New subject: Extremely serious issue with magically disappearing databases

13 Jan 2011


      I have an embedded (soon to be unembedded) Base-X database supporting a 
website written in Scala. I have a very basic caching mechanism.
The structure of the DB is such:
<data>
   <cache>
     <page>...</page>
     <page>...</page>
   </cache>
   <somethings>
     <something>...</something>
   </somethings>
   <otherthings>
     <otherthing>...</otherthing>
   </otherthings>
</data>
I have a huge (1000+ lines) XQuery that takes all the somethings and 
otherthings recursively and constructs a <page> item that includes the 
html content of the page thus:
<page uri="/" expires="">
   <name>home</name>
   <content>
     <html>
      <head>...</head>
      <body>...</body>
     </html>
   </content>
</page>
This page is then saved into /data/cache.
The way this works is simple. The main XQuery checks
/data/cache/page[@uri=$uri]
If it finds an unexpired page, it returns it. If it doesn't, it looks to 
see if such a page exists (in pieces) in the DB. If it does, it 
constructs the html page, inserts it into a <page> element along with 
some metadata (expires, etc.) and returns that. If no such page exists, 
it returns a 404 error page.
Beauty, eh?
And it works extremely well. Until I have an error in my XQuery, which, 
naturally, happens all the time during development.
Just to make clear, here is the Scala that runs the query (from a 
Circumflex web app):
get("(.)*".r) = {
     response.contentType("text/html")
val url = uri.get(0).getOrElse("/")
val out = executeQuery(Rx.pageQry(url)) match {
       case "" => "<p>Houston, we have a problem.</p>"
       case page if (page.substring(0,2) == "<p") =>
         executeQuery("delete node /data/cache/page[@uri='" + url + "']")
         executeQuery("insert node " + page + " into /data/cache")
         executeQuery(Rx.pageQry(url))
       case out => out
     }
"<!DOCTYPE html>\n" + out
   }
What this does is very simple. executeQuery simply runs the supplied 
query. Rx.pageQry just loads the huge query from a file and adds the 
call "local:getPage('/')" with the url inserted as a parameter. Right?
Then the match looks to see if nothing at all comes back (case ""), or 
if a page that begins "<p" comes back (meaning a newly constructed 
query). The default is to spit the thing out. So in case one, we're 
screwed. In the default case, we serve the right page *or* a 404 error 
page. And in case two, we get the <page> for caching. In that instance, 
we delete any old page that might be in the cache with that URI, then 
insert the new page. Then we pull the html from the newly inserted page 
and serve that. I hope this is clear even if you don't write Scala.
HERE IS THE PROBLEM:
When I make an error in the big XQuery (the one loaded by Rx.pageQry), I 
get an error message from Base-X with the line number and the type of 
error. Beauty! So I fix the error and rerun the query AND MY DATABASE IS 
EMPTY.
That's right: the entire database has been wiped. Running a / query 
gives me back <data>  </data>. OUCH!
If this happens in production (and this app is running in production 
right now), I might as well step in front of a bus.
What, please, could be causing the database to dump itself? This has 
happened dozens of times! I'm willing to send the huge query off-list if 
anyone thinks that would help, but I don't see how the query *failing* 
and destroying the database could have anything to do with the query itself.
I've looked at the Scala a million times and maybe I'm missing 
something, but the delete XQuery is /data/cache/page[etc.]. How can this 
destroy anything outside of /data/cache ? As for the insert query, how 
can inserting *anything* into /data/cache destroy things outside of 
/data/cache ?
Am I missing something really basic, or is there a nasty bug in Base-X? 
Hopefully the former, but if it's the latter, I thought you should know. 
Could this be related to the embedded nature of the database?
Thanks!
Chas.