Alex Rivera | Logout

Is it ok to initialize exports asynchronously in a node.js module?

Asked 2011-06-21T12:34:09.313
10

As MongoDB database access and initialization is asynchronous on Node.js, I would like to define one module per collection that exports wrapped db calls after db initialization.

Such a "Cars.model.js" module looks like that:

var db = require("mongodb");
db.collection("cars", function(err, col) {
    exports.getCars = function(callback) {
        col.find({}, callback);
    };
});

so that other modules can run:

var carModel = require("Cars.model.js").getCars;
getCars(err, cars) {
    // (do something with cars here...)
};

It happened to me that getCars was undefined, because db access was not yet initialized at the time my second module was run.

How do you deal with creating such asynchronous db models?

model-view-controller design-patterns mongodb asynchronous node.js
Edit
Report

2 Answers

11

You cannot write to exports after you've left the file. You must be blocking. To avoid being blocking I would use lazy loading of resources. 

var carCol;
var carEmitter = new require("events").EventEmitter;


exports.getCars = function(callback) {
  // if no car collection then bind to event
  if (carCol === undefined) {
    carEmitter.on("cars-ready", function() {
      callback(carCol);
    });
  } else {
    // we have cars, send them back
    callback(carCol);
  }
}

db.collection("cars", function(err, col) {
  // store cars
  carCol = col;
  // tell waiters that we have cars.
  carEmitter.emit("cars-ready");
});

Use event emitters to emulate lazy loading. You may want to generalize to a LazyLoadedCollection class/object to make the code neater / more DRY.

answered 2011-06-21T14:05:41.120
0

I believe this simple solution works: replace the asynchronous call to getCars() by a synchronous call to a collection cache that would be populated before models can be called.

The main.js application starter module:

var db = require("mongodb");
exports.collectionCache = {};
db.collection("cars", function(err, col) {
    exports.collectionCache["cars"] = col;
    // ready => start application logic (incl. models) here
});

Thus "Cars.model.js" would look like that:

var col = require("main").collectionCache; // populated
exports.getCars = function(callback) {
    col["cars"].find({}, callback);
};

This solution moves the asynchronous problem from models to the root database model, making models simpler to develop.

answered 2011-06-22T08:14:31.413

Your Answer