I have a client-server system where the client is spawning 100s of 1000s of
jobs on the client. The client polls the servers to see when each server’s task
queue is ready for more jobs. This all works fine.
Logically this polling is a while-true() loop that will continue until either
all the servers are offline or all the tasks to be submitted are consumed.
In a procedural language this is trivial, but in XQuery 2 I’m not finding a way
to do it that works. In XQuery 3 I could use the new iterate operator but that
doesn’t seem to be available in MarkLogic 9.
My first attempt was to use a recursive process, relying on tail recursion
optimization to avoid blowing the stack buffer. That worked logically but I
still ran into out-of-memory on the server at some point (around 200K jobs
submitted) and it seems likely that it was runaway recursion doing it.
So I tried using a simple loop with xdmp:set() to iterate over the tasks and
use an exception to break out when all the tasks are done:
try {
for $i in 1 to 1000000 (: i.e., loop forever :)
if (empty($tasks))
then error()
else submit-task(head($tasks))
xdmp:set($tasks, tail($tasks))
} catch ($e) {
(: We’re done. (
}
Is there a better way to do this kind of looping forever?
I’m also having a very strange behavior where in my new looping code I’m
getting what I think must be a pending commit deadlock that I didn’t get in my
recursive version of the code. I can trace the code to the xdmp:eval() that
would commit an update to the task and that code never returns.
Each task is a document that I update to reflect the details of the task’s
status (start and end times, current processing status, etc.). Those updates
are all done either in separately-run modules or via xdmp:eval(), so as far as
I can tell there shouldn’t be any issues with uncommitted updates. I didn’t
change anything in the logic that updates the task documents, only the loop
that iterates over the tasks.
Could it be that the use of xdmp:set() to modify the $tasks variable (a
sequence of <task> elements) would be causing some kind of commit lock?
Thanks,
Eliot
--
Eliot Kimber
http://contrext.com
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