Are RSS guids actually expected to be _globally_ unique?

Question

Just trying to clear up what level of uniqueness the <guid> element in an RSS feed is actually supposed to have. I understand that one of its main purposes is to be something that software can use to identify the item for such purposes as read/unread tracking. But am I right that:

• once a guid has been used, it should never be used again, even if the last instance was removed from the feed ages ago?
• it should be unique not only within a feed, but also across multiple feeds and even (to the extent it can be achieved) unrelated websites?
• one of the reasons it's common to use URLs as guids is to help achieve the above?

Moreover, if a program does encounter the same guid twice in different feeds, what should happen?

• it treats them as distinct RSS items, since they are in different feeds?
• they are considered to be one and the same item, just published in multiple places (similar to Usenet crossposts)?
• it depends on whether they're on the same site/domain?
• the behaviour is undefined?

Answer 1

It's a good question and the answer is No.

It was a bad choice of terminology.

The guids only have to be unique to the feed.

The goal in adding them was to have a way for an aggregator to know for sure whether or not it's seen the item before. A locally-unique id suffices for that purpose.