[OSLC] resource: storage vs representation--specifying variations in the form of the return from a GET
Martin Nally
nally at us.ibm.com
Mon Jan 18 12:59:44 EST 2010
I agree that accept headers are often the right solution, but it's less
clear to me that it's the right way to handle the example that Andy brings
up. Ideally, accept headers would only be used to choose between
representations that are equivalent, like XML, JSON and N3. A quartile
approximation of a distribution is not equivalent to a percentile
approximation - if I use PUT to update the distribution through the
quartile approximation, I will not subsequently be able to GET the
percentile representation and retrieve a useful result. If I were doing
Andy's example, I would create an URL for the distribution, and a separate
URL for quartile and percentile approximations of it, rather than using
content negotiation. the URLs might be <distributionURL> and
<distributionURL>?samplingRate=4 and <distributionURL>?samplingRate=100 or
something,
By the reasoning above, you would conclude that we should not have used
content negotiation for the "compact" resource format used for fly-over
help (I forget the right name for these). If we were doing that over, I
would recommend using a different URL, not accept headers
Best regards, Martin
Martin Nally, IBM Fellow
CTO and VP, IBM Rational
tel: (949)544-4691
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|Geoffrey M Clemm/Lexington/IBM at IBMUS |
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|Andrew J Berner/Dallas/IBM at IBMUS |
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|community at open-services.net, community-bounces at open-services.net |
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|01/17/2010 07:54 PM |
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|Re: [OSLC] resource: storage vs representation--specifying variations in the form of the return from a GET |
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|community-bounces at open-services.net |
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With HTTP, one requests a variant of the representation with the ACCEPT
header.
If one cannot encode the variant information in the ACCEPT header, I
suppose one could define a URL parameter for it.
Cheers,
Geoff
From: Andrew J Berner/Dallas/IBM at IBMUS
To: community at open-services.net
Date: 01/17/2010 06:04 PM
Subjec [OSLC] resource: storage vs representation--specifying variations
t: in the form of the return from a GET
Sent community-bounces at open-services.net
by:
An issue that has come up in several of the workgroups I'm attending: The
storage of a resource may be very different from the XML representation of
the resource you GET from the URL. The spec defines the representation you
GET and PUT, not the underlying storage mechanism. The XML representation
specified in the OSLC spec may be derived by the server from the data
stored.
This raises a question about the spec: Have we been specifying ways of
GETting alternate representations of the same resource? I'm wondering if
there shouldn't be a somewhat standardized way to do this, although the
specifics will be resource dependent.
As I look over some of the specs, it seems we are defining a single XML
format for each resource. In some cases, that may make sense--the resource
IS the XML document. But in other cases, the resource is more abstract.
Even if we expect the stream returned by an OSLC GET call to always be an
XML document, there may be variations in the XML representation of a
particular resource that could be important to be specified by the request.
One example of this is will show up in the Estimation OSLC spec: the
representation of estimated duration (or other key metrics) of a project.
It's not a single number, but rather a probability distribution, so you can
say the probability of completing the project in that amount of time. It's
a formalized representation of something like this: "it's 50% likely we
can complete it in 24 months, but if you want to be 75% confident, plan for
35 months and it's barely possible it could take over 4 years. There's a
25% chance you can get lucky and finish in 16 months, but the 12 month
schedule you asked for is impossible! The least it will be is 14 months."
Although "probability distribution" is an abstract mathematical construct,
the XML representation that will be in the estimation OSLC spec is a
quantile representation, which will be "good enough" for consumers. It
will be an XML representation of some of the values of the probability
distribution, but it can be at various granularities. It will list the
values of the distribution for evenly spaced percentages as a sequence of
values in the XML document--the client can figure out which percentage each
represents based on the number of elements and the order of the elements.
Here's an example of the informal description above, based on a similar
example from the WIKI:
<ems:QuantileFunction
ems:metric="http://open-services.net/software-metrics/duration"
ems:unitOfMeasure="month" ems:numberOfQuantiles="4">
<ems:low>14</ems:low>
<ems:quantile>16</ems:quantile>
<ems:quantile>24</ems:quantile>
<ems:quantile>35</ems:quantile>
<ems:high>50</ems:high>
</ems:QuantileFunction>
Now, for some purposes, a client may want a representation of the
distribution at a fairly high granularity, like the one above, at the 25%,
50% and 75% probabilities, maybe to get a ballpark of how risky the
schedule will be. For other purposes, a client may want a representation
at a detailed granularity, getting each percentage probability, perhaps to
answer the question, "what's the probability I can finish by Dec 1?" and
get an answer like "68%". I'll spare you the similar but much longer XML
representation of the same resource (the probability distribution) but with
100 quantiles :-).
This is just an example; I suspect each workgroup has examples like this,
where there can be different levels of detail or other variations returned
by a GET, all formatted by the interface provider from the same abstract
resource. It's related, but not quite the same I think, as the standard
issue of "partial retrieves" in queries (which I've started to hear
described as the "shape of the returned set"). So there needs to be a way
for the consumer to negotiate with the provider, or, more simply, just tell
the provider how the consumer wants the resource represented, even if all
the representations are in XML.
Even though the "choices" of representation are resource dependent (e.g.
"number of quantiles" for a probability distribution), is there a general
technique we'll use in the various OSLC specs to specify requested
variations?
Andy Berner
Lead Architect, ISV Technical Enablement and Strategy
IBM Rational Business Development
972 561-6599
ajberner at us.ibm.com
Ready for IBM Rational software partner program -
http://www.ibm.com/isv/rational/readyfor.html
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