On (C) I definitely concur. I am not against simplicity and HTTP GET requests.
I just want to make sure that the approach is discussed and that one doesn't
fall into the trap of believing HTTP GET is a panacea of simplicity. These
URLs that have been posted are pretty complex and aren't the kinds of things
that anyone but expert users will be crafting by hand. There will be a client
implementation in front of them and they will need to be updated if the server
processing API behind them changes. In this case, the client implementation
will have to change in tandem with the server side processing API. This will be
true regardless of whether the request is GET, POST, PUT, etc. One benefit of
GET is an embeddable link, to my knowledge this isn't easily done with POST or
PUT.
Our group uses WPS. We had issues with some holes with some implementation and
the specification so we made a choice to join on to the WPS 2.0 SWG.
There are advantages to the WPS specification. Implementations can list a set
of supported operations and processes using the GetCapabilities request (a GET
or POST, we use GET). Each process can be queried for it's API including
supported inputs and outputs (name, mime-type and schema if xml) using a
DescribeProcess request (GET or POST, we use GET). If you know the arguments
and types you can parse the DescribeProcess response and automatically generate
a UI. We have implemented this in JavaScript for our Web-based brokering
services. There are python clients as well as an Arc plugin in-progress
(completed?) by ERSI and 52n, also a qGIS plugin among others. Processes can
be executed with an Execute request (a GET or POST request, we use POST). POST
for us because we deal with some pretty complex inputs (WFS calls with server
side geometry filtering by reference to a GET or POST request; or Base64
encoded shapefiles sent in-line). These would bump us into some URL length
restrictions we have dealt with in the past. We don't have to use these
complex inputs but since WPS offers this flexibility we are happy to leverage
it. When we execute processes we have the options to execute them
synchronously or asynchronously (and an implementation can control these
options by advertising them per process.) We can query the executing process
for it's completion state (POST, don't know if GET is possible as I haven't
looked into it). We can request executions results in-line with the response
or by reference. We provide inputs to WPS calls as the results of other WPS
calls. WPS processing implementations can be complex or simple. Given our use
cases we made an architectural decision to leverage some of the more advanced
components of the specification. We've developed some complex processing that
does some cool and useful things that we are able to leverage in other projects
and share with other groups. With our processing endpoints we can add a
process and have it automatically be displayed in our UIs. One of the benefits
of WPS was processing end-points became self-documenting.
Now, the WPS execute by GET is pretty tricky as it requires so double URL
encoding. We are happy using POST and didn't delve too much into GET. If there
was a need and someone wanted to look at this with me (ahem, Roy?) I would be
more that happy to submit some change requests to simplify the specification
for some use cases. In my experience with the OGC standards almost everything
can be done with GET, it's when you get into the outlying use cases you have to
represent your requests with POST.
WPS is an OGC specification. I think the last 2 words of the previous sentence
instantly turn people off. But there's some real value to the work that's been
done. We've used it as a thin wrapper on process execution. Our initial cut
at processing involved using simple GET-based services. We found we had to
generate a whole suite of utility/supporting GET-based services relying on
clients to perform operations with correct ordering. The architecture was
becoming difficult to maintain and document. A large number of tasks have now
been implemented with the OGC standards suite and available standards
implementations. This has saved our group a lot of development time and in
turn taxpayer dollars.
Tom Kunicki
Center for Integrated Data Analytics
U.S. Geological Survey
8505 Research Way
Middleton, WI 53562
On Jul 1, 2012, at 11:34 PM, Gerry Creager wrote:
> Roy,
>
> That's a good explanation, and one I can live with. However, I also agree
> with Jeff's later comments, that A) in general, the same interpreter can
> handle GET and POST, and B) file uploads can't happen with a GET.
>
> And, most important: C) KISS is a good mantra.
>
> I'll sit back and listen to the debate again.
