check_mongodb returns python error on MongoDB 4.x database size check

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While applying mongodb monitoring checks on new MongoDB servers, the following error was showing up when the database_size action (check type) was used:

root@mon:/usr/lib/nagios/plugins# ./ -H -P 30000 --all-databases -A database_size
Traceback (most recent call last):
  File "./", line 1651, in <module>
  File "./", line 247, in main
    return check_all_databases_size(con, warning, critical, perf_data)
  File "./", line 927, in check_all_databases_size
    all_dbs_data = con.admin.command(son.SON([('listDatabases', 1)]))
  File "/usr/local/lib/python2.7/dist-packages/pymongo/", line 439, in command
    uuid_subtype, compile_re, **kwargs)[0]
  File "/usr/local/lib/python2.7/dist-packages/pymongo/", line 345, in _command
    msg, allowable_errors)
  File "/usr/local/lib/python2.7/dist-packages/pymongo/", line 163, in _check_command_response
    raise AutoReconnect(errmsg)
pymongo.errors.AutoReconnect: not master and slaveOk=false

I was unable to find relevant issues in the check_mongodb's Github repository, however I came across one (as of this writing) open issue stating that requires python3.

By simply executing the plugin, the script interpreter in the first line is chosen. Which is referring to env python:

root@mon:/usr/lib/nagios/plugins# head -n 1
#!/usr/bin/env python

On this monitoring server, running Ubuntu 16.04 Xenial, there are python2 and python3 co-existant with the python command pointing to python2:

root@mon:~# ll /usr/bin/python
lrwxrwxrwx 1 root root 9 Nov 24  2017 /usr/bin/python -> python2.7

Note: This is the default until very recent Linux distributions. Only from this year (2020) on, python2 will be gone from new distro releases.

By adjusting the script interpreter to use python3, the plugin should now work, right?

root@mon:/usr/lib/nagios/plugins# head -n 1
#!/usr/bin/env python3

root@mon:/usr/lib/nagios/plugins# ./ -H -P 30000 --all-databases -A database_size
No module named 'pymongo'

Ah! OK now that the script was changed to use python3, the plugin's requirement (pymongo module) also needs to be installed for python3, not just for python2!

root@mon:~# apt install python3-pip
root@mon:~# pip3 install pymongo

Let's try this again, shall we?

root@mon:/usr/lib/nagios/plugins# ./ -H -P 30000 --all-databases -A database_size
CRITICAL - Total size: 545850 MB; Database db1 size: 78 MB; Database db2 size: 10398 MB; Database metrics size: 531602 MB; Database open size: 351 MB; Database mief size: 2320 MB; Database wuff size: 7 MB; Database local size: 1094 MB; Database nagios size: 0 MB

And the plugin works (again)! What I'm still not sure of is why it stopped working on python2 in the first place. It might be working on MongoDB 2.x and 3.x with python2 but not on MongoDB 4.x.

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