They are ids to lookup closures in a database. They time out to stop the database overflowing ;) It's called continuation-based web development , popular with Lisp and Smalltalk-based web servers (because who else has continuations?)
The fnids do not expire randomly due to restarts; they expire when there are too many or they timeout so memory doesn't fill up with these continuations. Personally, I don't like this continuation-based approach since "Unknown or expired link" is a really bad user experience.
Way back I wrote a bunch of documentation on the Arc web server if you want details: http://www.arcfn.com/doc/srv.html
Look at harvest-fnids which expires the fnids.
The fnid is an id that references the appropriate continuation function in an Arc table. The basic idea is that when you click something, such "more" or "add comment", the server ends up with the same state it had when it generated the page for you, because the state was stored in a continuation. (Note that these are not Scheme's ccc first-class continuations, but basically callbacks with closures.)
(The HN server is written in Arc, which runs on top of Racket (formerly known as PLT Scheme or mzscheme))
Edit: submitted in multiple parts to avoid expired fnids. Even so, I still hit the error during submission, which seems sort of ironic.
Racket (arc's host language) keeps continuations on the filesystem, or you can write your own "stuffer" to do what you want with them (store them in a database or whatever). But you have to keep them somewhere or else (assuming the server uses continuations) you can't keep track of the user's path through your code as they click through links and such.
Racket does have an option to serialize the continuations, gzip them, sign them with HMAC, and then send all of that to the client so the server doesn't have to keep track of anything, but HN doesn't use it.
Then where is the data that is associated with "b7VO4wED8MRumCeiX5fCnF" stored? How is that data requested? There certainly is a database, it is just most likely not a traditional database that most people think of.
I never understood why the continuation couldn't instead be addressed by a URL path. It could even get constructed from URL/query data if it moves out of memory, so keeping them in memory would only be a caching mechanism.
Yes, but the way hn works, the continuations map uri's to browser sessions so they can be expired on a more granular basis than all at once or lru or what have you. I'm guessing here, though, I've not looked through the code.
[addendum] I Reread your comment and realized that wasn't what you meant at all. What would be the benefit of path based uris over query string params in hn's case? I only see how they be equal, not better.
I'm just like you. I don't sub-vocalize or hear words when reading and I can consequently read significantly faster than people speak (I'm assuming most people read at this rate if they're 'hearing' words -- though I'm actually not sure).
I have the exact same experience with character names -- that's a great example.
Bit tongue in cheek... but this sets the clock ticking for Microsoft going 'oh, crap, someone is defining a new computing paradigm. Let's get on that before they're way ahead of us again'. Any bets as to when Microsoft announces Windows for Eyewear?
The compiler doesn't pass any size information when you pass an array into a function. The function just gets a pointer. If you take the sizeof the array, you'll get the size of a pointer on your system:
[eric@rangely foo]$ cat foo.c
int test (int arr)
printf ("%lu\n", sizeof (arr));
int main (int argc, char *argv)
Wouldn't you need to put your sizeof test outside of every call to your function? Inside the function it would only know the declared parameter's type, so it would have no idea what sized array you actually passed.