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What are some good use cases for Server Sent Events

I discovered SSE (Server Sent Events) pretty late, but I can't seem to figure out some use cases for it, so that it would be more efficient than using setInterval() and ajax.
I guess, if we'd have to update the data multiple times per second then having one single connection created would produce less overhead. But, except this case, when would one really choose SSE?
I was thinking of this scenario:
A new user comment from the website is added in the database
Server periodically queries DB for changes. If it finds new comment, send notification to client with SSE
Also, this SSE question came into my mind after having to do a simple "live" website change (when someone posts a comment, notify everybody who is on the site). Is there really another way of doing this without periodically querying the database?

Nowadays web technologies are used to implmement all sort of applications, including those which need to fetch constant updates from the server.
As an example, imagine to have a graph in your web page which displays real time data. Your page must refresh the graph any time there is new data to display.
Before Server Sent Events the only way to obtain new data from the server was to perform a new request every time.
Polling
As you pointed out in the question, one way to look for updates is to use setInterval() and an ajax request. With this technique, our client will perform a request once every X seconds, no matter if there is new data or not. This technique is known as polling.
Events
Server Sent Events on the contrary are asynchronous. The server itself will notify to the client when there is new data available.
In the scenario of your example, you would implement SSE such in a way that the server sends an event immediately after adding the new comment, and not by polling the DB.
Comparison
Now the question may be when is it advisable to use polling vs SSE. Aside from compatibility issues (not all browsers support SSE, although there are some polyfills which essentially emulate SSE via polling), you should focus on the frequency and regularity of the updates.
If you are uncertain about the frequency of the updates (how often new data should be available), SSE may be the solution because they avoid all the extra requests that polling would perform.
However, it is wrong to say in general that SSE produce less overhead than polling. That is because SSE requires an open TCP connection to work. This essentially means that some resources on the server (e.g. a worker and a network socket) are allocated to one client until the connection is over. With polling instead, after the request is answered the connection may be reset.
Therefore, I would not recommend to use SSE if the average number of connected clients is high, because this could create some overhead on the server.
In general, I advice to use SSE only if your application requires real time updates. As real life example, I developed a data acquisition software in the past and had to provide a web interface for it. In this case, a lot of graphs were updated every time a new data point was collected. That was a good fit for SSE because the number of connected clients was low (essentially, only one), the user interface should update in real-time, and the server was not flooded with requests as it would be with polling.
Many applications do not require real time updates, and thus it is perfectly acceptable to display the updates with some delay. In this case, polling with a long interval may be viable.

What's the best way to retrieve messages from the server for a chat application

I'm writing a chat application from scratch, and I'm trying to figure out the best way to retrieve messages from the server in real time. I've done a fair amount of research, and have come up with this: I could use web sockets, and I've seen it done before successfully. I could use long polling, but I'm wondering if that places greater stress on the server, or maybe is just not right for a chat application. I could poll the server every second, but that just seems like a waste of bandwidth. Suggestions? Thank you!

The websocket protocol is ideal for something like a chat application for several reasons.
As you've already identified, continous polling of the server is quite a waste of resources.
Using traditional HTTP requires the use of bulky headers which waste valuable bandwidth. Websockets allow for lean messaging.
Most importantly for your application, unlike HTTP, websockets are bi-directional, meaning that your server can independently send messages to your chat client. With HTTP, all communication must be initiated by the client, and you are restricted to a request-response type of communication. With websockets, if your client receives a message from another user on your chat service, the server can immediately and independently relay that information to the intended recipient.
So to answer your question. You should definitely go with web sockets. Since you lack traditional headers, you will have to do a bit of work when it comes to formatting the messages sent over the connection, but the efficiency is well worth the minimal effort it takes to set up your messaging model.

What? Do you need to and from a database on a dedicated server?
I would recommend Ajax with JavaScript or jQuery but i like to do my own coding so JavaScript.
If your showing data back and forth match i would probably use MySQL.
For example to query up the last known query if matches current then would not be updated.
If doesn't match loop up from last to rent match would be DESC if in Order by type. Anyways...
Hope this helps you decide on what you should use.
Although this is what is common on games, blogs, forums, chats with MySQL/SQL.

