Hi,
I've been following the conversation... A couple of comments in
general, then to a couple specific to this message.
Years ago at the OPeNDAP developers meeting I made a plea for the
community to help define a syntax for server-side functions. We formed
a working group and had essentially this entire conversation
(application specific syntax vs a functional language, GET vs POST,
synchronous vs asynchronous) and so on. We even wrote the conversation
down in the Wiki
(http://docs.opendap.org/index.php/Server-side_Functions). The date on
the document is 2007. In the end, we couldn't agree on the right
approach, got tired and stopped working on the problem. I don't know
what lesson is to be learned from that experience except that I'm
probably not the right person to lead the effort to form a consensus on
the right approach.
Our product F-TDS will always allow transformations to be defined using
Ferret syntax. However, if there is a consensus on a functional
language, I would be thrilled to implement it for F-TDS.
As for Ben's comments on forming the URL, the idea when we build F-TDS
was that an ordinary Ferret user would be able to key in simple
transformations in their desktop clients. Instead of opening a data
set, they could open the data set with a new variable defined that was a
transformation of existing variables. However, the reality of stuffing
the Ferret syntax into the URL is ugly and complicated. The Ferret
scripting language wasn't designed for transmission on the URL so all
kinds of syntax that is significant to Ferret is also significant to
HTTP clients. Therefore things have to be very carefully encoded and
even then we had to make some extensions to the Ferret syntax to make it
work. So a functional language that was part of the DAP spec and URL
safe would be a big win. Some folks in our group think it's still
viable for folks to type Ferret syntax into URLs, but I don't.
However, our web application (LAS) uses the F-TDS syntax to do all of
the transformation users request from the Web UI (average, sum, min, max
for now) and to automatically request that a variable be re-gridded when
there is a request to compute the difference between two variables. All
the fussy preparation of the URL is handled by software and this has
been a big win for us. LAS is faster and more capable because of this.
To make sure this works universally, LAS will "wrap" a remote data
source in a local F-TDS URL so it can make the same transformation
requests of remote data albeit without the significant performance win
of doing the transformation local to the data. So, if we develop a
common functional language we would jump on that straight away -- both
to implement the functions it defines in F-TDS and to allow LAS to make
requests from remote data with the language.
Roland
On Mon, Jul 2, 2012 at 8:17 AM, Ben Domenico <bendomenico@xxxxxxxxx
<mailto:bendomenico@xxxxxxxxx>> wrote:
Hi all,
Just a quick note to emphasize a "use case" that I am especially
interested in. That is the case where an end user wants to invoke a
server side process from within an html document. Being able to
specify the process in a URL makes this possible.
On the other hand, having the user construct the URL by hand is not
practical. Roy's approach allows the user to set up the process
using a browser-based client to set up the process interactively and
them offers the resulting URL for the user to embed in a document.
From the user viewpoint, this combination is very powerful, but
I'm not sure how much it limits the complexity of the process that
can be specified.
-- Ben
On Sun, Jul 1, 2012 at 11:55 PM, Tom Kunicki <tkunicki@xxxxxxxx
<mailto:tkunicki@xxxxxxxx>> wrote:
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 len
gth 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 a
dd 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 <mailto: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
<mailto:Roy.Mendelssohn@xxxxxxxx> (Note new e-mail address)
> >> voice: (831)-648-9029 <tel:%28831%29-648-9029>
> >> fax: (831)-648-8440 <tel:%28831%29-648-8440>
> >> 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
<mailto:Roy.Mendelssohn@xxxxxxxx> (Note new e-mail address)
> voice: (831)-648-9029 <tel:%28831%29-648-9029>
> fax: (831)-648-8440 <tel:%28831%29-648-8440>
> 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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