TAP Implementation Issues: Final Comment: TAP and UWS, sync and async

[Tom McGlynn]

Hi Guy,

I'm not sure there is a big area of disagreement here.  In terms of the 
text that users write and get back in the TAP asynchronous interface I'm 
not suggesting a that a single byte needs to be changed.  It's all in 
what tasks are doing the processing.  I've put in some text that I hope 
clarifies what I was saying below in context.

	Tom

Guy Rixon wrote:
> Tom,
> 
> whenever you use UWS in a service definition, you have to say what  
> parameters it takes when setting up a job and the work done by that  
> job. That's the "application of the UWS pattern" to use the terms from  
> the UWS standard.

I'm not sure I understand this.  While there is talk of JDL and such in 
the UWS standard, I don't see any requirements that show it actually 
being used in any way.  So while a given UWS implementation might 
restrict parameters being used, I don't see how that is done within the 
UWS protocol itself.

> 
> The UWS specification is supposed to be reusable between applications;  
> hence the U in the title. Therefore, it can't specify the application- 
> specific parameters.
>

Right.  I'm not suggesting that.  What I'm saying is that it's easy to 
write a UWS that can handle any parameters -- as indeed you suggest you 
have done already below.

> It's possible to specify a UWS-conforming service for more than one  
> application. CEA does this. The modern interface of this kind is  
> called UWS-PA ("UWS for parameterized applications") and its fore- 
> runner (which is SOAPy) is the Common Execution Connector. In these  
> kind of services, the applications are pluggable.
> 
Sounds like the kind of thing I was looking at.  I suggested in the 
original message that this has likely come up in earlier discussions.

> AstroGrid DSA/Catalogue has had a CEC interface for years. It uses a  
> generic CEC implementation and passes the requests through to an ADQL- 
> query application plugged inside it.
> 
> The downside of generalizing a job-control service in this way is  
> complication and divergence from the synchronous case. TAP/UWS is  
> quite like asynchronous TAP: you do an asynchronous query by POSTing  
> the same parameters you could use for a synchronous query. If you try  
> to use CEC or UWS-PA to start a TAP query then you have a different  
> interface. Because that interface is more general, it's not as simple,  
> either to implement in a service or to call from a client.
> 
Here's where I think I'm getting a little lost.  My suggestion is the 
that I have a UWS service running above TAP that is simply a proxy for 
the TAP synchronous service.  So by definition, they could not get 
disassociated.  I'm getting the sense that for you, the UWS service 
needs to know about the parameters it's going to pass along to whatever 
it calls when it does a run.  However, as far as I can see a UWS service 
can be entirely agnostic about parameters.  It can simply take whatever 
parameters the user specifies and pass it along, leaving it to the 
underlying synchronous call to handle validity.  In fact, for TAP that's 
pretty much the case since the names of the parameters used in TAP are 
not bounded.


> I think that the current boundary between TAP and UWS is just where we  
> need it for the simplest implementations.

I'm not so much concerned with boundaries as in the sense in which UWS 
is instantiated.  Let me give a concrete example.  I have a TAP service 
with a base URL of http://tap/, so http://tap/sync is the synchronous 
access point and http://tap/async is the async access point.

What happens when someone references the later URL?  In my current 
implementation, a TAP servlet starts up, notes that I'm using an 
asynchronous request and calls the appropriate methods and classes that 
TAP has defined for this.  If I had multiple asynchronous services these 
would likely be in a nice little UWS library.  All is copacetic: UWS is 
a layer within TAP.  It works fine but TAP and the UWS layer are pretty 
tightly coupled.

What I think I'm going to do when I get back from the IVOA is a bit 
different.  When I invoke http://tap/async I start a servlet whose only 
knowledge of TAP is that there is a synchronous service at 
http://tap/sync.  It knows nothing of the internals of TAP and is 
completely independent of it.  At some point the user does  a 
http://tap/async/id/phase?phase=run and this UWS service takes the 
parameters that the user has specified for this job and invokes the 
http://tap/sync URL with those parameters.  The results get saved 
somehow and whenever the user sends the appropriate URL the results are 
sent back.   The only thing the UWS service ever knows about TAP is the 
base URL.  Everything else is supplied by the user.