Advice on which technology to use for real time notifications

I have X amount of activity sensors connected to a server that inserts data to a database everytime a sensor is triggered. What I'm trying to do is create a web interface with a blue print of the facility (svg) and whenever a sensor is triggered, besides the db insert, I want it to show some sort of alert in my blue print. For that I need to keep an open connection to the server I think.
I was thinking of using web sockets, but it might be overkill since I only need to retrieve data from the server. But running an ajax call every second doesn't sound very efficient either. Are there any other alternatives?
Thank you

Some potential choices include:
WebSocket
Adobe® Flash® Socket
AJAX long polling
AJAX multipart streaming
Forever Iframe
JSONP Polling
Which actual transport you end up using will depend on the your requirements for browser support and what technology you are using on the server to handle these requests. The transport choice may also depend on your network topology - what types of load balancers you need to integrate with, proxies, etc.
There are many libraries available on both the client and server sides, many of which support more than one of these transports.
For example (not an exhaustive list):
socket.io for nodejs
WebSocket
Adobe® Flash® Socket
AJAX long polling
AJAX multipart streaming
Forever Iframe
JSONP Polling
SignalR for an asp/.net backend
WebSockets
Server-Sent Events
ForeverFrame
Long Polling
Atmosphere for a java backend
WebSockets
Server Side Events (SSE)
Long-Polling
Forever frame
JSONP
IMO - Websockets is NOT overkill for this type of problem and would lend itself nicely to this type of application.

Without specifically discussing frameworks or knowing what is running in the backend of your server(s), we have a few options to consider for the frontend:
Websockets
Websockets are designed for bidirectional communication, although it is kind of shocking how many users are surfing the web in a browser that doesn't support websockets. I always recommend a fallback for this, such as the other methods listed below.
SSE
SSE is an HTML5 spec and is still shaky at best. Try scrolling on a page while when an SSE event fires... It may be a little easier on the backend, put it sometimes hangs on the client side since it runs inside the same thread that the DOM is running in.
Long Polling
Keeps your connection open. It doesn't scale well with PHP, but performs swimmingly with Python+Twisted on the backend, or Node.Js
Good Old Ajax
Keep your requests small, and you still have a scalable solution. Yes, a full GET request is the most expensive, but is supported in just about every browser rolled out the past ten years. It is also worth noting that GET requests are easy to scale horizontally with more hardware.
In a perfect world:
You would break up your application into a few components, operating behind a reverse proxy such as Nginx. Then use Node.Js + Socket.IO handle the realtime aspects of your app.
Another option would be to use small Ajax requests, and offer websocket support for the browsers that support it. This is advice specifically for PHP in the backend.

WebSocket is certainly not overkill. On the contrary. With websockets, you have a bi-directional communication channel; this means, that the server can initiate communication whenever it seems fit (e.g. when sensor data changes).
In a previous project, I have used node.js together with socket.io, to monitor 50+ sensors. Data was updated in real-time in a browser. The data was visualized using smoothie.js.
Whenever a sensor value was updated, it was communicated to the browser. Some sensors only updated once a minute, others once a second, ...
Polling would have been overkill, because it would retrieve all data for all sensors, even from those that were not updated yet.

I had a similar problem and did a lot of research on this. As I understand it, there are three main options:
Short polling: Have an endpoint that your javascript client pings every second. This is the worst option, because the pings add latency up to one second to your communication, and depending on how you implement, the endpoint could query the database every second, adding unnecessary overhead.
Long polling: Have an endpoint that your javascript client pings that holds the connection until a) the event occurs or b) the connection times out. If the endpoint returns a response, the client gets the event information. If the endpoint does not return a response, no event has occurred, and the client sends a new request. This is a good option because the events can immediately trigger the response to the client, assuming you have an asynchronous interprocess communication layer (like 0MQ) to send the message without any sort of polling.
Websocket: Have your javascript client connect to a websocket server, which will send a message to your client immediately upon the event trigger.
I think a websocket is your best option, because it accommodates immediate communication of the event without all the request/response overhead. And most importantly, this is exactly what websockets are designed to do! As such, you will probably have to write the least amount of custom code with this solution.

