[Paraview] Visualizing a Cone with PVW - Connection can't be established

Mon Jul 21 11:02:15 EDT 2014

Hi Dmitry,

First of all you need to understand the role of apache here in order to see
how the configuration should be.
As you may know, you can use ParaViewWeb without Apache, but such setup is
not meant for production.

The list below is what you get when you add Apache:
- Apache can be use to serve the static content (html, js, png, css...)
- Apache provide a single entry point host + port. This prevent
cross-domain issue or firewall limitation.
- Apache will relay the WebSocket communication to the proper ParaViewWeb
instance on the backend based on a session ID. This mean that the
ParaViewWeb process DOES NOT need to be on the same machine as the Apache
Web Server.

Then you need to understand how the launcher works and its role.
In ParaViewWeb a launcher is used to start a new visualization process for
a given user. This simply mean running a command line with some dynamic
parameters like the port number on which this specific instance should
listen.
So when a user connect to the system and request a new visualization
session, it is the responsibility of the launcher to start the process and
let the client know on which WebSocket URL it should connect to in order to
reach the requested visualization session.

In a setup that DOES NOT contain Apache, we can not use the same port as
the main server (the process would not start). Hence, we start it with some
port and we let the user know that it should connect to
"ws://{host}:{port}/ws" where host and port get replaced with the set that
was used to create the process.

See section "resources" in launcher configuration.

BUT with Apache that is different, now we have a process which is meant to
relay the communication to the backend using the port and host that Apache
is using. So the launcher should let the client know what URL they should
use so Apache could handle it and forward it to the backend. Hence the
ws://{apache_host}/proxy?sessionId=${id}.
In the same time to keep track on which host/port a process is running the
launcher generate a text file with the proper mapping.
That mapping file is then used inside Apache to figure out how it should
establish the connection to the backend.

This should help you understand how to fix your configuration.
Moreover, here is the remaining set of questions:

> can we specifically log and debug Rewrite rule actions?

Don't know, Google will be your friend here...

> do we actually need ProxyPassReverse in Apache?

Don't know, Google will be your friend here as well as the documentation on
how to setup ParaViewWeb on Ubuntu 14LTS.

> what particular modules in Apache are required?

Read the documentation, the list is minimum and provided in it.

> in launch.json there are two particular lines host/resources:

So the "host" is to select the proper network card when you have several
for the actual launcher server.

Then the resources are use for process management and URL generation for
the client. Which mean, what name should be used by Apache to connect to
the machine that actually run a ParaViewWeb process.

Seb

On Mon, Jul 21, 2014 at 8:01 AM, Dmitry Duplyakin <duplyakin at uchicago.edu>
wrote:

