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Hi again,
OK, I can reproduce the problem reliably. It wasn't very reliable when I
was doing it by hand. Now I'm running a script that continually makes
GetCapabilities requests one after the other. While that script is
running I make GetMap requests and abort them with a ctrl-C. I also have
Tomcat configured to just 3 threads. (Thanks Heiko for figuring all that
out! Works like a charm.)
I'm catching the GetCapabilities request that is going to fail with a
conditional breakpoint ("response.isCommitted() == true") at the
beginning of the WMSController.handleRequestInternal() method. As we
might expect, it looks like the response is committed from the time the
org.apache.coyote.http11.Http11Processor creates the request and
response objects.
I haven't figured out how to reliably catch the problem during the abort
of the GetMap request. Seems like a Tomcat issue rather than a TDS/WMS
issue. I would think Tomcat should handle things even if we are dealing
with an exception poorly.
I'll dig around a bit more tomorrow. See if I can figure out why the
response doesn't get reset properly.
Ethan
On 3/17/2010 11:03 AM, Ethan Davis wrote:
> Hi Jon, Heiko,
>
> Jon, forgot to mention, we have code in the TDS WMS controller for
> catching SocketException-s and ClientAbortException/IOException-s
> similar to the change you reference.
>
> Have either of you seen this with ncWMS? Or is it so far just showing up
> in TDS?
>
> Most of the references to this exception that I've found deal with
> exceptions interrupting JSP processing and causing a switch to a JSP
> error page that then can't create a session object. Since the problem
> happens in one of our main JSP pages (capabilities_xml.jsp) and the
> ClientAbortException-s are happening in a different thread, I'm leaning
> towards the "thread in an invalid state" idea as well.
>
> One thing we could try is to set our JSP pages to not create session
> objects: "<%@ page session='false' ...%>". But since we don't know
> if/how the threads might be invalid, I'm afraid that would just make
> this problem even harder to see.
>
> I'm going to try for a bit to get things setup so I can debug into
> Tomcat code and see if I can catch this in action.
>
> Ethan
>
> On 3/17/2010 1:24 AM, Ethan Davis wrote:
>> Hi Jon, Heiko,
>>
>> I'm having a hard time reproducing the problem reliably. I've only seen
>> it twice so far. And haven't been able to catch it in action. I did get
>> the IllegalStateException from the Jasper JspFactoryImpl both times:
>>
>> "java.lang.IllegalStateException: Cannot create a session
>> after the response has been committed"
>>
>> Googling on the above exception message brings up a fair number of hits
>> but none of them seem totally relevant. Or when they sound somewhat
>> relevant, there isn't a solution.
>>
>> More tomorrow.
>>
>> Ethan
>>
>> 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
>>>>>
>>>>> _______________________________________________
>>>>> thredds mailing list
>>>>> thredds@xxxxxxxxxxxxxxxx
>>>>> For list information or to unsubscribe, visit:
>>> http://www.unidata.ucar.edu/mailing_lists/
>>>>
>>
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