>
> gerry
>
> On Sun, Jul 1, 2012 at 3:13 PM, Roy Mendelssohn <roy.mendelssohn@xxxxxxxx>
> wrote:
> BTW - a discussion we have been having around these parts is can you do
> enough in the way of server-side functions without a POST (ie the URL
> defines the function). That is why I would like to hear more from people who
> are running F-TDS and GDS - how many requests do they get for server side
> functions, but is the usual response time and download for these request, how
> large are the usual expressions? And then contrast it with a WPS or WCPS
> approach. I clearly believe in one approach, but I would welcome people
> who are using some of these other approaches to describe what they have done,
> the benefits of doing things that way, and what it means for a client.
>
> Thanks,
>
> -Roy
>
> On Jul 1, 2012, at 11:25 AM, Dennis Heimbigner wrote:
>
> > Roy-
> >
> > > ... One comment. I think you misunderstood my point about
> > > Matlab and R. I am not interested in Matlab specific
> > > implementations. The point was because the URL completely
> > > defines the request, I can implement scripts in any application
> > > that can send an URL and receive a file in terms of functions
> > > built-in to that application - that is my clients do not break as
> > > the application or operating system change.
> >
> > Not quite sure I understand. This phrase "...receive a file in
> > terms of functions built-in to that application" sounds
> > like you are creating an association between functions defined
> > on the client side and functions defined on the server side.
> > Can you elaborate?
> >
> > > Why I strongly prefer, if it is at all reasonable, services that
> > > only use GET, not POST.
> >
> > Again, that is only possible if you keep your requests
> > short enough to not violate the URL length restrictions.
> >
> > =Dennis Heimbigner
> > Unidata
> >
> >
> >
> > Roy Mendelssohn wrote:
> >> Hi Dennis:
> >> Thanks. One comment. I think you misunderstood my point about Matlab and
> >> R. I am not interested in Matlab specific implementations. The point was
> >> because the URL completely defines the request, I can implement scripts in
> >> any application that can send an URL and receive a file in terms of
> >> functions built-in to that application - that is my clients do not break
> >> as the application or operating system change.
> >> While I understand why this occurred, a few years ago we had straight
> >> OPeNDAP implementations. We had a lot of users using scripts we developed
> >> for Matlab, running under Windows. Due to updates in both Windows and
> >> Matlab, the OPeNDAP files for Windows stopped working (at least for
> >> Matlab). We had a lot of users that were left stranded and stranded for
> >> quite a long time. Developing and maintaining clients, particularly
> >> clients that are working within an application for which you have to write
> >> code, very quickly becomes a non-trivial exercise.
> >> Since we switched to a service where the URL completely defines the
> >> request, our Matlab and R scripts have survived quite nicely any number of
> >> updates both to the applications themselves and to the operating systems.
> >> That is because the clients now only use functions built into the
> >> applications.
> >> Why I strongly prefer, if it is at all reasonable, services that only use
> >> GET, not POST.
> >> -Roy
> >> On Jun 28, 2012, at 1:03 PM, Dennis Heimbigner wrote:
> >>>> I am old and slow, but suppose I am in OpeNDAP, are you proposing
> >>>> to separate say constraint expressions and server-side function
> >>>> requests basically the same (ie I just scan what is after each
> >>>> comma) or do you propose some method that signifies in the URL
> >>>> that what follows is an expression? In F-TDS and GDS the form of
> >>>> the URL is:
> >>> First, I am proposing to subsume DAP constraints.
> >>> Second, I am proposing, like DAP, to put the expressions
> >>> in the query part of the URL (i.e. after the '?').
> >>>
> >>>> http://machine:port/thredds/dodsC/dataset_expr_{dataset2,dataset3,...}{expression1;expression2;...}.URLsuffix?constraint
> >>> So, I would rewrite this as something more-or-less like this:
> >>> http://machine.../dataset?expression1,expression2,...
> >>> Where the expressions would include the references to dataset2, dataset3,
> >>> and the constraint.
> >>>
> >>>> BTW, the reason I have asked about the experience of people who
> >>>> are using F-TDS and GDS on whether synchronous requests can cover
> >>>> the large majority of cases, is because I am very partial to
> >>>> systems where the URL completely defines the request, and hence
> >>>> essentially use GET as the verb.
