Mongoose and commander

Question

Mongoose and commander

I'm writing some scripts for some command-line manipulation of Mongoose models with commander.js (eventually, I'd like to run these tools using Cron).

Now, I've written several scripts with commander and they all work fine, but if I connect to the MongoDB database using mongoose, they script just hangs after it's done. Now, I figured the database connection is keeping node alive, so I added a mongoose.disconnect() line and it still hangs.

The only thing I found that allows me to shutdown is to use process.exit(), but I'm reluctant to just terminate the process. Is there something in particular that I should do to trigger a graceful shutdown?

node.js
mongodb
mongoose

Answer 1

My reading of the API docs implies that .disconnect() must be given a callback function. It looks like it's called for each that's disconnected and may be passed an error.

There is a check in the code to make sure it's not called if it doesn't exist when things work out, but that check isn't being run on errors, so if Mongoose received an error message from the MongoDB client, it may be leaving a connection open and that's why it's not stopping execution.

If you're only opening a single connection to the database, you may just want to call [Connection object].close() since that function correctly inserts a no-op "callback" if no callback is given, and looks like it will correctly destruct things.

(The more I look into Mongoose, the more I want to just write a thin wrapper around the MongoDB client so I don't have to deal with Mongoose's "help.")

Answer 2

Shutdown the node program directly is hiding the symptoms, not fixing the problem!

I finally isolated the problem and found it to be with Mongoose schema definitions. If you try to shutdown the connection too soon after Mongoose schemas are defined¹, the application hangs and eventually produces some weird MongDB-related error.

Adding a small timeout before running the program.parse(argv) line to run the commander application fixes the problem. Just wrap the code like so:

var program  = require('commander')
  , mongoose = require('mongoose')
  , models   = null
;

// Define command line syntax.
program
  .command(...)
;

mongoose.connect(
    ..., // connection parameters.

    function() {
        // connected to database, defined schemas.
        models = require('./models');

        // Wait 1 second before running the application code.
        setTimeout(function(){
            program.parser(process.argv);
        }, 1000);
    }
);

1: This is my initial interpretation, I have not (yet) extensively tested this theory. However, removing Mongoose schema definitions from the application successfully prevents the application from hanging.

Actually, just using process.nextTick() instead of the setTimeout() call fixes the situation nicely!

Answer 3

I use the async "Series" to perform operations and then call mongoose.connection.close() on completion. It prevents callback hell and allows you to neatly perform operations either one at a time or parallel followed by a function when all the other methods have completed. I use it all the time for scripts that require mongoose but are meant to terminate after all mongoose operations are finished.