TAP Implementation issues (cont'd): UWS

[Tom McGlynn]

By the by,  presumably this issue of whether we can count on getting an 
up to date version of a UWS resources accessed via a GET call applies 
not just to the job list but to any resource called by GET.  They are 
all dynamic.  The phase request in particular seems designed to be 
called repeatedly in a loop and might be especially liable to caching. 
So this needs to be resolved clearly.  The text that you quoted suggests 
that the proposed UWS standard is on pretty thin ice and that we are 
relying on the vagaries of current server implementations.  I was 
surprised that CGI like requests are specifically exempted from being 
cached so that they are not subject to this unless the user explicitly 
sets a future date for expiration.  Perhaps going back to a more CGI 
like syntax

    $rooturl/$jobid?phase
rather  than
    $rooturl/$jobid/phase

should be considered since that seems to get the HTTP standard a little 
closer our side.  Or warn users to specify $rooturl/$jobid/phase?junk if 
they wish to be careful about avoiding caching.

Though I'm not sure that's really enough since even for a CGI-like GET 
there's nothing that the current standard says that precludes a service 
from setting an expiration date in the future.  It seems that to do any 
GET requests properly the protocol needs to say something more proactive 
to ensure that caching is turned off.


	Regards,
	Tom


Tom McGlynn wrote:
> I can't really see the difference here between the create case (i.e., my
> use of GET to create a job) and the job list case.  In fact the text you
> quote seems to suggest that the job list is more likely to be cached
> than a job creation request with parameters.
> 
> In both cases we start out with some URL (in fact the same URL for both
> cases).  There are two differences in the processing at that HTTP sees:
> 
> The return code is different (303 for a creation versus 200 for a job
> list).  There is nothing that I know of in the HTTP protocol that
> suggests that a 200 is less likely to get a Expiration time header
> associated with it than a 303 (or more likely for that matter).
> 
> The more significant difference is that the job creation URL is going to
> have job creation parameters (well at least for me!).  That suggests --
> looking at the text you quoted below -- that it is less likely to have a
> problem with an expiration time being associated with it.  It's the job
> list that is more likely.
-- I got slightly confused here in discussing the consequences  The 
expiration time header is a side issue.  The question is whether the 
server (or some proxy in the path) is allowed to cache.  The words below 
suggest it is for a URL that does not include ? (e.g., a request to list 
the jobs or get the current phase) when either there is no expiration 
date or the expiration date is in the future.  For CGI like requests 
with parameters, caching is allowed only when there is an expiration 
date in the future.
> 
> Now you may be entirely correct that the servlet containers that you've
> used take care of this very nicely.  But it doesn't sound like it's
> required by the protocol.  So to ensure that users get an up to date job
> list I think the protocol needs  to specify that any job list specify an
> immediate expiration to disable caching - the very details that you were
> leery of below.
> 
>         Regards,
>         Tom
> 
> 
> Guy Rixon wrote:
>> FWIW, this is the passage in RFC 2616 that restricts the use of GET for
>> state-changing requests:
>>
>>
>>       13.9 Side Effects of GET and HEAD
>>
>> Unless the origin server explicitly prohibits the caching of their
>> responses, the application of GET and HEAD methods to any resources
>> SHOULD NOT have side effects that would lead to erroneous behavior if
>> these responses are taken from a cache. They MAY still have side
>> effects, but a cache is not required to consider such side effects in
>> its caching decisions. Caches are always expected to observe an origin
>> server's explicit restrictions on caching.
>>
>> We note one exception to this rule: since some applications have
>> traditionally used GETs and HEADs with query URLs (those containing a
>> "?" in the rel_path part) to perform operations with significant side
>> effects, caches MUST NOT treat responses to such URIs as fresh unless
>> the server provides an explicit expiration time. This specifically means
>> that responses from HTTP/1.0 servers for such URIs SHOULD NOT be taken
>> from a cache. See section 9.1.1
>> <http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1> for
>> related information.
>>
>>
