac OSX       - 10.10.5
Mongo Version - 3.2.4

Is this expected? Has anyone else seen this?

Thanks.

[Jason Carey]

Mayank,

You may be seeing the Monitor Thread.  Drivers are required by the MongoDB Server Discovery and Monitoring Specification to do a variety of background health checks to monitor the topology behind a given MongoDB URI.  This catches things like replica set failover, the addition of new mongos' in sharding, etc.

Most drivers handle this with a background thread, those that don't are forced to periodically block during processing instead.

-Jason

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[Mayank Jain]

Thanks :)