Is Comet obsolete now with Server-Sent Events and WebSocket?
or do Server-Sent Events and WebSocket replace Comet techniques?
Answer
I will approach this answer from both a terminology and historical perspective.
As I wrote in this other answer, we can use one of the several umbrella terms to refer to the set of technologies available to asynchronously send events from a web server to a web client (and vice versa). The "Push Technology" term has been used for fifteen years (for a short history of Push Technology you can see this old white paper I wrote many years ago—full disclosure: I am the creator of Lightstreamer). Now, the "Web Streaming" term is gaining consensus among the IT analysts (see Gartner, "Cool Vendors in Application and Integration Platforms, 2012", by Massimo Pezzini and Jess Thompson, 11 April 2012).
The important aspect is that we are talking about Web-based communication, that is, leveraging Web protocols. There are tons of messaging protocols and technologies that are not Web based (most of MOMs, for example) and we do not consider them as part of Push Technology (or Web Streaming).
That being said, you can distinguish between two sub-categories of Push Technology (or Web Streaming):
- HTTP based
- WebSockets based
Both HTTP and WebSockets are Web protocols.
If you explode the HTTP-based push mechanisms, you can identify:
- HTTP Streaming
- HTTP Long Polling
- HTTP Polling
Traditionally, the "Comet" term (coined in 2006 by Alex Russell) has been referring to both HTTP Streaming and HTTP Polling. But consider that the first implementations of HTTP Streaming go back to 2000, well before the Comet term was coined (examples are Pushlets and Lightstreamer).
Now WebSockets make it simpler to implement Web Streaming, especially for the "backward" channel (messages sent from the browser to the server). For a more detailed explanation on the peculiarities of the backward channel over HTTP, see the final part of this article I wrote for CometDaily: http://cometdaily.com/2011/07/06/push-technology-comet-and-websockets-10-years-of-history-from-lightstreamers-perspective/
As pointed out by Phil, Comet is still necessary and will probably be for some more years, as there are not only old browsers around (including IE9, which does not support WebSockets…) but also infinite pieces of network intermediaries that do not speak WS. For example, we have seen that some mobile carriers in some countries (for example Vodafone Italy) support WSS but block WS. So a world without the Comet "hacks" is still far away… And let me add, on a personal note, that I've never loved the term "hack" applied to Comet (or, from a more correct historical point of view, applied to HTTP Streaming and HTTP Long Polling). Having worked on these techniques for 12 years now, I can say we have been able to refine them so much that they have become a full-blown technology themselves, completely reliable and used every day in many critical production scenarios (in finance, aerospace, and military, to name a few industries).
Now, let's imagine a world where WebSockets are universally supported and Comet is no more necessary. What do you get exactly? Well, just a bi-directional transport, nothing more... On the top of it you need to build everything: a messaging protocol (perhaps based on pub/sub), a server-side interface to talk to your server code, and a good set of optimization techniques and algorithms to manage the data flow, including bandwidth management, data conflation, automatic throttling, delta delivery, etc. The good thing is that both the messaging protocols and the optimization mechanisms have already been implemented by good Comet solutions. So, extending former Comet servers to support WebSocket is the natural evolution that all of us vendors have implemented.
So, in a nutshell, in a not-so-near future WebSockets might make Comet transports obsolete, but will need to suck in all the higher layers already implemented and well tested on traditional Comet servers.