> Thank you very much, Sebastien!
>
> Launcher side:
>>
>> "sessionURL": "ws://YOUR_HOST_NAME_TO_REPLACE(the one used to connect to apache)/proxy?sessionId=${id}",
>>
>> "proxy_file": "/data/proxy.txt"
>>
>> This is exactly what we have in our launcher:
>
>   "sessionURL": "ws://${host}:${port}/ws",
>
>   "host": "localhost",
>
>   "port": 8080,
>
> Your sessionURL looks different. Do you actually use "proxy?sessionId="
> there? Ours came directly from the script you've suggested earlier:
> http://www.paraview.org/gitweb?p=ParaViewSuperbuild.git;a=blob_plain;f=Scripts/pvw-setup.py;hb=HEAD
>
>
>> Apache side
>>
>> RewriteMap  session-to-port  txt:/data/proxy.txt
>>
>> In Apache we have:
> RewriteEngine On
> RewriteMap session-to-port txt:/pv/conf/proxy.conf
> RewriteCond %{QUERY_STRING}     ^sessionId=(.*)$ [NC]
> RewriteRule    ^/proxy.*$  ws://${session-to-port:%1}/ws  [P]
>
>
>> Then when a session start you can check the content of that
>> /data/proxy.txt and that should one line by running session with
>> {sessionId} localhost:yyyyy
>>
>
> Verified that those entires are added to the file and have the right
> format.
>
>
>> On the other hand you forgot that line in the config of your cone app:
>> "ready_line" : "Starting factory"
>>
>> Thanks! Added it and now the app looks like this:
>
> "cone": {"cmd": ["${python_exec}",
> "/pv/src/VTK/Web/Applications/Cone/server/vtk_web_cone.py", "--port",
> "${port}"], "ready_line" : "Starting factory"}},
>
>
>> You can check is any pvpython process is running, on which port, see if
>> that match in the proxy file (/data/proxy.txt) and also looking at the log
>> directory with the output of the pvpython process.
>>
>> A common error could also be that the apache does not have the right to
>> read the /data/proxy.txt file or that the launcher is not able to write it.
>>
>
> Verified that launcher can write it and that entries are added.
>
> These are the rest of our questions (we need to sort these out because
> they have caused some confusion and we want to clear things up and do not
> come back to these issues anymore):
>
> - can we specifically log and debug Rewrite rule actions?
> - do we actually need ProxyPassReverse in Apache?
> - what particular modules in Apache are required? So far we have:
> proxy_module, proxy_http_module, and rewrite_module. We are unsure whether
> we need proxy_wstunnel_module or not.
> - in launch.json there are two particular lines:
> "host": "localhost",
> "resources": [{"port_range": [9901, 9999], "host": "localhost"}],
> where we have some confusion about using "localhost" vs. "0.0.0.0". What
> exactly should we use in either case?
>
> Appreciate your help!
>
>
>> Good luck,
>>
>> Seb
>>
>>
>> On Sun, Jul 20, 2014 at 10:12 AM, Dmitry Duplyakin <
>> duplyakin at uchicago.edu> wrote:
>>
>>> Hi all,
>>>
>>> My name is Dmitry Duplyakin. I am a PhD student helping to build a
>>> ParaViewWeb system at University of Chicago.
>>>
>>> We are building ParaView 4.1.0 (with OSMesa) on Amazon EC2 Ubuntu 12.04
>>> instances.
>>>
>>> We have verified that PV builds without errors and that PVW launcher is
>>> functioning (new entries are added to the proxy file, vis sessions are
>>> created and are listening to ports in the specified range, etc.)
>>>
>>> We are trying to visualize a simple cone:
>>> (added app in launch.json)
>>> "cone": {"cmd": ["${python_exec}",
>>> "/pv/src/VTK/Web/Applications/Cone/server/vtk_web_cone.py", "--port",
>>> "${port}"]}},
>>>
>>> We have verified that vtk_web_cone.py actually gets called. In order to
>>> test it, we used /pv/www/apps/TestApp/index.html as a template and created
>>> /pv/www/apps/Cone/index.html where we have:
>>> ...
>>>   var config = {
>>>     sessionManagerURL: vtkWeb.properties.sessionManagerURL,
>>>     application: "cone"
>>>   };
>>> ...
>>>
>>> Now when we access this app via a browser at: http://<hostname>/apps/Cone/
>>> we either get a timeout or "connection can't be established" message.
>>>
>>> In the apache error log we see:
>>> [Sun Jul 20 16:04:15 2014] [error] [client 128.135.188.231] proxy: Error
>>> reading from remote server returned by /paraview/, referer:
>>> http://54.211.22.94/apps/Cone/
>>> [Sun Jul 20 16:04:15 2014] [error] [client 128.135.188.231] File does
>>> not exist: /pv/www/ws
>>>
>>> It appears that it has do to with Apache configuration, where we have:
>>> ProxyPass        /paraview http://localhost:8080/paraview
>>> ProxyPassReverse /paraview http://localhost:8080/paraview
>>> RewriteEngine On
>>> RewriteMap session-to-port txt:/pv/conf/proxy.conf
>>> RewriteCond %{QUERY_STRING}     ^sessionId=(.*)$ [NC]
>>> RewriteRule    ^/proxy.*$  ws://${session-to-port:%1}/ws  [P]
>>>
>>> Do these Rewrite commands look right? How can their actions be logged
>>> and verified?
>>>
>>> Do we actually need ProxyPassReverse here?
>>>
>>> Is it possible that we are missing a module or some additional
>>> configuration in Apache?
>>> # apachectl -t -D DUMP_MODULES
>>> ...
>>>  proxy_module (shared)
>>>  proxy_http_module (shared)
>>>  proxy_wstunnel_module (shared)
>>>  rewrite_module (shared)
>>> ...
>>> Syntax OK
>>>
>>> Any debugging advice?
>>>
>>> Your help will be much appreciated and will allow to move forward with
>>> our isntallation.
>>>
>>>
>>> -------------------------------------------------------------------------------
>>>
>>> Dmitry Duplyakin
>>> PhD student, CS at University of Colorado - Boulder
>>>
>>>
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>>>
>>
>
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