Feather can sync events happening in two different Node processes or even servers in real-time. For example: an event happens in server A, the user connected to server B is instantly notified of that event.
This is done through a central Redis or Mongo collection or through a websocket libraries' clustering library.
Through services, Feathers provides instant CRUD functionality, it also can easily expose a RESTful and real-time API through HTTP/HTTPS and websockets.
Since Feathers itself is built on top of Express (it's a thin wrapper over socket.io, primus and Express) and because of Feathers' highly modular nature, it's very easy to integrate Feathers in an existing Express project.
Feathers makes extensive use of modules (which are called Feathers services). These services work like ExpressJS middleware and can be used with app.use('/path', serviceObject).
Services help developers keep their applications modular and as minimal as possible, without any unnecessary libraries or bloat.
FeatherJS uses primus which is a universal wrapper for real-time frameworks. Through primus you can easily choose which socket transport you want to use.
Meteor was developed with simplicity in mind, even for beginners who have just started using JavaScript frameworks. One of the reasons that beginners should start with Meteor is that Meteor is a full-stack framework, this way they can get the complete learning experience when it comes to web development (back-end and front-end development), all by using a single platform and a single language.
Furthermore, Meteor does not have complex and esoteric concepts that may be hard to grasp by a beginner, it has a clear documentation and well-established coding conventions.
There's also a very useful resource for learning Meteor in the form of a book: Discover Meteor, by the authors of many Meteor packages.
It's common for the front-end and back-end to lose sync if an internet connection is flaky. Even though the connection should be in real-time, if the connection is weak, you may lose that real-time sync. For example, in chat applications you may have to refresh the page to get the latest updated data from the server.
Database collection subscriptions when partially loaded on one view might get added as it is to other view subscription to same collection. That means you need to properly manage subscription with strict conditions to same collection on different UI. This becomes annoying when you see unwanted data on other view, which might be raised as a bug by client.
A Meteor package can supply code for both the client and the server, and for mobile (Cordova) apps. For example, an autocomplete package supplies both server code to search a collection, and client code to display the results. The mdg:camera package supports the native camera if the app is built for mobile, or the HTML getUserMedia API to take pictures from the browser.
Every time a change in the source file is saved, all connected clients will refresh automatically - browser tabs, mobile apps running in the simulator or on the physical device.
Or, deploy a Meteor app (meteor deploy myapp) and all clients, plus all mobile apps with the server set to myapp.meteor.com will automatically reload to use the code changes.
This drastically reduces the development cycle for apps in the App Store, where a regular update can wait for one to two weeks before being approved.
Meteor is built on top of Node.js and jQuery on the client.
Meteor enables the client and server to communicate data seamlessly, in real-time. You don't have to write any REST API or pub/sub code - Meteor takes care of it all automatically for you.
Meteor can generate mobile (iOS and Android) apps from the same codebase as the web app, using the Cordova (PhoneGap) library (which brings native device functionality to JavaScript applications).
Meteor-generated mobile apps are JavaScript, HTML and CSS bundles that run in a UIWebView (on iOS) or WebView (on Android). Apps can be run locally in the iOS/Android emulator, or on physical devices. You can also publish them to Google Play Store or Apple's App Store.
Moreover, these hybrid mobile apps benefit from hot code push, which dramatically accelerates the development cycle.
Meteor also has an official testing framework called Velocity. Velocity enables real-time unit testing and integration with Jasmine or Mocha syntax. Tests are automatically run when code is saved and the testing result is indicated by a green or red dot in the upper right corner of the app.
The community behind Meteor has created a useful application for debugging Meteor apps that will automatically delete collections and display client-side documents, allow you to control subscriptions and lets you modify the documents.
Meteor also has great support for server-side debugging, and WebStorm has also released full support for Meteor, including debugging capabilities.
MongoDB advertises scalability but only if you don't care about data-integrity. There are other backend options but none of them are officially supported.
Meteor does not have support for server-side rendering of views, which is extremely helpful when it comes to SEO. However, there's a third-party server-side rendering package available for download. The Meteor team has also said that server-side rendering is on the roadmap.
Though lately Google has announced that the search engine can render JS and CSS files just like modern web browsers
Derby seems to have a small community of followers, at least compared to other Node full-stack frameworks. So it could be difficult for a beginner to find examples or help if they get stuck somewhere in their development.
Derby renders it's client-side views on the server. This means that there's no "delay" in serving the content to the browser when the user requests it. It's also very helpful for SEO purposes.