When developing a web application with Node.js (Express) + MongoDB structure, I encountered the following error when starting the app (node-dev app.js).

The web app couldn’t connect to mongo at all. I could connect from CLI though.

[ERROR] 21:57:58 Error
Error: failed to connect to [localhost:27017]
    at null. (/Users/your_username/git/com/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/server.js:540:74)
    at EventEmitter.emit (events.js:106:17)
    at null. (/Users/your_username/git/com/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:140:15)
    at EventEmitter.emit (events.js:98:17)
    at Socket. (/Users/your_username/git/com/node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection.js:478:10)
    at Socket.EventEmitter.emit (events.js:95:17)
    at net.js:830:16
    at process._tickCallback (node.js:415:13)

After trying various solutions without success, I ended up reinstalling mongodb.

% brew uninstall mongodb                                             
Uninstalling /usr/local/Cellar/mongodb/2.4.8...

% brew install mongodb

If installation doesn’t take much time, it would be fine, but mongodb takes several minutes to compile, so reinstalling every time is a bit painful.

By the way, here’s what I tried troubleshooting:

I tried deleting mongod.lock and restarting, and also tried —repair, but nothing worked.

% ps aux | grep mongo                  
your_username           572   0.0  0.4  2721360  36264   ??  S    10:00PM   0:00.61 /usr/local/opt/mongodb/mongod run --config /usr/local/etc/mongod.conf
your_username           896   0.0  0.0  2432784    600 s003  S+   10:04PM   0:00.00 grep mongo

% launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.mongodb.plist

% ps aux | grep mongo        
your_username           982   0.0  0.0  2432784    608 s003  S+   10:07PM   0:00.00 grep mongo

% rm /usr/local/var/mongodb/mongod.lock

% launchctl load ~/Library/LaunchAgents/homebrew.mxcl.mongodb.plist

% ps aux | grep mongo                                                
your_username          1060   0.0  0.0  2432784    604 s003  S+   10:11PM   0:00.00 grep mongo
your_username          1052   0.0  0.4  2712144  34696   ??  S    10:11PM   0:00.06 /usr/local/opt/mongodb/mongod run --config /usr/local/etc/mongod.conf

If anyone knows the solution, I would appreciate your help.

Update: Error: failed to connect to [localhost:27017] Resolved!

Still unresolved, and when this symptom occurs, I’m dealing with it by “System Shutdown” → “Startup” (restart doesn’t work).

Successfully resolved! (2013/12/17)

The cause was that the application was trying to connect to localhost:27017, but there was nothing written in /etc/hosts, so it couldn’t connect to localhost.

It could be resolved by either changing the mongodb connection settings in the application to 127.0.0.1, or fixing /etc/hosts.

I was using a Mac app called “hoster” to manage the hosts file, but this might have been causing trouble, so I dunked it into the trash.

That’s all from the Gemba.