>> Reading the details of this, we /could /have written UWS to start
>> queries using GET, but then we'd have to qualify it with "and then you
>> must do x, y and z with the response headers to turn off caching". This
>> is extra work for the implementor and quite unecessary because a POSTed
>> request is naturally uncached. Creating a sub-resource is exactly what
>> POST is for.
>>
>> The particular caching cock-up I had in mind works like this:
>>
>> 1. You GET the job list with query-starting parameters.
>> 2. The service starts a query and returns 303 "See other" redirecting to
>> the new job
>> 3. The web server tacks on some default cache-expiration because, hey,
>> it's a GET response and we're suppose to cache those.
>> 4. You GET the job list again with parameters for another query.
>> 5. The web-cache sends you back the 303 for the first query and the TAP
>> service never sees the request.
>>
>> Maybe this only catchs you out if the queries in steps 1 and 4 are the
>> same - i.e. same URL including query string. Not sure about the details
>> there...meaning that it's subtle and scary and dangerous to depend on
>> this kind of usage. On the other hand, using POST makes it 100% certain
>> that the cache won't swallow your request.
>>
>> In respect of job lists getting cached what sees to happen with Java is
>> this: when you GET a response from a servlet or JSP, everything works as
>> expected with dynamic content; you never seem to get a stale page.
>> However, when the same servlet engine delivers a static HTML page, or a
>> CSS stylesheet or (particularly) and XSLT for in-browser transformation
>> then it caches like hell and you can easily miss updates. (This bit me
>> repeatedly when I was setting up the browser-side styling of the job
>> resources for my TAP implementation.) Something in the servlet engine
>> seems to know when the content should be dynamic and adjusts the HTTP
>> headers appropriately.
>>
>> Cheers,
>> Guy
>>
>>
>> On 2 Nov 2009, at 16:56, Tom McGlynn wrote:
>>
>>> Guy Rixon wrote:
>>>> Tom,
>>>> if you want to get the job list then go ahead and do HTTP-GET. That's
>>>>  part of UWS (although implementations may restrict the set of you
>>>>  reported to be those owned by the caller). What you can't do is use
>>>>  HTTP-GET to submit a query via UWS. If you want to use GET to do a
>>>>  query then you're doing a synchronous query by definition.
>>>> Cheers,
>>>> Guy
>>> But I recall from earlier in this discussion some clever fellow said...
>>>
>>>> GET responses can be cached, and the caching is out of your control
>>>> as a service provider - it may be on the user's LAN (HTTP proxy) or
>>>> in their client (browser cache). If you send the same query twice
>>>> then via GET,
>>>> for the second request you could get the response for the first,
>>> pulled from the cache,
>>>> and no new job.
>>> If what you said earlier is correct, then I can't rely on what I get
>>> back from a GET call.  I might be getting a cached response and not
>>> the current state of the system.  If caching is truly an issue, then
>>> there doesn't seem to be any reliable way to get the job list.
>>>
>>> Tom
>>>
>>>> On 2 Nov 2009, at 14:13, Tom McGlynn wrote:
>>>>> Paul Harrison wrote:
>>>>>> Guy has already done a good job of answering most of these points - I
>>>>>> The UWS design of the two stage process is for two principal reasons
>>>>>> a) to be able to manipulate job metadata parameters before the job is
>>>>>> run - e.g. the DestructionTime - and receive the feedback from the
>>>>>> service whether it is prepared to honour such requests before  actually
>>>>>> committing the job.
>>>>>> b) to allow complete parameter namespace freedom on the job creation
>>>>>> step - i.e. if PHASE is used by UWS then it could not be a parameter
>>>>>> for the implementation protocol.
>>>>>> So if for a particular implementation using UWS there is no problem
>>>>>> with meeting that second condition, then there is no particular  reason
>>>>>> why job metadata parameters could not be included with the initial  UWS
>>>>>> job creation step if desired - this would require revision of the UWS
>>>>>> specification to include this possibility - I think that this is a
>>>>>> small enough change to be added into the document as part of the  RFC -
>>>>>> it does have a larger impact on possible service implementers however
>>>>>> - their code might not be structured to allow this change easily. For
>>>>>> a generalized UWS client the change would not be so great, all that
>>>>>> would happen is that after the initial submission a job object would
>>>>>> be returned with the PHASE=EXECUTING, and general clients should not
