[Ur] interfacing with C or other languages - example system

[Marc Weber]

Excerpts from Adam Chlipala's message of Sun Nov 28 21:58:18 +0100 2010:
> That's right.  The runtime system will also restart if an optimistic SQL 
> concurrency violation is detected, when certain errors are signaled, etc..

Don't know yet whether that's smart behaviour.
How exactly is determined how much heap will be allocated for a request?

There are some cases where you don't want to retry;
Some payment systems redirect customers using a "return" url
which tells the web system that the customer has entered all the data.

Then the web system is supposed to verify and confirm the data - because
the return url parameters can be manipulated by the user.

However you can't run that confirmation query - which is typically a
http request - twice.

The second time they'll return an error telling you "that transaction id
has been used previously".

Worse: Because urweb rolls back everything db related you don't even
have traces about it. So the custmer has to pay - and you don't have an
order.

I can see 3 solutions:
a) write a layer between urweb and the payment service which is caching
  the confirmation results. Bad: You can't use urweb for writing it.
  You could do this caching in C as well - but I'd like to use a higher
  order language to get my jobs done..

b) tell urweb to use a huge heap size so that its very unlikely that the
   issue appears - is there a project configuration option specifying a
   min heap size?

c) never fail for heap reasons. Use a linked list of heap blocks.
   If one is full allocate another one which is large enough and
   continue.

About the pointer; I'm lucky that it worked. 

http://gitorious.org/some-urweb-utility-libraries/uw-process/commit/ff620ea22b3209c73482c50ec24909ea581ac4ab

Marc Weber