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What are reading lists?

Sunday, May 24, 2009 by Dave Winer.

First an update on today's earlier blog post. Apparently Google Reader does not support reading lists. I think when I asked the question, the 140-character limit on Twitter made it impossible for an accurate answer.  

No matter what, maybe it would be a good idea now to try to give a complete technical explanation of what a reading list is.  

1. There are many kinds of "feed consumer" apps, all of them are capable of supporting reading lists, not just feed readers or aggregators. In the rest of this piece I'll use the shorthand "FC" to refer to a feed consumer app. 

2. When you subscribe to a feed you're telling the FC that you want it to periodically read the contents of that feed and somehow act on the new items in the feed.  

3. A reading list contains a set of feeds. The format of a reading list is exactly the same as the OPML-based subscription list format that's supported by many FCs. 

4. When a FC subscribes to a readling list it does not import the feeds. 

5. When the FC checks for updates, it checks for new items in the feeds in the reading list. Therefore it must keep a record of the feeds in the reading lists it is subscribed to.  

6. If a new feed appears in the reading list, it does whatever it does for a new feed. Many FCs will consider all the items in the feed as "old" the first time the feed is read, esp if it's a podcatcher. 

7. If a feed that was in the reading list has been removed, then the feed is not read and all record of the feed is erased from FC database, with the following exception. If a feed appears in two or more reading lists, a reference count must be maintained, and the feed is erased only when the reference count goes to zero. 

8. Obviously the user can subscribe to as many reading lists as he or she likes. 

9. The behavior I've described is how the NewsRiver aggregator that runs in the OPML Editor works. I suppose it's possible that other FCs work differently. If so, it would be great to hear about them.  

I think that pretty much covers it.