Mongo::ConnectionTimeoutError possible problem with threading?

[Michael Lawlor]

Hi there,

I've been running into frequent Mongo::ConnectionTimeoutError exceptions when running MongoMapper behind Sidekiq.

I'm running a Sidekiq server with 30 concurrent actors on MRI 1.9.3. I've configured Mongo to use a connection pool with 40 connections and a 15 second timeout, figuring that should more than cover me. However I hit these timeouts constantly:

Mongo::ConnectionTimeoutError: could not obtain connection within 15.0 seconds. The max pool size is currently 40; consider increasing the pool size or timeout.

As far as I've read online, MongoMapper is thread safe, but I'm wondering if the connections to mongo are not being closed when a Sidekiq thread finishes?

There is a gem for Mongoid that adds some Sidekiq worker middleware to close connections when a worker finishes: https://github.com/mongoid/kiqstand

Is something similar necessary for MongoMapper? Any advice would be extremely appreciated, and I'm happy to implement the solution myself, if someone could point me in the right direction.

Thanks!

[John Nunemaker]

I would assume MM would need the same type of stuff as Mongoid, but I'm not familiar with how sidekiq works beyond that it uses celluloid.

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[Michael Lawlor]

Thanks John,

Right, Sidekiq uses Celluloid and threads to run its workers. It has a middleware concept similar to Rack that lets you wrap middleware around thread creation/destruction. Kiqstand is one such middleware.

I'm not familiar with Mongoid at all, but it apparently has a concept of "sessions" which appear to be mongodb connections that have to be disconnected when a thread finishes. If you have a look at the kiqstand code, it's really just two lines that run after a thread completes:

::Mongoid::IdentityMap.clear

::Mongoid.disconnect_sessions

Is there anything analogous in MongoMapper?

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[Michael Lawlor]

another example is the Sidekiq ActiveRecord middleware that calls ::ActiveRecord::Base.clear_active_connections! after a worker thread finishes.

https://github.com/mperham/sidekiq/blob/master/lib/sidekiq/middleware/server/active_record.rb

[John Nunemaker]

MM just wraps the mongo connection class. I imagine it has connect/disconnect methods. The connection is stored in MongoMapper.connection.

[Michael Lawlor]

okay I will dig into the mongo gem a bit then. Thanks.