Chris Wellons, creator of Elfeed, comments on RSS' guid field for items and their flaws:
Not only did it not appear until RSS 2.0, but the guid tag is not required. In practice an RSS client will be rereading the same feed items over and over, so it’s critical that it’s able to identify what items it’s seen before.
I too was surprised to learn that that the guid field for RSS items was not required. I first came across this while build rss_py. I defaulted to using the link value, as RSS guid's are normally permalinks. That brings me to Chris Wellons second flaw of RSS guid's:
The bigger flaw of the guid tag is that, by default, guid tag content is not actually a GUID! This was a huge oversight by the specification’s authors. By default, the content of the guid tag must be a permanent URL. Only if the isPermalink attribute is set to false can it actually be a GUID (but even that’s unlikely).
The rest of the article has some interesting comparisons between RSS and Atom and what makes Atom superior.
I discovered this article through Simon Willison's quote from Nelon Minar, when Chris' article was shared on lobste.rs.
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