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blog名称:Eternal Spark
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建立时间:2004年12月23日 |
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 [social network][转载]What is an RSS Channel, Anyway?
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Spark 发表于 2005/7/25 22:31:24 |
| 转载自: http://www.mnot.net/rss/channel.html
What is an RSS Channel, Anyway?
An examination of the uses of RSS channels, to better understand their nature and to move towards a rigerous definition of them.
In The Beginning
Although its history is much disputed, most people can agree that RSS was born for practical purposes, as good formats tend to be. The RSS 0.91 spec starts with the words "Files must be 100% valid XML," and doesn't go much loftier than that. What started to be known as headline syndication drove development and refinement of the format.
What is syndication? The dictionary says
To sell (a comic strip or column, for example) through a syndicate for simultaneous publication in newspapers or periodicals.
The RSS community started calling what it did 'syndication' (as in, "Really Simple Syndication") because it was all about distributing content for reuse. However, RSS syndication isn't usually paid for, and it isn't just for republication; specialized clients called aggregators made content available directly to end users, so that they could mix-and-match their channels at will.
The headline syndication view of RSS is straightforward; a channel is a reverse-chronological sorted list of links to stories, along with metadata for each one indicating the title and sometimes a description. This is the model that most developers and users of RSS have in mind to this day, and it is a useful one.
However, a few people quickly noticed that it was easy to extend RSS to say other things about those links; in fact, you could say almost anything just by adding an element as a child of .
This metadata summary view of RSS ("Rich Site Summary" or "RDF Site Summary") treated the channel as a container for any kind of statement - from market data to mp3 playlists to event calendars or even order systems - as long as what they were talking about could be arranged in a list. The modularity of RSS1.0 enables its use in a variety of contexts, from Wall Street to Open Source software distribution.
Last but not least, Weblogs have been using RSS for something completely different - content syndication. Instead of just saying things about the channels' links, they reproduce the content at the other end, so that a Web resource can be replicated in whole in an aggregator or on another site.
All of these views of RSS use the channel to group items together. None of them, however, establish what a channel actually is. In other words, although items are somewhat well-understood (having identifiers and metadata associated with them), the relationships between the items, in the context of a channel, hasn't been explored so much as it has been assumed.
Flipping Channels
The basic conceit of RSS is a channel. This is a term from information theory; the dictionary defines it as
A course or pathway through which information is transmitted
This emphasises the changing nature of the information; it flows. Another oft-used term for an RSS channel is a feed, and its definition reinforces this:
The transmission or conveyance of a local radio or television program, as by satellite, on the Internet, or by broadcast over a network of stations
Thinking of an RSS channel as a flow is interesting for a number of reasons. Many RSS aggregators will keep a history of a channel and add to it over time, reconstructing the complete output of the flow. Some will also remember the last snapshot of a flow that the user has seen, so that they can show what has changed since.
More fundamentally, the flow nature of RSS needs to be aligned with that of the Web, as explained by Representational State Transfer (REST). The Web according to REST is embodied by three components - identifiers (URIs), protocols (e.g., HTTP) and formats (e.g., HTML; RSS, is a format as well).
Formats represent the state of a resource (as identified by a URI) in a snapshot (called a representation) that is sent using a Web protocol. But RSS is very often a partial snapshot; it describes the last few items in the feed, not the complete flow to date. Although other formats don't always give the complete state of a resource (think CNN's HTML home page; it shows you the latest news, not all the news they've ever published), RSS gives us the tantalizing ability to do just that; reconstruct the flow over time, enabling new features and applications of RSS. Some of these are already in use today (as noted above), but they're put together on an ad-hoc basis.
A well-defined concept of a channel would allow other uses of RSS beyond simple headlines; aggregators could make more intelligent decisions about how to display a feed, enabling more content - from my Netflix queue to my Amazon shopping cart to a personal todo list. just as Weblogs repurposes RSS from headline syndication to their own purposes, so to can we find new applications of RSS, without sacrificing its legacy. Rather, if we explicitly state what's implied about feeds now (reverse-chronological ordering), we enable new applications as well as new functionality for existing ones.
Taking Collection
So, what would a channel definition look like, and how can we use this to add all of these nifty features?
For a while, I thought of an RSS feed as a list - a sequence of items with some relationship implied by the sequence. Then I realised - with some prodding from others - that it isn't necessarily a sequence; sometimes, a channel is just a blob of things without an implicit ordering. For example, you might want to use an RSS channel to keep track of the state of a group of software packages (IIRC, RedHat uses RSS like this; can anyone provide a link?).
Thinking about it a bit more, I believe the following statements can be made;
An RSS Channel is a container for an arbitrary number of items of a similar type, meaning that they share the same potential metadata signature. E.g., if one has a |
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回复:[转载]What is an RSS Channel, Anyway?
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