Why do I like this better?  Well it makes the TAP code simpler.  It 
makes it easy for me to provide UWS functionality to all of my web 
services.  E.g., I'd have a UWS interface to SkyView by simply changing 
the syncrhonous URL. And if UWS changes so that, e.g., there's now a 
security resource, I can plug it in without any change whatsoever to my 
TAP servlet.   For me it will be a big win.

I'm not suggesting that this implementation be required.  It would be 
fine to keep things coupled in one TAP implementation.  However if the 
paradigm (and here I mean it in its literal sense of exemplar) is a UWS 
service runs on top of a TAP service then the way to describe the 
relationship between TAP and UWS changes. In particular I think it then 
makes a lot more sense to simply say that a UWS service can be used to 
provide asynchronous access to a TAP service.  The standard can require 
that if we  decide async access is mandatory (as I think we have).  So 
the TAP document becomes simpler -- and far less tightly coupled with 
the UWS document.

> 
> Cheers,
> Guy
> 
> On 6 Nov 2009, at 15:33, Tom McGlynn wrote:
> 
>> I'm sure everyone will be happy to see the word 'Final' in the  
>> title...
>>
>> In the past couple of days I've gotten the UWS asynchronous  
>> implementation of TAP working (though doubtless still bug-ridden).
>>
>> When I read and implemented the TAP and UWS standard I had the sense  
>> of UWS as being a layer within TAP.  In retrospect I think it would  
>> have been better (for my implementation at least), if I had  
>> distinguished them more clearly.
>>
>> Suppose we think of UWS not as an interface layer but as the  
>> definition of how to build an asynchronous proxies.  UWS becomes a  
>> service definition, not an access protocol.  The proxy accepts and  
>> caches input parameters from the users, starts the underlying  
>> request when told to, caches the response and sends it back to the  
>> user when requested.  [I haven't followed the discussions of UWS  
>> earlier in the Grid list, so my apologies if I just discovering what  
>> everyone already knows....]
>>
>> If I think of things this way, then I can implement UWS completely  
>> independently of the underlying application. Indeed the binding to  
>> the underlying application could be dynamic: I can provide a UWS  
>> layer over any number of distinct synchronous applications.    I  
>> don't need to know anything about what parameters they use, just  
>> some root URL.  The one piece of the  specification that might cause  
>> problems is the desire to support multiple outputs as well as a  
>> single result.  That's not at issue in TAP, but even this could  
>> easily be handled by returning a list of the outputs -- which is  
>> what UWS does now anyway.
>>
>> UWS is not described this way in its standards document: it is shown  
>> as a layer within some bigger application, not as a separable  
>> entity. Similarly TAP shows the asynchronous interface tightly  
>> coupled within the rest of the TAP.
>>
>> In this new view, the TAP document would say very little about the  
>> asynchronous interface.  TAP itself would be synchronous, but if we  
>> want asynchronous access to be mandatory then the requirement is  
>> that a TAP implementation must specify a corresponding UWS service  
>> through which the TAP implementation can be invoked.  We could still  
>> have a TAP service that is only available asynchronously: we allow  
>> that this TAP service is not directly callable: Only the associated  
>> UWS service can access it.  I'm not trying to take sides here in the  
>> sync/async wars.
>>
>> Changes to the UWS document would be rather more subtle, noting that  
>> the interface can implemented without reference to the underlying  
>> implementation, and perhaps explicitly supporting the kind of  
>> dynamic association with the underlying synchronous service  
>> mentioned above. Maybe provide a convenience resource to get the  
>> output in the single output case (rather than having to parse the  
>> output list).
>>
>> The advantage had we taken this approach before, is that it largely  
>> decouples TAP and UWS.  The TAP standard is shorter and simpler.   
>> The UWS standard is largely unchanged.  We can change UWS in the  
>> future without worrying about any impact on TAP.
>>
>> This is probably a bridge too far in terms of the TAP standard.  For  
>> UWS it's really a change in tone more than content -- hints to the  
>> user -- so perhaps it is doable were it to be thought a good idea.   
>> Regardless, I do anticipate revising my own implementation to use  
>> this approach after the Interop.
>>
>> 	Tom McGlynn
> 
>