Technological Thoughts by Jerome Kehrli

Wikipedia's definition does a pretty great job in introducing comet:

This says it all. For a long time, Comet - which is more an umbrella term regrouping several techniques - was pretty much the only way to get as close as possible to Server Push in Web application, plain HTML/Javascript applications on top of the HTTP protocol.
All methods have in common that they rely on browser-native technologies such as JavaScript, rather than on proprietary plug-ins.

History

In the early days of the World Wide Web, the browser would make multiple server requests: one for the page content and one for each page component. Examples of page components include images, CSS files, scripts, Java applets, and any other server-hosted resource referenced in the page.

Ajax - Asynchronous JavaScript and XML - went a long way towards making this model evolve. It allowed far greater control over page content by providing the ability to send server requests for as little, or as much data as the browser needed to update. In addition, its asynchronous nature supported multiple simultaneous calls - even while other elements downloaded.
One problem that Ajax did not adequately solve was the issue of data synchronization between the client and server. Since the browser would not know if something had changed on the server, Web applications typically polled the server on a periodic basis to ask if new information was available. The only possible way as to use Polling where the browser would poll the serve rat regular intervals to find out about new events and updated data.

To circumvent this very limitation, developers started to imagine techniques aimed at getting closer to server push, either using the Forever Hidden iframe technique or the Long Polling XMLHttpRequest technique. Both these techniques are grouped under the umbrella term Comet or Bayeux Protocol.
Now of course these techniques have respective advantages and drawbacks that I will be discussing later in this article.

Comet

For many reasons, mostly robustness and universality of the solution, I am favoring the Forever Hidden iframe approach and I have designed quite some time ago a Comet framework making use of this technique to provide Server-Push to Plain HTML/javascript web applications.

The forever hidden iframe technique is the one I found most seducing for one very good and essential reason : it's the most robust one from a technical perspective. It has drawbacks of course in comparison with other techniques, but I still deem it the most solid one, and in some situations it was even the only one I could make work.
Its principle is straightforward to understand: an iframe is opened on a server resource and the HTTP response stream is never closed by the backend. The iframe keeps loading more javascript instructions as the server pushes them in the response stream.
Having said that, I have to admit ... It blocks a freaking amount of threads in the java backend, it shows the annoying loading icon on the browser, managing errors is a nightmare ... but it always works, in every situation, and at the end of the day considering the kind of business critical applications I usually work on, this is what matters the most to me.

Now of course, WebSockets tend to rend all Comet tecnniques kind of legacy.
But still, I end up deploying comet techniques instead of WebSockets in many circumstances. You may wonder why ?

WebSockets are no magic silver bullet

• Most importantly, when we have to support plain HTTP connection and bad HTTP proxies which let a WebSocket being opened but don't let anything pass through
• Implementing WebSockets efficiently on the server side is more complicated than Comet and requires most of the time specific libraries, sometimes pretty incompatible with some Application Servers
• With WebSockets you are forced to run TCP proxies as opposed to HTTP proxies for load balancing
• When I have to support old version of Internet Explorer, such as IE9, in banking institutions that have a bad tendency to use pretty old version of software
• Many other reasons I am detailing below ...

The first problem above is the darkest one regarding WebSockets in my opinion. This proxy server issue is quite widespread.
Nonetheless, "You must have HTTPS" is the weakest argument against WebSocket adoption since I want all the web to be HTTPS, it is the future and it is getting cheaper every day. But unfortunately there are some context where we have to integrate our web application on plain HTTP and one should know that weird stuff will definitely happen you deploy WebSockets over unsecured HTTP.

lightweight, simple and robust Comet framework

This is the main rational behind the Lightweight and simple Comet Framework for Java that I am introducing here. I am using this framework, that I've designed long ago and somewhat maintained over the years, each and every time I encounter issues with WebSockets.
To be honest, I always consider it somewhat a failure since the standardization of the WebSocket specification should prevent me from reverting to this Comet Framework so often, but surprisingly it doesn't and I end up returning there pretty often.
Again, the forever hidden iframe Comet technique always works!

Anyway, this Lightweight and simple Comet Framework for Java is pretty handy and I am presenting it here and making the sources available for download.