There are two great commercial services that might work for you.
Firebase - a javascript hierarchical database and realtime
messaging/ synchronization platform, uses websockets and has other fallbacks
PubNub - a real time message passing and queue system, uses websockets

My Understanding of HTTP Polling, Long Polling, HTTP Streaming and WebSockets

I have read many posts on SO and the web regarding the keywords in my question title and learned a lot from them. Some of the questions I read are related to specific implementation challenges while others focus on general concepts. I just want to make sure I understood all of the concepts and the reasoning why technology X was invented over technology Y and so on. So here goes:
Http Polling: Basically AJAX, using XmlHttpRequest.
Http Long Polling: AJAX but the server holds on to the response unless the server has an update, as soon as the server has an update, it sends it and then the client can send another request. Disadvantage is the additional header data that needs to be sent back and forth causing additional overhead.
Http Streaming: Similar to long polling but the server responds with a header with "Transfer Encoding: chunked" and hence we do not need to initiate a new request every time the server sends some data (and hence save the additional header overhead). The drawback here is that we have to "understand" and figure out the structure of the data to distinguish between multiple chunks sent by the server.
Java Applet, Flash, Silverlight: They provide the ability to connect to socket servers over tcp/ip but since they are plugins, developers don't want to depend on them.
WebSockets: they are the new API which tries to address the short comings of above methods in the following manner:
The only advantage of WebSockets over plugins like Java Applets, Flash or Silverlight is that WebSockets are natively built into browsers and does not rely on plugins.
The only advantage of WebSockets over http streaming is that you don't have to make an effort to "understand" and parse the data received.
The only advantage of WebSockets over Long Polling is that of elimination of extra headers size & opening and closing of socket connection for request.
Are there any other significant differences that I am missing? I'm sorry if I am re-asking or combining many of the questions already on SO into a single question, but I just want to make perfect sense out of all the info that is out there on SO and the web regarding these concepts.
Thanks!

There are more differences than the ones you have identified.
Duplex/directional:
Uni-directional: HTTP poll, long poll, streaming.
Bi-direcitonal: WebSockets, plugin networking
In order of increasing latency (approximate):
WebSockets
Plugin networking
HTTP streaming
HTTP long-poll
HTTP polling
CORS (cross-origin support):
WebSockets: yes
Plugin networking: Flash via policy request (not sure about others)
HTTP * (some recent support)
Native binary data (typed arrays, blobs):
WebSockets: yes
Plugin networking: not with Flash (requires URL encoding across ExternalInterface)
HTTP *: recent proposal to enable binary type support
Bandwidth in decreasing efficiency:
Plugin networking: Flash sockets are raw except for initial policy request
WebSockets: connection setup handshake and a few bytes per frame
HTTP streaming (re-use of server connection)
HTTP long-poll: connection for every message
HTTP poll: connection for every message + no data messages
Mobile device support:
WebSocket: iOS 4.2 and up. Some Android via Flash emulation or using Firefox for Android or Google Chrome for Android which both provide native WebSocket support.
Plugin networking: some Android. Not on iOS
HTTP *: mostly yes
Javascript usage complexity (from simplest to most complicated). Admittedly complexity measures are somewhat subjective.
WebSockets
HTTP poll
Plugin networking
HTTP long poll, streaming
Also note that there is a W3C proposal for standardizing HTTP streaming called Server-Sent Events. It is currently fairly early in it's evolution and is designed to provide a standard Javascript API with comparable simplicity to WebSockets.

