[Pywps-dev] A process with JSON inputs and outputs?

[Jáchym Čepický]

Hi Derek,

another long-time-waiting mail in my stack - I'm very sorry for letting 
you wait so long and even it might be too late, I try to answer hoping, 
it will be still valid:

Dne 10.1.2017 v 13:32 Derek Hohls napsal(a):
> Hi all
>
> I am just starting out with pyWPS, and am struggling to get a basic Process working, that takes JSON data as input and produces JSON data as output.
>
> Any feedback on this from the community would be appreciated!
>
> Thanks
> Derek
>
> ********
> PART ONE: My Example
>
> I have posted an example of what I am trying to achieve on github:
>
> https://gist.github.com/gamesbook/3c3ff83f9348a01fff3750f53346db08
>
> I first tried to call the Process with this:
>
> wget --content-on-error -O - 'http://localhost:5000/wps?service=WPS&request=Execute&version=1.0.0&identifier=say_goodbye&datainputs=person={"firstname": "Derek","surname":"Richards","title": "Mr"}'
>
> I get this response:
>
> !DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
> <title>500 Internal Server Error</title>
> <h1>Internal Server Error</h1>
> <p>The server encountered an internal error and was unable to complete your request.  Either the server is overloaded or there is an error in the application.</p>
>
> The server log from the demo server shows this error trace:

[...]

>   File "/usr/local/lib/python2.7/dist-packages/pywps-4.0.0-py2.7.egg/pywps/app/Service.py", line 452, in create_complex_inputs
>     frmt = data_input.supported_formats[0]
> TypeError: 'NoneType' object has no attribute '__getitem__'

In your process, the input has default_format=[] -> change it to 
supported_formats

>
> As there is no reference point in the above trace to any line in the code I wrote, its not obvious what is triggering this error?
>
> I then created a simple JSON file (`test.json`):
>
> {
>     "firstname": "Derek",
>     "surname":"Richards",
>     "title": "Mr"
> }
>
> When I try and call the WPS with this command:
>
> curl -H "Content-Type: application/json" -X POST -F 'datainputs=person=@test.json'  http://localhost:5000/wps?service=WPS&request=Execute&version=1.0.0&identifier=say_goodbye
>
> I get this response:
>
>
> <ows:ExceptionReport xmlns:ows="http://www.opengis.net/ows/1.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.opengis.net/ows/1.1 http://schemas.opengis.net/ows/1.1.0/owsExceptionReport.xsd" version="1.0.0">
>   <ows:Exception exceptionCode="NoApplicableCode" locator="" >
>       <ows:ExceptionText>Start tag expected, '<' not found, line 1, column 1</ows:ExceptionText>
>   </ows:Exception>
> </ows:ExceptionReport>

WPS assumes, it must be XML input (see some examples in 
pywps-demo/static/requests/


>
> I assume the POST syntax is wrong, but cannot see what to change?
>
> ********
> PART TWO: General Comments
>
> I have struggled a bit with the documentation, as it seems to go to a certain point and then be incomplete.  For example, the page at https://github.com/geopython/pywps-demo/blob/master/docs/flask.rst ends by saying:
>
> "NB it is recommended to use HTTP POST requests for invoking WPS Execute operations as normally DataInputs will be more complex."
>
> But then it does not give any example of such an HTTP POST request for a Process with a ComplexInput. It would really help to have examples that work via:
>  * command line
>  * JavaScript
>  * Python

Well, examples of the request are stored in

pywps-demo/static/requests

and

PyWPS/tests/requests/

You might be right in this point, ti is assumed, that you are familiar 
with the OGC WPS spec (http://opengeospatial.org/standards/wps), 
therefore the documentation is a bit spare at this point.

>
> Similarly, there is a fairly detailed example of creating a Process in http://pywps.readthedocs.io/en/latest/process.html , but it also stops short of showing how this process is actually invoked.

Hm, you might be right at this point too - the documentation continues 
in pywps-demo project, but it would be good to have it linked there at least

>
> The test code in https://github.com/geopython/pywps-demo/blob/master/tests/test_execute.py runs tests for some of the demo processes; but not all of them; it would be helpful to see how the more complex ones are invoked.

As I already said, more complex processes should use POST request 
encoded to as XML files - you might have link to OGC WPS and request 
examples already.

>
> The other issue I have is how to debug a new Process; the XML responses from the Server seem quite minimal and I cannot find any guidelines for interpreting them.  Is there a way to ensure that a new Process is constructed properly; are there obvious mistakes and pitfalls to avoid; etc.?


The server response is not very verbose, that is true, we try to put as 
much as possible into server logs.

I'm not aware of many pitfalls or mistakes, if you follow the docs a 
bit, it should be working (but as you can see on your github, some bugs 
are still there)

Sorry one more time for such late response

your contribution to documentation would be appreciated, if you go any 
further

Jachym

-- 
Jachym Cepicky
e-mail: jachym.cepicky at gmail.com
twitter: @jachymc