> >>> The synchronous/asynchronous issue is, for me, a separable issue.
> >>> I should note that GET has a limit on the size of URLS, so
> >>> there needs to be ways to deal with that. Two possibilities are
> >>> 1) use POST or PUT, or 2) provide a way to upload a long expression
> >>> in parts USING multiple GETs.
> >>>
> >>>> The reason for this is long
> >>>> experience. where client code has broken with changes in
> >>>> operating system and/or application, fixes were slow in coming,
> >>>> so many users were left with nothing working. In a system where
> >>>> the URL completely defined the request, say ERDDAP, in Matlab:
> >>>>
> >>>>>> link='http://coastwatch.pfeg.noaa.gov/erddap/griddap/erdBAsstamday.mat?sst[(2010-01-16T12:00:00Z):1:(2010-01-16T12:00:00Z)][(0.0):1:(0.0)][(30):1:(50.0)][(220):1:(240.0)]';
> >>>>>> F=urlwrite(link,'cwatch.mat');
> >>>>>>
> >>>> Will get the related file, and the entire command is in Matlab,
> >>>> no extra code required. The same in R is:
> >>>>
> >>>>>> download.file(url="http://coastwatch.pfeg.noaa.gov/erddap/griddap/erdBAsstamday.nc?sst[(2010-01-16T12:00:00Z):1:(2010-01-16T12:00:00Z)][(0.0):1:(0.0)][(30):1:(50.0)][(220):1:(240.0)]",
> >>>>>> destfile="AGssta.nc",mode='wb')
> >>>>>>
> >>>> again, "download.file" is an R command.
> >>> I think that we do not want to be R/MATLAB specific
> >>> in a proposal to put stuff in URLs. I would rather
> >>> propose to allow uploading of R/MATLAB scripts to serve
> >>> as additional, user-defined functions.
> >>>
> >>> I would prefer to
> >>>> maintain this simplicity and cover 80% of the cases if possible,
> >>>> than cover the rest but where more complex, application specific
> >>>> code would have to be developed and maintained.
> >>> Agreed. However my assumption is the the output of any function that
> >>> is not assigned to a single-assignment variable will be returned as part
> >>> of the response; but other ways of specifying this are possible within
> >>> the functional framework I am proposing.
> >>>
> >>> =Dennis Heimbigner
> >>> Unidata
> >> **********************
> >> "The contents of this message do not reflect any position of the U.S.
> >> Government or NOAA."
> >> **********************
> >> Roy Mendelssohn
> >> Supervisory Operations Research Analyst
> >> NOAA/NMFS
> >> Environmental Research Division
> >> Southwest Fisheries Science Center
> >> 1352 Lighthouse Avenue
> >> Pacific Grove, CA 93950-2097
> >> e-mail: Roy.Mendelssohn@xxxxxxxx (Note new e-mail address)
> >> voice: (831)-648-9029
> >> fax: (831)-648-8440
> >> www: http://www.pfeg.noaa.gov/
> >> "Old age and treachery will overcome youth and skill."
> >> "From those who have been given much, much will be expected" "the arc of
> >> the moral universe is long, but it bends toward justice" -MLK Jr.
>
> **********************
> "The contents of this message do not reflect any position of the U.S.
> Government or NOAA."
> **********************
> Roy Mendelssohn
> Supervisory Operations Research Analyst
> NOAA/NMFS
> Environmental Research Division
> Southwest Fisheries Science Center
> 1352 Lighthouse Avenue
> Pacific Grove, CA 93950-2097
>
> e-mail: Roy.Mendelssohn@xxxxxxxx (Note new e-mail address)
> voice: (831)-648-9029
> fax: (831)-648-8440
> www: http://www.pfeg.noaa.gov/
>
> "Old age and treachery will overcome youth and skill."
> "From those who have been given much, much will be expected"
> "the arc of the moral universe is long, but it bends toward justice" -MLK Jr.
>
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