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SSE or WebSockets for Stackoverflow-like instant notification

What is the best technology to use if I want to make instant user notification,
like StackOverflow has?
I consider SSE and WebSockets. What are pros and cons of each solution?
Should I use socket.io or better to use WebSockets directly?

The main difference is that with SSE you can only receive messages from the server. You cannot send messages to the server. Everything doable with SSE is doable with WebSockets. But not vice versa - WebSocket is capable of sending data to the server. So from that point of view WebSockets win. I can't really see any advantage of SSE (perhaps performance?), but then again I don't have much experience with it.
Note that StackOverflow uses WebSockets. They might have some fallback for older browser, I don't know about that.
As for the third question: perhaps you should ask what language you want to use in the first place? I've been working with WebSockets and Python and it worked really well. You could work with WebSockets directly. The advantage of using socket.io is mostly the fallback (assuming it matters - it does not IMHO): if WebSockets are not available it can automatically switch to other ways of communication (like Flash or long polling). The disadvantage is that it is Node.js (in the sense that you have to restrict yourself to one language) plus there are some performance issues, i.e. socket.io does not scale well beyond one machine.

You might consider using a library like SocketIO that abstracts out the transport layer, so you don't have to worry about the mechanics of how the real-time connection is maintained. This will save you a TON of headaches.

Recommendation for converting from Qt app to Web app

I currently have a Qt-based GUI application that simply provides a graphical dashboard (guages, graphs, and such) displaying real-time data provided by another application via a TCP connection. For architectural and security reasons, I have been told that my TCP connection is no longer allowed, and that an HTTPS websocket will be my only conduit. It was suggested that I convert my app to a webapp using HTML5 and Javascript. However, I'm not a web programmer, but may consider this an opportunity to learn a new technology. I want to understand the landscape a little better before making a decision. So, I think my choices are:
Convert my app to a web app, giving me all the advantages/disadvantages of web-based apps. More work for me, as it's a new technology and I already have the Qt/TCP version working fine. If this is the suggested approach, any suggestions on a development environment/tools would be appreciated.
Convert my current Qt app to us a client HTTPS connection rather than a TCP connection. Not sure this is possible. From what I've read, this may not be possible with QtWebKit. Seems strange?
Maybe there's another choice I'm not considering?

You can also use a mixed approach, using WT library. It's c++ (and QT inspired) based.
It has a nice toolkit and supports websockets.
http://www.webtoolkit.eu/wt/examples/

Porting requires a lot of work. On Qt side there are at least two implementations of WebSockets:
QtWebSocket, Qt-based websocket server implementation and alpha-state implementation of websocket client.
WebSocket++, Boost-based websocket server and client implementation.

I just went through a similar exercise for work recently. We settled on the Google Web Toolkit (GWT) framework for doing our web apps. It's completely java-based, meaning you write (almost) everything in Java and the GWT compiler converts it to Javascript for you. We used the GWT-EventService plugin for pushing data from the server to the clients.
I wish I had know about this WT library before we started though, that looks interesting.

You can also have a look at QWebSockets, which is a pure Qt implementation of websockets, both for client and server use.
Update: this library is now an integral part of Qt

Websocket API to replace REST API? [closed]

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I have an application whose primary function works in real time, through websockets or long polling.
However, most of the site is written in a RESTful fashion, which is nice for application s and other clients in the future. However, I'm thinking about transitioning to a websocket API for all site functions, away from REST. That would make it easier for me to integrate real time features into all parts of the site. Would this make it more difficult to build applications or mobile clients?
I found that some people are already doing stuff like this: SocketStream