Some great answers from others that cover a lot of ground. Here's a little bit extra.
The only advantage of WebSockets over plugins like Java Applets, Flash or Silverlight is that WebSockets are natively built into browsers and does not rely on plugins.
If by this you mean that you can use Java Applets, Flash, or Silverlight to establish a socket connection, then yes, that is possible. However you don't see that deployed in the real world too often because of the restrictions.
For example, intermediaries can and do shutdown that traffic. The WebSocket standard was designed to be compatible with existing HTTP infrastructure and so is far less prone to being interfered with by intermediaries like firewalls and proxies.
Moreover, WebSocket can use port 80 and 443 without requiring dedicated ports, again thanks to the protocol design to be as compatible as possible with existing HTTP infrastructure.
Those socket alternatives (Java, Flash, and Silverlight) are difficult to use securely in a cross-origin architecture. Thus people often attempting to use them cross-origin will tolerate the insecurities rather than go to the effort of doing it securely.
They can also require additional "non-standard" ports to be opened (something administrators are loathe to do) or policy files that need to be managed.
In short, using Java, Flash, or Silverlight for socket connectivity is problematic enough that you don't see it deployed in serious architectures too often. Flash and Java have had this capability for probably at least 10 years, and yet it's not prevalent.
The WebSocket standard was able to start with a fresh approach, bearing those restrictions in mind, and hopefully having learned some lessons from them.
Some WebSocket implementations use Flash (or possibly Silverlight and/or Java) as their fallback when WebSocket connectivity cannot be established (such as when running in an old browser or when an intermediary interferes).
While some kind of fallback strategy for those situations is smart, even necessary, most of those that use Flash et al will suffer from the drawbacks described above. It doesn't have to be that way -- there are workarounds to achieve secure cross-origin capable connections using Flash, Silverlight, etc -- but most implementations won't do that because it's not easy.
For example, if you rely on WebSocket for a cross-origin connection, that will work fine. But if you then run in an old browser or a firewall/proxy interfered and rely on Flash, say, as your fallback, you will find it difficult to do that same cross-origin connection. Unless you don't care about security, of course.
That means it's difficult have a single unified architecture that works for native and non-native connections, unless you're prepared to put in quite a bit of work or go with a framework that has done it well. In an ideal architecture, you wouldn't notice if the connections were native or not; your security settings would work in both cases; your clustering settings would still work; your capacity planning would still hold; and so on.
The only advantage of WebSockets over http streaming is that you don't have to make an effort to "understand" and parse the data received.
It's not as simple as opening up an HTTP stream and sitting back as your data flows for minutes, hours, or longer. Different clients behave differently and you have to manage that. For example some clients will buffer up the data and not release it to the application until some threshold is met. Even worse, some won't pass the data to the application until the connection is closed.
So if you're sending multiple messages down to the client, it's possible that the client application won't receive the data until 50 messages worth of data has been received, for example. That's not too real-time.
While HTTP streaming can be a viable alternative when WebSocket is not available, it is not a panacea. It needs a good understanding to work in a robust way out in the badlands of the Web in real-world conditions.
Are there any other significant differences that I am missing?
There is one other thing that noone has mentioned yet, so I'll bring it up.
The WebSocket protocol was designed to a be a transport layer for higher-level protocols. While you can send JSON messages or what-not directly over a WebSocket connection, it can also carry standard or custom protocols.
For example, you could do AMQP or XMPP over WebSocket, as people have already done. So a client could receive messages from an AMQP broker as if it were connected directly to the broker itself (and in some cases it is).
Or if you have an existing server with some custom protocol, you can transport that over WebSocket, thus extending that back-end server to the Web. Often an existing application that has been locked in the enterprise can broaden it's reach using WebSocket, without having to change any of the back-end infrastructure.
(Naturally, you'd want to be able to do all that securely so check with the vendor or WebSocket provider.)
Some people have referred to WebSocket as TCP for the Web. Because just like TCP transports higher-level protocols, so does WebSocket, but in a way that's compatible with Web infrastructure.
So while sending JSON (or whatever) messages directly over WebSocket is always possible, one should also consider existing protocols. Because for many things you want to do, there's probably a protocol that's already been thought of to do it.
I'm sorry if I am re-asking or combining many of the questions already on SO into a single question, but I just want to make perfect sense out of all the info that is out there on SO and the web regarding these concepts.
This was a great question, and the answers have all been very informative!

