Geert,
Maybe it is a bit harsh, but I think I stand by it. I know what
deadlocks are, and now how to avoid them in MarkLogic Server. I just
think that in a service-oriented software package that, by design, will
crash in the event of X (taking all other App Servers, WebDAV and XCC
connections with it), then X shouldn't necessarily be available by
default. The developer should explicitly (as if signing a consent form)
say "I know what deadlocks are, I understand the risks, I won't crash
the server, promise."
Eric
Geert Josten wrote:
Hi Eric,
You sound quite harsh on your judgement. Note that xdmp:eval is not a commonly
used function, and the isolation option is most likely used even less. And also
note that a deadlock will happen only in specific situations. I will give an
example below..
About update statements, there should be examples mentioned somewhere. It sais
update statements are calls to xdmp:document-insert and alike. And if an Xquery
script contains them somewhere, the transaction of the Xquery script switches
to update mode. In that case it will request read and write locks on resources
that are being accessed, but only at the moment they are accessed.
If you are in an update-mode transaction, and you update a document named
'/mydoc.xml', then use xdmp:eval with isolation = different-transaction and the
deadlock-prevention to false, while updating the same document from within the
eval, the eval will wait for the calling transaction to release the update
lock, but that will never happen as the calling transaction is waiting for the
eval to finish first.
Once you understand it, it is not that hard to prevent.. ;-)
Kind regards,
Geert
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
Eric Palmitesta
Sent: vrijdag 20 februari 2009 17:44
To: General Mark Logic Developer Discussion
Subject: Re: [MarkLogic Dev General] dynamic module import
Yep, read the dev guide, and have wondered why <prevent-deadlocks> is
false by default. Sounds like:
"There's a situation in which ML will undetectably deadlock
and you'll
have to kill your services while you restart the server. Oh, but you
can completely avoid the deadlock and have a nice warning printed for
you instead by setting this option to 'true', but we'll allow you to
shoot yourself in the foot by default."
Why wouldn't the server-breaking-behavior be constrained to those who
explicitly wished to venture into deadlock territory by setting
<prevent-deadlocks> to 'false' (from a default of 'true')?
(Don't get me wrong, there's likely a good explanation, and
it's likely
over my head, but this is what <prevent-deadlocks> defaulting
to false
"sounds like")
Also, a question about what an update statement is. A "statement" is
described as "a unit of XQuery code", and an "update statement" is "a
statement which has the potential to perform updates".
declare function do-update() { node-replace(...), 'done' };
let $x = doc('blah')/somenode/text()[1]
let $y = do-update()
return concat($x, $y)
Is that whole thing an update statement? Is that a query statement,
except for the first statement of the do-update() function,
which is an
update statement? What exactly constitutes a "unit" of XQuery code?
Actually, I can test this myself using xdmp:request-timestamp()!
EP
Geert Josten wrote:
Hi Eric,
I am sure there are other ways to achieve what you want,
but if you really would like to do something like that, you
could always resort to xdmp:eval (most likely with
<isolation>different-transaction</isolation> within the options).
Be aware of the transaction mechanism if you start using
eval like that, it can easily put you on a wrong foot if you
haven't at least read the dev guide about it.. ;-)
Kind regards,
Geert
Drs. G.P.H. Josten
Consultant
http://www.daidalos.nl/
Daidalos BV
Source of Innovation
Hoekeindsehof 1-4
2665 JZ Bleiswijk
Tel.: +31 (0) 10 850 1200
Fax: +31 (0) 10 850 1199
http://www.daidalos.nl/
KvK 27164984
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From: [email protected]
[mailto:[email protected]] On Behalf Of
Eric Palmitesta
Sent: donderdag 19 februari 2009 18:50
To: ML Developer Mailing List
Subject: [MarkLogic Dev General] dynamic module import
I'm fairly sure this isn't possible, but I might as well ask.
Is there any way to import a particular module at runtime?
I know this won't work, but it shows the idea:
import module namespace blah = "http://blah"; at $filename;
It won't work because import statements must come before not
only functions (and code, in the case of a main module), but
before variable declarations as well.
Can someone confirm?
Thanks,
Eric
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