Not to say that the other answers here don't have merit, they make some good points. But I'm going to go against the general consensus and agree with you that moving to websockets for more than just realtime features is very appealing.
I am seriously considering moving my app from a RESTful architecture to more of an RPC style via websockets. This is not a "toy app", and I'm not talking about only realtime features, so I do have reservations. But I see many benefits in going this route and feel it could turn out to be an exceptional solution.
My plan is to use DNode, SocketIO, and Backbone. With these tools, my Backbone models and collections can be passed around from/to client and server by simply calling a functions RPC-style. No more managing REST endpoints, serializing/deserializing objects, and so forth. I haven't worked with socketstream yet, but it looks worth checking out.
I still have a long way to go before I can definitively say this is a good solution, and I'm sure it isn't the best solution for every application, but I'm convinced that this combination would be exceptionally powerful. I admit that there are some drawbacks, such as losing the ability to cache resources. But I have a feeling the advantages will outweigh them.
I'd be interested in following your progress exploring this type of solution. If you have any github experiments, please point me at them. I don't have any yet, but hope to soon.
Below is a list of to-read-later links that I've been collecting. I can't vouch that they are all worthwhile, as I've only skimmed many of them. But hopefully some will help.
Great tutorial on using Socket.IO with Express. It exposes express sessions to socket.io and discusses how to have different rooms for each authenticated user.
http://www.danielbaulig.de/socket-ioexpress/
Tutorial on node.js/socket.io/backbone.js/express/connect/jade/redis with authentication, Joyent hosting, etc:
http://fzysqr.com/2011/02/28/nodechat-js-using-node-js-backbone-js-socket-io-and-redis-to-make-a-real-time-chat-app/
http://fzysqr.com/2011/03/27/nodechat-js-continued-authentication-profiles-ponies-and-a-meaner-socket-io/
Tutorial on using Pusher with Backbone.js (using Rails):
http://blog.pusher.com/2011/6/21/backbone-js-now-realtime-with-pusher
Build application with backbone.js on the client and node.js with express, socket.io, dnode on the server.
http://andyet.net/blog/2011/feb/15/re-using-backbonejs-models-on-the-server-with-node/
http://addyosmani.com/blog/building-spas-jquerys-best-friends/
http://fzysqr.com/2011/02/28/nodechat-js-using-node-js-backbone-js-socket-io-and-redis-to-make-a-real-time-chat-app/
http://fzysqr.com/2011/03/27/nodechat-js-continued-authentication-profiles-ponies-and-a-meaner-socket-io/
Using Backbone with DNode:
http://quickleft.com/blog/backbone-without-ajax-part-ii
http://quickleft.com/blog/backbone-without-ajax-part-1
http://sorensen.posterous.com/introducing-backbone-redis
https://github.com/cowboyrushforth/minespotter
http://amir.unoc.net/how-to-share-backbonejs-models-with-nodejs
http://hackerne.ws/item?id=2222935
http://substack.net/posts/24ab8c

HTTP REST and WebSockets are very different. HTTP is stateless, so the web server doesn't need to know anything, and you get caching in the web browser and in proxies. If you use WebSockets, your server is becoming stateful and you need to have a connection to the client on the server.
Request-Reply communication vs Push
Use WebSockets only if you need to PUSH data from the server to the client, that communication pattern is not included in HTTP (only by workarounds). PUSH is helpful if events created by other clients needs to be available to other connected clients e.g. in games where users should act on other clients behaviour. Or if your website is monitoring something, where the server pushes data to the client all the time e.g. stock markets (live).
If you don't need to PUSH data from the server, it's usually easier to use a stateless HTTP REST server. HTTP uses a simple Request-Reply communication pattern.

I'm thinking about transitioning to a WebSocket api for all site functions
No. You should not do it. There is no harm if you support both models. Use REST for one way communication/simple requests & WebSocket for two way communication especially when server want to send real time notification.
WebSocket is a more efficient protocol than RESTful HTTP but still RESTful HTTP scores over WebSocket in below areas.
Create/Update/Delete resources have been defined well for HTTP. You have to implement these operations at low level for WebSockets.
WebSocket connections scale vertically on a single server where as HTTP connections scale horizontally. There are some proprietary non standards-based solutions for WebSocket horizontal scaling .
HTTP comes with a lot of good features such as caching, routing, multiplexing, gzipping etc. These have to built on top of Websocket if you chose Websocket.
Search engine optimizations works well for HTTP URLs.
All Proxy, DNS, firewalls are not yet fully aware of WebSocket traffic. They allow port 80 but might restrict traffic by snooping on it first.
Security with WebSocket is all-or-nothing approach.
Have a look at this article for more details.

The only problem I can using TCP (WebSockets) as your main web content delivery strategy is that there is very little reading material out there about how to design your website architecture and infrastructure using TCP.
So you can't learn from other people's mistakes and development is going to be slower. It's also not a "tried and tested" strategy.
Of course your also going to lose all the advantages of HTTP (Being stateless, and caching are the bigger advantages).
Remember that HTTP is an abstraction for TCP designed for serving web content.
And let's not forget that SEO and search engines don't do websockets. So you can forget about SEO.
Personally I would recommend against this as there's too much risk.
Don't use WS for serving websites, use it for serving web applications
However if you have a toy or a personal websites by all means go for it. Try it, be cutting-edge. For a business or company you cannot justify the risk of doing this.