>>>>>> make any assumptions about state in their coding, so should probably
>>>>>> still be able to react appropriately.
>>>>>> Just as a side note to show that UWS is not so strange in this
>>>>>> multiple interaction between client and server - consider what  happens
>>>>>> when a web browser loads a web page - it does the initial get of the
>>>>>> html, then parses this html and then gets images, javascript etc.
>>>>>> before the page is shown to the user.
>>>>> I trust the goal is not to require that UWS services need to have
>>>>>  the complexity of an interactive visual Web browser.  The protocol
>>>>>  should cater to simple applications as well.
>>>>>
>>>>> I'd be perfectly happy with a change that made it clear that the
>>>>>  request that created the job could return it in any state.  In
>>>>> fact,  even without the desire to be able to start jobs at creation,
>>>>> that  is probably needed to accommodate the situation where there is
>>>>> a  problem detected in creating the job but we want the user to be
>>>>> able  to parse the error with IVOA protocol level error handling.
>>>>>
>>>>> The specific text that I find problematic is in 2.1.3:
>>>>>
>>>>> PENDING: the job is accepted by the service but not yet committed
>>>>>  for execution by the client.  In this state the job quote can be
>>>>>  read and evaluated.  This is the state into which a job enters when
>>>>>  it is first created.
>>>>>
>>>>> in conjunction with
>>>>>
>>>>> 2.2.3.1 Creating a job.
>>>>>
>>>>> Posting a request to the job list creates a new job (unless the
>>>>>  service rejects the request).
>>>>>
>>>>>
>>>>>
>>>>> I would suggest something like the following changes
>>>>> -
>>>>> In 2.1.3 add somewhere
>>>>>
>>>>>  Phases are ordered with PENDING before QUEUED, QUEUED before
>>>>>  EXECUTING and EXECUTING before the trio of COMPLETED, ABORTED and
>>>>>  ERROR.  The state of a job may change only to a later state but
>>>>> need  not pass through any intermediate state.  A job may be created
>>>>> in  any state.
>>>>> -
>>>>> Delete the last sentence in the definition of PENDING.
>>>>> -
>>>>>
>>>>>
>>>>> I think it would be desirable to suggest how a job could be created
>>>>>  in the run state even if this is not required by the standard  It
>>>>>  would be possible to do this without polluting the parameter name
>>>>>  space by specifying a new URI for that.  E.g., ${jobs} creates a
>>>>> new  request but does not start it.  ${jobs}/run could create and
>>>>> start  the job if that is permitted in the given service.  However I
>>>>> find  the worry about pollution of the parameter name space less
>>>>> than  compelling, since we require certain parameters to be used in
>>>>> calls  to start the job running or to alter other aspects of the
>>>>> job.  It  would be poor practice to have phase= mean one thing in a
>>>>> job  creation request and mean something else in a job update
>>>>> request.   In effect, these parameters are reserved already.
>>>>>
>>>>>
>>>>> This discussion brought up another thing that's not really clear.
>>>>>  2.2.3.1 has the little parenthetical phrase "(unless the service
>>>>>  rejects the request)" which is neither explained, nor is the action
>>>>>  implied in rejection specified.  I think something like:
>>>>>
>>>>>  Errors may occur in the creation of the job.  Where possible a job
>>>>>  should be created in the ERROR phase with a error message that
>>>>>  describes the problems.  If this is not possible, an HTTP 500 error
>>>>>  must be returned.
>>>>>
>>>>> would be clearer for implementation and should replace that phrase.
>>>>>
>>>>>
>>>>> Finally,  one new issue/concern.  [This probably reflects a lack of
>>>>>  understanding on my part but if so then perhaps it could be
>>>>>  clarified in the standard.]:  It doesn't seem like there is any
>>>>>  valid way to get the current job list.
>>>>>
>>>>> I can't do a GET request for  /{$jobs} because that's cacheable and
>>>>>  the list is dynamic.  And I can't do a POST request for it since a
>>>>>  post to /{$jobs} means that I'm creating a new job and I'm supposed
>>>>>  to be redirected to the job information for the newly created job.
>>>>>
>>>>> So how do I get to it?
>>>>>
>>>>> In my TAP implementation I assume that any request to create a new
>>>>>  job needs to have a REQUEST= parameter and if I don't see this I
>>>>>  return the job list.  If I do, I create the new job.  However this
>>>>>  doesn't seem to be literally correct.  I suppose you could say that
>>>>>  I'm 'rejecting' the request and since that behavior is undefined I
>>>>>  can do anything I want. Relying on undefined behavior doesn't seem
>>>>>  satisfactory.
>>>>>
>>>>> Tom
>>>>>
>>>>>
> 
>