If I may ask one additional thing: I came across in an article somewhere that says that http streaming may also be cached by proxies while websockets are not. what does that mean?
(StackOverflow limits the size of comment responses, so I've had to answer here rather than inline.)
That's a good point. To understand this, think about a traditional HTTP scenario... Imagine a browser opened a web page, so it requests http://example.com, say. The server responds with HTTP that contains the HTML for the page. Then the browser sees that there are resources in the page, so it starts requesting the CSS files, JavaScript files, and images of course. They are all static files that will be the same for all clients requesting them.
Some proxies will cache static resources so that subsequent requests from other clients can get those static resources from the proxy, rather than having to go all the way back to the central web server to get them. This is caching, and it's a great strategy to offload requests and processing from your central services.
So client #1 requests http://example.com/images/logo.gif, say. That request goes through the proxy all the way to the central web server, which serves up logo.gif. As logo.gif passes through the proxy, the proxy will save that image and associate it with the address http://example.com/images/logo.gif.
When client #2 comes along and also requests http://example.com/images/logo.gif, the proxy can return the image and no communication is required back to the web server in the center. This gives a faster response to the end user, which is always great, but it also means that there is less load on the center. That can translate to reduced hardware costs, reduced networking costs, etc. So it's a good thing.
The problem arises when the logo.gif is updated on the web server. The proxy will continue to serve the old image unaware that there is a new image. This leads to a whole thing around expiry so that the proxy will only cache the image for a short time before it "expires" and the next request goes through the proxy to the web server, which then refreshes the proxy's cache. There are also more advanced solutions where a central server can push out to known caches, and so on, and things can get pretty sophisticated.
How does this tie in to your question?
You asked about HTTP streaming where the server is streaming HTTP to a client. But streaming HTTP is just like regular HTTP except you don't stop sending data. If a web server serves an image, it sends HTTP to the client that eventually ends: you've sent the whole image. And if you want to send data, it's exactly the same, but the server just sends for a really long time (like it's a massively gigantic image, say) or even never finishes.
From the proxy's point of view, it cannot distinguish between HTTP for a static resource like an image, or data from HTTP streaming. In both of those cases, the client made a request of the server. The proxy remembered that request and also the response. The next time that request comes in, the proxy serves up the same response.
So if your client made a request for stock prices, say, and got a response, then the next client may make the same request and get the cached data. Probably not what you want! If you request stock prices you want the latest data, right?
So it's a problem.
There are tricks and workarounds to handle problems like that, it is true. Obviously you can get HTTP streaming to work since it's it's in use today. It's all transparent to the end user, but the people who develop and maintain those architectures have to jump through hoops and pay a price. It results in over-complicated architectures, which means more maintenance, more hardware, more complexity, more cost. It also means developers often have to care about something they shouldn't have to when they should just be focussing on the application, GUI, and business logic -- they shouldn't have to be concerned about the underlying communication.

HTTP limits the number of connections a client can have with a server to 2 (although this can be mitigated by using subdomains) and IE has been known to enforce this eagerly. Firefox and Chrome allow more (although I can't remember of the top of my head exactly how many). This might not seem like a huge issue but if you are using 1 connection constantly for real-time updates, all other requests have to bottleneck through the other HTTP connection. And there is the matter of having more open connections from clients puts more load on the server.
WebSockets are a TCP-based protocol and as such don't suffer from this HTTP-level connection limit (but, of course, browser support is not uniform).

AJAX and Client-Server Architecture with JavaScript

I have to program websites, but I rather don't like the static HTML nature. I prefer more of a client-server architecture.
Now I've figured, that with XMLhttp, you can basically dynamically update your page and send/request for information/action to/from a server. So this would basically cover the client area.
But to complete a client-server architecture, it is necessary for the server to send/request information, too, without being queried.
Is there any way, for example for a chat server, to send back a received message to all clients (the clients use a web browser) without that the clients have to query in a fixed interval? I want to implement that one can see while you type something in.