I learned a little lesson (the hard way). I made a number crunching application that runs on Ubuntu AWS EC2 cloud services (uses powerful GPUs), and I wanted to make a front-end for it just to watch its progress in realtime. Due to the fact that it needed realtime data, it was obvious that I needed websockets to push the updates.
It started with a proof of concept, and worked great. But then when we wanted to make it available to the public, we had to add user session, so we needed login features. And no matter how you look at it, the websocket has to know which user it deals with, so we took the shortcut of using the websockets to authenticate the users. It seemed obvious, and it was convenient.
We actually had to spend quiet some time to make the connections reliable. We started out with some cheap websocket tutorials, but discovered that our implementation was not able to automatically reconnect when the connection was broken. That all improved when we switched to socket-io. Socket-io is a must !
Having said all that, to be honest, I think we missed out on some great socket-io features. Socket-io has a lot more to offer, and I am sure, if you take it in account in your initial design, you can get more out of it. In contrast, we just replaced the old websockets with the websocket functionality of socket-io, and that was it. (no rooms, no channels, ...) A redesign could have made everything more powerful. But we didn't have time for that. That's something to remember for our next project.
Next we started to store more and more data (user history, invoices, transactions, ...). We stored all of it in an AWS dynamodb database, and AGAIN, we used socket-io to communicate the CRUD operations from the front-end to the backend. I think we took a wrong turn there. It was a mistake.
Because shortly after we found out that Amazon's cloud services (AWS) offer some great load-balancing/scaling tools for RESTful applications.
We have the impression now that we need to write a lot of code to perform the handshakes of the CRUD operations.
Recently we implemented Paypal integration. We managed to get it to work. But again, all tutorials are doing it with RESTful APIs. We had to rewrite/rethink their examples to implement them with websockets. We got it to work fairly fast though. But it does feel like we are going against the flow.
Having said all that, we are going live next week. We got there in time, everything works. And it's fast, but will it scale ?

I would consider using both. Each technology has their merit and there is no one-size fits all solution.
The separation of work goes this way:
WebSockets would be the primary method of an application to communicate with the server where a session is required. This eliminates many hacks that are needed for the older browsers (the problem is support for the older browsers which will eliminate this)
RESTful API is used for GET calls that are not session oriented (i.e. not authentication needed) that benefit from browser caching. A good example of this would be reference data for drop downs used by a web application. However. can change a bit more often than...
HTML and Javascript. These comprise the UI of the webapp. These would generally benefit being placed on a CDN.
Web Services using WSDL are still the best way of enterprise level and cross-enterprise communication as it provides a well defined standard for message and data passing. Primarily you'd offload this to a Datapower device to proxy to your web service handler.
All of this happen on the HTTP protocol which gives use secure sockets via SSL already.
For the mobile application though, websockets cannot reconnect back to a disconnected session (How to reconnect to websocket after close connection) and managing that isn't trivial. So for mobile apps, I would still recommend REST API and polling.
Another thing to watch out for when using WebSockets vs REST is scalability. WebSocket sessions are still managed by the server. RESTful API when done properly are stateless (which mean there is no server state that needs to be managed), thus scalability can grow horizontally (which is cheaper) than vertically.

Do I want updates from the server?
Yes: Socket.io
No: REST
The downsides to Socket.io are:
Scalability: WebSockets require open connections and a much different Ops setup to web scale.
Learnin: I don't have unlimited time for my learnin. Things have to get done!
I'll still use Socket.io in my project, but not for basic web forms that REST will do nicely.

WebSockets (or long polling) based transports mostly serve for (near) real-time communication between the server and client. Although there are numerous scenarios where these kinds of transports are required, such as chat or some kind of real-time feeds or other stuff, not all parts of some web application need to be necessarily connected bidirectionally with the server.
REST is resource based architecture which is well understood and offers it's own benefits over other architectures. WebSockets incline more to streams/feeds of data in real-time which would require you to create some kind of server based logic in order to prioritize or differentiate between resources and feeds (in case you don't want to use REST).
I assume that eventually there would be more WebSockets centric frameworks like socketstream in the future when this transport would be more widespread and better understood/documented in the form of data type/form agnostic delivery. However, I think, this doesn't mean that it would/should replace the REST just because it offers functionality which isn't necessarily required in numerous use cases and scenarios.

I'd like to point out this blog post that is up to me, the best answer to this question.
In short, YES
The post contains all the best practices for such kind of API.

That's not a good idea. The standard isn't even finalized yet, support varies across browsers, etc. If you want to do this now you'll end up needing to fallback to flash or long polling, etc. In the future it probably still won't make a lot of sense, since the server has to support leaving connections open to every single user. Most web servers are designed instead to excel at quickly responding to requests and closing them as quickly as possibly. Heck even your operating system would have to be tuned to deal with a high number of simultaneous connections (each connection using up more ephemeral ports and memory). Stick to using REST for as much of the site as you can.

