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Re: [thredds] ncWMS GetCapabilities return sometimes empty document

It seems unlikely that production-quality Tomcat code wouldnt deal correctly with a common situation of a client abort. But you could search the tomcat archives for any other reports of this problem. what version of tomcat is this on?
My best guess is that you are seeing some kind of thread-safety issue in our code. But apparently triggered or made more likely by client abort exceptions ??
what is this exception: "When the capabilites document with 0 byte size is fetched, the earlier mentioned exception is thrown." ? 

On 3/16/2010 9:05 AM, Jonathan Blower wrote:

Hi Heiko, Ethan,

I've been trying to figure out what might be going on here - thanks for
your great diagnosis Heiko.  I'm not sure what can be done in the
THREDDS/WMS code to prevent this, but I've modified some code in the
ncWMS WmsController class to attempt to catch exceptions that are thrown
when clients disconnect:

http://www.resc.rdg.ac.uk/trac/ncWMS/browser/branches/tds-refactor-2010-
01-09/src/java/uk/ac/rdg/resc/ncwms/controller/WmsController.java#L250

Heiko - you mentioned that it's possible that poor exception handling
could cause a tomcat thread to be left in an invalid state.  I can't
reproduce your problem reliably so I can't test this myself, but I
wonder if you could find out whether this is happening in your setup?
The VisualVM tool
(http://java.sun.com/javase/6/docs/technotes/guides/visualvm/) can give
a thread dump from your running Tomcat system - perhaps you could run
this when an error occurs and see if anything is suspicious?

Ethan - are you able to reproduce Heiko's problem?

Cheers, Jon


-----Original Message-----
From: Ethan Davis [mailto:edavis@xxxxxxxxxxxxxxxx]
Sent: 05 March 2010 23:32
To: Heiko Klein
Cc: Jonathan Blower; thredds@xxxxxxxxxxxxxxxx
Subject: Re: [thredds] ncWMS GetCapabilities return sometimes empty
document

Hi Heiko,

That is great. Thanks.

I'm a bit swamped through next week but I will jump on this the
following week before I get to some other WMS work.

Thanks,

Ethan

On 3/5/2010 4:44 AM, Heiko Klein wrote:

Hi,

finally, I managed to generate a simple case how to reproduce this

error:

a) start tomcat
b) abort fetching some maps (3-4 times on a tomcat with
10thredds/HTTP1.1 connector), i.e. call
GET


'http://localhost:8081/thredds/wms/data/met.no/hirlam12/wam_nsea.fc.2009
0604.nc?REQUEST=GetMap&LAYERS=significant_wave_height&PALETTE=redblue&SE
RVICE=WMS&FORMAT=image/png&VERSION=1.3.0&CRS=EPSG:4326&BBOX=-30,-60,30,9
0&WIDTH=400&HEIGHT=400&STYLES=BOXFILL/ncview'

/dev/null

and abort this call before it is finished. (Ctrl-C)
c) get some 'getCapabilities' documents:
while [ 1 ]; do GET


'http://localhost:8081/thredds/wms/data/met.no/hirlam12/wam_nsea.fc.2009
0604.nc?service=WMS&version=1.3.0&request=GetCapabilities'

testCap.xml; ls -l testCap.xml; done

...
-rw-r--r-- 1 heikok heikok 172126 2010-03-05 12:11 testCap.xml
-rw-r--r-- 1 heikok heikok 172126 2010-03-05 12:11 testCap.xml
-rw-r--r-- 1 heikok heikok 172126 2010-03-05 12:11 testCap.xml
-rw-r--r-- 1 heikok heikok 0 2010-03-05 12:11 testCap.xml
-rw-r--r-- 1 heikok heikok 172126 2010-03-05 12:11 testCap.xml
-rw-r--r-- 1 heikok heikok 172126 2010-03-05 12:11 testCap.xml


When the capabilites document with 0 byte size is fetched, the earlier
mentioned exception is thrown.


Since step b) is the crucial step to reproduce the 0 byte document, I
guess there is somewhere a badly caught

ClientAbortException/IOException

when writing to the responses OutputStream, leaving a tomcat-thread in
an invalid state.


We can solve this problem currently by adding a proxy in front of
thredds (apache mod_proxy) which seems to swallow all
client-connection-aborts before they reach tomcat.

Best regards,

Heiko

On 2010-02-19 16:54, Heiko Klein wrote:

Hi,

I'm still trying to track down this problem, so here a short update:

The ncWMS/TDS crashes seems to be connected to concurrency: The

problem

occurs when reloading the wms-client (openlayers in firefox), that

means

GetCapabilites and GetMap are run in parallel. We're downloading

tiles,

so reloading will fetch one capability doc + approx 12 maps.

* If I configure tomcat to only use one thread, and use the default
'HTTP/1.1' connector, the problem disappears (but response-times are
very slow).
* Using 4 threads + 'HTTP/1.1', the problem appears very fast. (one

or

two reloads)
* Using the 'Nio' connector and 4 threads, the problem appears, but

not

so fast (3-4 reloads)
* Using the 'Nio' connector with only one thread, is about the same

as

Nio with several threads.


Best regards,

Heiko

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