Using (and reusing) multiple mongoose database connections on express.js

Is this approach is efficient? Are there any bottlenecks/pitfalls I'm missing?

This all seems generally correct to me

What about the model registering?

I agree with @narc88 that you don't need to register models in middleware.

For lack of a better term, I would use a factory pattern. This "factory function" would take in your sub-domain, or however you decide to detect tenants, and return a Models object. If a given middleware wants to use its available Models you just do

var Models = require(/* path to your Model factory */);...// later on inside a route, or wherevervar models = Models(req.sub/* or req.tenant ?? */);models.Project.find(...);

For an example "factory", excuse the copy/paste

var mongoose = require('mongoose');var fs = require('fs');var debug = require('debug')('app:middleware:registerModels');var models = [];var conns = [];var path = __dirname + '/../schemas';function factory(tenant) {    // if the connection is cached on the array, reuse it    if (conns[tenant]) {        debug('reusing connection', tenant, '...');    } else {        debug('creating new connection to', tenant, '...');        conns[tenant] = mongoose.createConnection('mongodb://localhost:27017/' + tenant);    }    if(models[tenant]) {        debug('reusing models');    } else {        var instanceModels = [];        var schemas = fs.readdirSync(path);        debug('registering models');        schemas.forEach(function(schema) {            var model = schema.split('.').shift();            instanceModels[model] = conns[tenant].model(model, require([path, schema].join('/')));        });        models[tenant] = instanceModels;    }    return models[tenant];}module.exports = factory;

Aside from potential (albeit probably small) performance gain, I think it also has the advantage of:

• doesn't clutter up the request object as much
• you don't have to worry as much about middleware ordering
• allows more easily abstracting permissions for a given set of models, i.e. the models aren't sitting on the request for all middleware to see
• This approach doesn't tie your models to http requests, so you might have flexibility to use the same factory in a job queue, or whatever.