To John Kalucki's points first. I absolutely agree that the driver for this protocol is latency. But it is difficult to answer the question "what latency is necessary?", because we don't yet know what applications people will devise.
(An illustration of how latency changes everything, from a very different business: when my wife worked for Niman Ranch, I was amazed to hear that they dispatch steaks via FedEx (packed in ice and insulation, and sent overnight); this would be out of the question using the USPS and a three day delivery time.)
I believe that real-time web content feeds are a game changer. I call it the Streaming Web — a web where every piece of content is accessible via a URL and you can subscribe to be alerted immediately if a piece of content changes. Every page would become a potential feed, and there would be agents that allow us to collect and filter content we are interested in: be it a friend's photo album or the price of a plane ticket.
A huge effort is required to make the Streaming Web a reality. The first steps, the web content formats such as RSS and Atom, are already in place. The next step is to introduce a protocols so that subscribers are notified of changes as soon as they happen.
What experience can you offer with feeds at a 50ms push latency vs a 180,000ms pull latency? If a machine is consuming the feed, not much. If a human is immediately consuming a feed, perhaps a great deal.I agree that a human can benefit from low-latency content, although there is little benefit for content arriving faster than the human's think time — say 5,000ms. But if a computer is the consumer, ideal latencies span a broad spectrum: a mail server would operate more efficiently if it is allowed latencies in the minutes or hours, whereas an automated stock trading system needs information to arrive within 50ms.
Today, not much web content is of interest to automated stock trading systems. Most web content feeds today are textual — written by humans, and consumed by humans — but I believe that once we remove the latency constraints and introduce some standard protocols, we will start to see more structured data in feeds. Also, we will see algorithms for extracting information from textual feeds.
As for the right protocol for the job, I am not really the best judge, so I am going to punt for now, and focus on the architecture. Richard Taylor suggests XMPP. It seems to have the right qualifications, and I'm sure that it could be made to work technically. (And I see that XMPP is already a central part of the Twitter ecosystem.) It comes down to power versus simplicity: the power of an established standard versus the simplicity required to reach a new audience of developers.
I've been around long enough to see new approaches overturn "over-complex" existing technologies and then, in time, acquire the features that made their predecessors complicated. Take for example SOAP overturning CORBA, or PCs overturning minicomputers. I'm not going to take sides: these revolutions are part of the process of how technology moves forward. But it does seem that each revolution will only be successful if the new technology serves a new audience. And, to borrow Einstein's words, a protocol should be as simple as possible, but no simpler; otherwise, even if the technology finds its initial audience, it will not survive its growing pains.
I'm not a big fan of XML as a protocol for transmitting data over a network, mainly because it is bulky, and that makes it expensive to produce and consume at high data rates. But for this protocol, I would choose XML over a binary format. If you're a developer learning a new protocol, it's a lot easier to debug your code if you can read the messages being sent over the nextwork as text.
Which brings us to the audience for this protocol. I do agree with Stefan Tilkov that "[f]or the majority of use cases, [the polling] approach is vastly superior to a push model". That majority is already well served, so I'm focusing on the minority that need low latency. I think those use cases are important, and we'll all be using them if the "streaming web" thing catches on.
To achieve low latency feeds, push is more efficient than high-frequency polling, but it is still more expensive than low-frequency polling, which is what people are doing today. So, if every web content aggregator and RSS reader switched to a low-latency push protocol overnight, the system would collapse.
But luckily, there is no need for those millions of clients who would like to receive low-latency feed updates to connect using this new protocol. If those clients are humans, they will be happy to receive their updates via XMPP or SMS, or slower protocols like email. A single server could speak the streaming web feed protocol to various source feeds, and route the results to thousands of end users via XMPP or SMS. This approach means that each source feed is serving a modest number of downstream servers.
I'd describe it as a 'wholesale' architecture. A food producer has a central depot, where it loads its goods onto the trucks of several client stores. The food company allows consumers to buy from the depot, if they are prepared to buy their goods in bulk, but most consumers opt for the convenience of visiting a local store and buying their goods in smaller quantities.
(If you're Twitter, no problem is ever small, so that 'modest number' is probably in the tens of thousands. But I suppose that problem can be solved using multiple tiers of servers and fanning out streams between one tier and the next.)
The next step in the evolution of the architecture would be to introduce a query language. Queries present a more convenient interface for clients, but they would have architectural advantages. For example, using a query, a client can specify more precisely which content it is interested in. It would save CPU effort on the client and possibly the server, and bandwidth for everyone, so there would be a strong incentive to use queries rather than raw feeds.
Queries would also allow feeds to be virtualized: rather than talk directly to blogger and typepad, a client could talk to a third party that aggregates the content into a single feed.
Streaming SQL would be a good candidate for expressing these queries, but is by no means the only choice. And in fact the architecture and protocol would work well enough for clients that did not use queries and wanted to consume only raw feeds.
The resulting system, the Streaming Web, would enable applications yet to be imagined.