What limitations does converting question links to links with titles have?

Question

On this question there are more than 50 links to Stack Overflow questions. However, the last 6 links aren't converted to titles (http://facebook.stackoverflow.com/questions/6530420/control-the-image-that-facebook-shows-when-i-like-a-url should be converted to http://facebook.stackoverflow.com/questions/6530420/control-the-image-that-facebook-shows-when-i-like-a-url).

However: changing all http://facebook.stackoverflow.com/... into http://stackoverflow.com/... fixes this, both in the preview and the final result. Also, the preview won't show any title for http://facebook.stackoverflow.com/... links.

My question is, what limitations does this converting have? How are these question titles retreived? Via API? Through database?

^{_{I'm not sure about title, feel free to edit that}}

Answer 1

You had to ask – well, here goes.

The restrictions are currently as follows, but note that they do look a bit contrived, resulting from the fact that they have grown over time (from single site to cross-site, from "there are no subsites" to "facebook.SO", etc.), so it's not unlikely that we'll revisit them.

1. Links to questions on the same site are unlimited.
2. Links to other sites are limited to
   • a) 5 linked-to sites and
   • b) N questions per target site, where N is 200 on meta sites (was 40 at the time this question was asked) and 10 on non-metas (this is about the site that the post containing the link is posted on, not the site that is being linked to)
3. While same-site links always work, which other sites work depends on where you're posting:
   • a) On all subsites (currently none), links to the master site work, and vice versa.
   • b) On all meta sites, links to the corresponding main site work, and vice versa.
   • c) On Meta Stack Exchange, all sites work.
   • d) On all meta sites, links to Meta Stack Exchange work.
   • e) Anything else does not work.
4. For the purpose of 2 (in contrast to 1 and 3a), facebook.SO and SO were considered separate sites.

The reason that removing the facebook. part from links caused the auto-title to suddenly work, as you observed, is 2b in combination with 4: forty links to fb.so work; the 41st doesn't.

Point 4 is also the rule that makes the least sense, so don't expect that to stick around forever.

This will also work identical in the preview from the next build on, with the exception of 2a. That one is only applicable to Meta SO anyway (because of 3), it's non-trivial to add to the preview because of slight differences in the implementation, and it's a pathology to begin with.

To answer your other question in addition to what Arjan said: on the server-side, same-site links are retrieved from the database, cross-site links use an HTTP API. The latter used to happen via cross-DB queries, but this was dropped at some point, because having multiple database connections lying around is just crying for issues.

Answer 2

I'm not an authoritative source, but:

• While editing, Ajax calls to the API are used to fetch the data that is needed to change raw links into clickable links with their title.

• When saving, the HTML is "pre-cooked" (including sanitation and tag-balancing) from the Markdown source. This includes creating the clickable links, just like it includes creating <img> elements and so on. So, upon loading the page, the HTML fragment is simply fetched from a database or something like that. This implies that changes in a title are not visible in a post that links to it, unless the post that links to it is edited and saved too. (That applies to tags too.)

From this very Meta, this seems to work for links to all Stack Exchange sites: Why did my "goat cheese espuma" flop? But some titles might look odd, lacking MathJax on the linking site: Integrating $\int \frac{1}{1+e^x} dx$.

On other sites (and their Metas) it seems only links to that specific site work:

     

Bugs^† aside, there shouldn't be any other limitations. Apparently the facebook.stackoverflow.com subdomain causes some issues in your example (which you already tested works fine when removing that subdomain). That is a bug.

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^† Like:

• Bare links in parenthesis: (URL inside parenthesis are not automatically hyperlinked). That can be fixed by adding <...>, and thus using (<http://...>), like: (URL inside parenthesis are not automatically hyperlinked).

• Multiple URLs separated by just whitespace: Jon Skeet Facts The Many Memes of Meta