Real-time bi-directional JSON-RPC communication over HTTP

I am building a JSON-RPC server that accepts requests over HTTP. I would like to support bi-directional communication (both client and server can send requests), the specific use case being a publish/subscribe architecture where a client sends a subscribe(X) request and receives changed(X) requests in (almost) real-time. As far as I know, there are several ways to implement this with HTTP:
long polling
WebSockets
polling calls using a cookie-based session model
streaming (keeping the HTTP connection open)
a combination of some of the above
What I'm looking for is a solution that is based on accepted internet standards (if possible), usable from a web browser and easy to work with on the client side. So far, I favour the streaming thing (Twitter, CouchDB do it that way), but I'm not sure about how well this is supported within browsers and JSON-RPC libraries. Also, there may be other ways to do it that I'm not aware of.
Thank you in advance.

I think you should have a look at socket.io to accomplish your task. You could if you wanted to watch this video from the author: "Socket.IO Workshop: Guillermo Rauch". It is easy to work with on both server as client. I have created a simple sample pubsub using redis on top of socket.io.

To my knowledge, Streaming is supported by FF, Chrome (Has bufffering issues that require a datatype of application/octet-stream or a prelude to work) and IE8 (through a little XDomainRequest). I don't know about opera.
I don't really know of any comet industry standards, the Bayeux is probably the closest. It's hard to see how facebook/gmail/twitter do it as all the code is obfuscated, and it's exceedingly difficult to find much info on how all the browsers handle everything.
Even more difficult is that you will need to use a specialized server, keeping this many connections open will require thread pooling etc.. A normal server will blow up pretty fast.
It is a very powerful design if you can get it to work reliably though.

You should take a look at JSONRPC-bidirectional.It supports bidirectional RPC over WebSocket, Worker, WebRTC and HTTP and it is highly extensible.

If anyone is interested in a Java implementation I just wrote a sample app and a blog post about it. It uses Java, Maven, Comet, Bayeux, Spring.
http://jaye.felipera.cloudbees.net/
http://geeks.aretotally.in/thinking-in-reverse-not-taking-orders-from-yo

Web Sockets questions

I have a couple of questions concerning Web Sockets.
The latest Firefox 4.0 nightlies support Web Sockets. So does Webkit (Chrome 4 + Safari 4/5). Internet Explorer 9 is supposed to feature Web Sockets at some point according to Microsoft (before the stable release).
Anyway, my questions are:
I am building a JavaScript admin interface to manage a website. Should I use Web Sockets for the client-server communication instead of XMLHttpRequest if I told you that I do not need to care about browser compatibility?
Would Web Sockets result in faster save, deletion and update calls compared to the usual situation with XMLHttpRequest? Would the requests be more instant?
I am aware of the HTML5's navigator.online and window.addEventListener('offline', ...), but with Web Sockets (upon connection loss), am I able to detect connection issues more accurately and faster? I mean, when I turn off my Internet connection, or block it with my firewall, Firefox still seems to state that navigator.online is true. With Web Sockets, it seems, the connection to the server will be lost instantly, thus, I can detect connection problems more accurately?
Can I support Web Sockets server-side with pure PHP, so, that the code is portable with other web servers (no need to install any Apache modules or do other customization). I would like to distribute the software to a couple of places without having to ask people to install all sorts of modules to their HTTPD or so.
I wish you can answer to as many of those questions as possible. I am really interested in answers.

I am building a JavaScript admin interface to manage a website. Should I use Web Sockets for the client-server communication instead of XMLHttpRequest if I told you that I do not need to care about browser compatibility?
It appears to me that you want to use WebSockets just for the sake of it. The main reason to use WebSockets is when you want to push data from the server to the client. If your application does not need this, you should probably not use WebSockets.
Would Web Sockets result in faster save, deletion and update calls compared to the usual situation with XMLHttpRequest? Would the requests be more instant?
You could probably save some time on both ends (client and server) due to the absence of headers. But the gain is probably pretty small.
with Web Sockets (upon connection loss), am I able to detect connection issues more accurately and faster?
Yes, an event will fire instantly when the WebSocket closes. Alternatives would be long-polling or periodic XHRs. Or event client-side storage.
Can I support Web Sockets server-side with pure PHP, so, that the code is portable with other web servers
First I suggest you read through this. WebSockets don't work very well in a synchronous way. PHP and apache don't work very well in an asynchronous way. Although there are some implementations, many of them are outdated. I personally would use an other language for this, such as ruby, python, java or server-side javascript. Simply because the languages have better support for the asynchronous model and the WebSocket implementations are more sophisticated.
The WebSocket protocol is currently still a draft, it can change. Just like it did a few weeks ago. So your code may very well break.
My advice is: Do not use WebSockets just for the sake of it. If you have a real-time event-driven application, then it is probably the right choice. Make sure you understand what WebSockets are for and what it takes on the server side, also in terms of event-driven applications. Don't use it for anything production, it's way too fragile.