Why is pymongo 3 giving ServerSelectionTimeoutError?

[Mark Stephens]

http://stackoverflow.com/questions/31030307/why-is-pymongo-3-giving-serverselectiontimeouterror

I'm using:

• python 3.4.2
• pymongo 3.0.2
• mongolab running mongod 2.6.9
• uwsgi 2.0.10
• cherrypy 3.7.0
• nginx 1.6.2

uwsgi start params:

--socket 127.0.0.1:8081 --daemonize --enable-threads --threads 2 --processes 2

I setup my MongoClient ONE time:

    self.mongo_client = MongoClient('mongodb://user:p...@host.mongolab.com:port/mydb')
    self.db = self.mongo_client['mydb']

I try and save a json dict to mongo:

    result = self.db.jobs.insert_one(job_dict)

It works via a unit test that executes the same code path to mongodb. However when I execute via cherrypy and uwsgi using an HTTP POST, I get this:

pymongo.errors.ServerSelectionTimeoutError: No servers found yet

Why am I seeing this behavior when run via cherrypy and uwsgi? Is this perhaps the new thread model in pymongo 3?

update:

if I run without uWSGI and nginx by using the cherrypy built-in server, the insert_one() works.

update 1/25 4:53pm EST:

After adding some debug in pymongo, it appears that topology._update_servers() knows that the server_type = 2 for server 'myserver-a.mongolab.com'. However server_description.known_servers() has the server_type = 0 for server 'myserver.mongolab.com'

This leads to the following stack trace:

result = self.db.jobs.insert_one(job_dict)
File "/usr/local/lib/python3.4/site-packages/pymongo/collection.py", line 466, in insert_one
with self._socket_for_writes() as sock_info:
File "/usr/local/lib/python3.4/contextlib.py", line 59, in __enter__
return next(self.gen)
File "/usr/local/lib/python3.4/site-packages/pymongo/mongo_client.py", line 663, in _get_socket
server = self._get_topology().select_server(selector)
File "/usr/local/lib/python3.4/site-packages/pymongo/topology.py", line 121, in select_server
address))
File "/usr/local/lib/python3.4/site-packages/pymongo/topology.py", line 97, in select_servers
self._error_message(selector))
pymongo.errors.ServerSelectionTimeoutError: No servers found yet

[Bernie Hackett]

Are these two machines (myserver-a and myserver) in a replica set? If so, are both of those hosts reachable from where you are running nginx?

[Mark Stephens]

This is a sandbox instance on mongolab.  I can ping both hosts from my linux box on AWS.

[Bernie Hackett]

Are both hosts mongod instances? If so, are they in the same replica set?

On Wed, Jul 1, 2015 at 2:13 PM, Mark Stephens <markrs...@gmail.com> wrote:

This is a sandbox instance on mongolab.  I can ping both hosts from my linux box on AWS.

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[Mark Stephens]

It looks like this is just a single instance:

nslookup myserver.mongolab.com

 

Server: 192.168.1.254

Address: 192.168.1.254#53

Non-authoritative answer:

myserver.mongolab.com canonical name = myserver-a.mongolab.com.

myserver-a.mongolab.com canonical name = h000000.mongolab.com.

h000000.mongolab.com canonical name = ec2-54-92-xxx-xxx.compute-1.amazonaws.com.

Name: ec2-54-92-xxx-xxx.compute-1.amazonaws.com

Address: 54.92.xxx-xxx

[Bernie Hackett]

We think your problem may be related to this ticket, which we are investigating:

https://jira.mongodb.org/browse/PYTHON-961

[Mark Stephens]

I read that blog post a few days ago and ensured I was using a single MongoClient() instance.  I still see the same behavior.

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[Bernie Hackett]

I don't think it matters how many instances of MongoClient you create. The problem seems to have something to do process forking through the multiprocessing module. To test a hunch, could you pass connect=False when you create the MongoClient instance?

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[Mark Stephens]

That works, I'm able to persist data by calling insert_one() when passing connect=False to MongoClient()

[Bernie Hackett]

Great. That's very useful. I suspect there is some kind of race condition between the multiprocessing module forking the process and MongoClient spawning its monitor thread.

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