When you share a link on social media the platform fetches the page and includes a preview with your post:
I don't even see a request for
robots.txt in my logs.
But that's actually ok! The robots exclusion standard is for
"crawlers", or automated agents. When I share a link I want the
preview to be included, and so the bot fetching the page is acting as
my agent. This is the same reason why it's fine that WebPageTest and PageSpeed Insights also ignore
robots.txt: they're fetching specific pages at the user
s request so they can measure performance.
This puts Mastodon in an awkward situation. They do want to include
previews, because they're trying to do all the standard social network
things, and if they respected
robots.txt many sites
you'd want to be able to preview won't work. They also don't want
the originating instance to generate the preview and include it in the
post, because it's open to abuse:
You can trust
mastodon.mit.edu about what
@firstname.lastname@example.org says, but not about what
The approach Mastodon has gone with is to have each instance generate link previews for each incoming link. This means that while having a link shared on Twitter or Facebook might give you one automated pageview for the preview, on Mastodon it gives you one for each instance the link is federated to. For a link shared by a widely-followed account this might mean thousands of link-preview requests hitting a server, and I'd also now consider this traffic to be from an automated agent.
I'm not sure what the right approach is for Mastodon. Making the link
preview fetcher respect
robots.txt would be a good
Longer term I think including link previews when composing a post and
handling abuse with defederation seems like it should work as well as
the rest of Mastodon's abuse handling.
 Wouldn't that just add an extra request and increase the load on sites even more? No: everyone builds sites to make serving
robots.txt very cheaply. Serving HTML, however, often
involves scripting languages, DB lookups, and other slow operations.