Express Sub-App Demo

An extreme example of separating what may have been a single Expres app into multiple applications with a host to bring them back together.

About This Demo

Express is one of the most flexible web hosting libraries I have used in my career. I love how simple it can be, and how it allows you to grow your system as needed.

One of the areas that has continued to bother me in my own code, is my lack of ability to cleanly separate different parts of my express app while having those parts hosted in the same root domain name.

Prior Attempts To Separate Code

Previously, I had separated what may have been a /admin route into a completely separate sub-domain, like http://admin.example.com.

With the realization that Express application instances can be mounted into other Express application instances, however, that changed.

Mounting Sub-Apps Into A Host

While I do see value in moving /admin out to a separate sub-domain, still, I also see a lot of value in separating /admin into it's own project folder and still mounting it as /admin in the main Express app.

This repository is a demonstration Express' ability to do just that - to take what would have been cluttered code from multiple route paths, separate them into their own application instance, and bring them back together in a single application host.

On Being An "extreme" Example App

I call this an "extreme example" because I have purposely taken the separation out to the edge of what I might consider useful. Taking this to an extreme allows us to see the bounds of where this might be useful.

It is still possible to take this separation of apps further, creating more sub-applications to split further responsibilities apart.

For example, you could split the "development" vs "production" error handlers into separate app instances, and only mount the one you need.

However, this might really jump the bounds of usefulness... depending on your circumstances, of course. :)

Running The Demo

Clone the repository, then

npm install from the root folder
npm start from the root folder
- alternatively: run node host/bin/www

Project Structure

This demo project is split up into multiple Express application instances, each of which handles a specific portion of the over-all web application.

The primary application is:

host: The root express app and the actual web server into which the other apps are mounted

The sub-application instances include:

api: Mounted at /api, handles the JSON based API access for the app
web: Mounted at /, handles the core web pages for browser based usage
errors: Mounted in the root express app, handles root level 404 and route / middleware errors

You'll note in each of these projects, that various parts of the default Express app (as generated by the Express-Generator project) have been removed.

For example, the api project does not need to render any Jade views, so the view engine registration and views folders have been removed from this project. Similarly, none of the sub-applications need to have actual web server code from the bin/www file - only the host application needs this.

By separating the application into multiple sub-apps, code is easier to maintain as it contains less clutter from unrelated things.

Legal Junk

Distributed under MIT License.

dcurletti/express-sub-app-demo