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

[Guy Rixon]

Tom,

your suggested implementation has massive, inherent problems: it loses  
most of the benefits of a asynchronous interface!

By depending on a synchronous HTTP endpoint to run the query, your  
implementation breaks any time that synchronous thing times out, and  
it breaks if the network connection with the synchronous thing drops.  
In those cases, your UWS has no way to regain control of the query,  
even if the DB part is still running it. The UWS has to resubmit the  
query.

The whole point of UWS is not  to depend on a synchronous HTTP- 
connection for long-running jobs.

You can build synchronous TAP as a wrapper around a UWS (Pat D. has  
done so) but it doesn't work the other way around.

Guy


On 6 Nov 2009, at 17:33, Tom McGlynn wrote:

> 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

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