There are several different ways to accomplish this. Some of them are already answered here, but I wanted to include a few more as well as my thoughts on them.
1. Polling
Frequent requests are made to the server to check for new info. This is the worst way to do this, but probably the easiest. If your site will have a low number of users, it might be worth doing it this way.
This can be accomplished by using setInterval(myFunction, n) in javascript to send XMLHttpRequests to the server every n milliseconds. Then, on the server, you respond to this with your new info, when you have it, or some message that implies no new info.
2. Long Polling
When the page is loaded, it makes a request to the server for new info. The server holds the connection open until there is something to send back. This method reduces the amount of network traffic used, but increases the resources used on the server. You can use this for a small number of users, but it doesn't scale very well.
The easiest way to do this is to have the page that handles the AJAX request simply wait for new information to be available, then respond. This can tie up a lot connections on your server. So, use with care.
3. COMET
COMET is basically long polling, but the server is setup properly for it. It knows that these connections aren't "real" and it uses less resources to handle them. This is a great solution for this problem, but it requires that the server is explicitly setup for this purpose. There are COMET servers and COMET addins for other popular servers, but it will require some setup and sometimes some money.
Implementing this on .NET isn't the easiest thing in the world. You can pay for solutions, try to find someone else's code that does something similar, or try to write it yourself. I've not found any decent free solutions for this. If someone else has, please comment.
4. RIA
Another solution would be to include Flash, Silverlight, or Java Applet on your page. People often do this by using a 1x1 object so that they can use Flash or Silverlight to talk to the server. If you don't mind adding the dependency, this is a decent solution. If you already know Silverlight or Flash, it could be relatively simple to implement.
You can find tutorials on the internet for each of these options.
5. Web Sockets
If you are on the cutting edge, you can look into Web Sockets. It's only available in the latest builds of modern browsers. It was part of HTML5, but it might be its own spec now. Regardless, it means that older browsers won't be able to handle it. But, if you don't mind limiting yourself to the latest of browsers, you can use this amazing feature.
I believe that Chromium is the only browser that currently supports it. However, there is work being done to implement this in Firefox and WebKit.
I'll spare you the controversy and simply say that this does exactly what you want it to. The Abstract of the spec says it all.
This specification defines an API that enables Web pages to use the Web Sockets protocol for two-way communication with a remote host.
Special Mention
If you are interested in the world of Node JS, you can't go wrong with Socket IO. It will implement the best of whichever technology is available to the browser.
Conclusion
The best option is Socket.IO on Node JS. However, for an ASP.Net solution, go for COMET or Web Sockets, if you can. Otherwise, using Flash/Silverlight isn't terrible. Finally, polling and long polling should be last resorts. You could always support one of these, then fall back to another if there isn't support for it in the client's browser.

Yes, you can use COMET.

The client has to tell the server when the client-user begins typing. You've got a couple options here.
Frequent requests from the server for the latest activity. This would be taking place for each user involved in the chat. The same request could be used to send user-specific activity to the server as well: "Jonathan is typing..."
Long-polling. This essentially requests information from the server, and the server keeps the connection opened until it has something to send back. So your requests are minimized, but your connections stay opened longer.
Depending on your traffic, type of data being transmitted, server-environment, and many other factors, one of these options may shine more than the other.

You can use Silverlight for push notifications. Look at PollingDuplexHttpBinding. Since you are using ASP.Net MVC, adding Silverlight will be easy.
Look at this page for more information.

Based upon the REST architecture the html system is based upon, the servers role is to simply act as a resource for the client to pull from. I am generalizing but there are tools to implement this type of action on the client side, rather than on the server side.
You are better off writing/using a library that can request updates from the server periodically. You can encapsulate these types of requests in a javascript object that can fire events. This way your client side script can act like it's getting notified from the server. Review some common stuff with COMET you can probably find some tools to help you client side code.
HTML 5 has some tentative attempts at this type of functionality, but if you want your app to work on older browsers, your better off using more stable methods, like AJAX requested updates.