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What does context-by-value mean ( was RE: [ws-caf] context block or one element per type)
Title: Message
So
understood. It was only that your preparedness to entertain the alternative
seemed to indicate different implications on the use of Context in general
to what I had assumed was meant. (have I ever written a more tangled sentence
?)
My
email wasn't really concerened with the issue on format - so I've changed
the subject now, which I probably should have done before. Of course, if we were
to abolish context-by-value as a soap header, the question
evaporates.
Peter
-----Original Message-----
From: Mark Little
[mailto:]
Sent: 26 March 2004
17:43
To: Furniss, Peter; Greg Pavlik
Cc: Newcomer, Eric;
ws-caf
Subject: Re: [ws-caf] context block or one element per
type
Peter, I haven't had time to read your email
fully, but since this has come up a couple of times in other emails I just
want to make something clear: in my email I wasn't proposing we go one way or
another in terms of context format. What I was trying to lay out was the
issues that the editors has come across (with input from the Paris f2f) and
the different possible routes we could take.
Mark.
----- Original Message -----
From:
Furniss, Peter
To: Greg Pavlik ; Mark
Little
Cc: Newcomer, Eric ; ws-caf
Sent: Friday, March 26, 2004 5:10
PM
Subject: RE: [ws-caf] context block or
one element per type
Interesting.
I
had assumed the usual way of dealing with representing contexts and the
protocol-specific information would be as Greg describes - each context is a
soap header containing the stuff for its (combination of) protocol and there
can be multiple headers of base type wsctx:ctx. :
Mark's proposal startled me, partly because it raises a question
about context as a whole, at least context-by-value (this is partly a
reprise of some of the Paris discussion)
If
it is possible for a compound context to be broken up, with its
protocol-specific elements travelling in their own right, and a context
element to define the relationship, then presumably one could do the same
with simple (one protocol) context. In which case, what semantic is being
carried by the context in that case? Is it needed as a header at all ? And
then the same argument applies to a simple context in the
normal arrangement - if the protocol-specific content is
intelligible on its own, what has been gained by putting it inside a
context, in addition to making it a header ? This was one of my points in
Paris - context-by-value is not needed
Of
course, there is a semantic in the multi-protocol case : the context wrapper
(or references in the context) mean "these two are to be considered
together, in accordance with the rules defined in uri ....", the uri
being that of the context-type ( aka ALS-configuration-identifier, I
think)
But that raises another question with these multi-protocol contexts:
who is supposed to implement the combination rules ? The Context Service is,
as I understand it, unaware of what the different context types mean - it
just goes round all its enlisted ALS's asking if they would like to
make a contribution. If it is unaware of different types (i.e. one could
take a Context Service implmentation and use it with ALSes and a
context-type that were unknown when the Context Service was written), then
it can only have a very simple combination rule - put the stuff from each
ALS in the context in an arbitrary order.
If
anything more sophisticated is needed (order the requests to the ALSes,
structure the combining of their responses) then the Context Service has to
implement the combination, and the Context : ALS relationship is no longer
the use-agnostic interface it appeared. And is it really going to be common
that independent ALSes can just be combined without modifying the ALSes
themselves ? It seems much more likely that the Context+ALSes group will be
the implementation unit - in which case, separating them out in an
interoperable way isn't really worth it.
Which in turn means there is no such thing as a general
multi-protocol <context> element. There are contexts (in the
sense of soap headers on application messages that contain information
relevant to supporting protocols/mechanisms) but their internal content is
specific to the combination, in the same way as the content of a
single-protocol context is specific to the protocol. And it we are back to
considering why:
<wsctx:context>
<wsctx:type>uri_for_foo_and_bar_combination</wsctx:type>
<foo:contextinfo
xmlns:foo="foo_schema_uri"> ... <foo:data>
<bar:data xmlns:bar="bar_schema_uri">
... <bar:data>
</wsctx:context>
is easier to cope with as a header
than
<foobar:context
xmlns:foobar="uri_for_foo_and_bar_combination">
<foo:contextinfo xmlns:foo="foo_schema_uri"> ...
<foo:data>
<bar:data xmlns:bar="bar_schema_uri">
... <bar:data>
</foobar:context>
As
I mentioned, some of this was raised in Paris, but I don't remember a clear
answer, and it would be good have it on the archive if I've forgotten
it/failed to understand it.
Peter
-----Original Message-----
From: Greg Pavlik
[mailto:]
Sent: 25 March 2004
23:00
To: Mark Little
Cc: Newcomer, Eric;
ws-caf
Subject: Re: [ws-caf] context block or one element per
type
Attached find response to original mail from Mark
in RTF.
Mark Little wrote:
Agreed.
Mark.
-----
Original Message -----
From:
Greg Pavlik
To:
Newcomer, Eric
Cc:
Mark Little ; ws-caf
Sent:
Tuesday, March 23, 2004 5:17 PM
Subject:
Re: [ws-caf] context block or one element per type
I suggest we have a statement explaining why context is
useful with this structuring approach.
Newcomer, Eric
wrote:
It might be no harm to include the context wrapper as an
optional element (ignored by those systems that don't want to deal
with it but used by those that can) to indicate ordering - where
one may want to define an order of header processing
precedence.
-----Original Message-----
From: Mark
Little [mailto:]
Sent:
Monday, March 22, 2004 5:37 AM
To:
ws-caf
Subject: [ws-caf] context block or one element
per type
After thinking about the structure of
context further, there was some concerned that the current
structure may inhibit adoption by potential users of the
specification because it does require a change to existing context
structures (or at least to recipients of the context). If we take
the case of an activity that has security and transactions
involved, then the current format of that context would
be:
<SOAP:header>
<wsctx:ctx>
<security>
...
</security>
<transactions>
...
</transactions>
</wsctx:ctx>
</SOAP:header>
whereas normally you'd expect
(because this is how the relevant specs. would define it) the
context to be:
<SOAP:header>
<security>
...
</security>
<transactions>
...
</transactions>
</SOAP:header>
We've said from the start that
existing services should just be able to plug in to Context and be
used as-is. If we take the case of ALSs then that may well be the
case (the difference between the formats of context can be hidden
from the individual ALSs by the context service). However, that's
not the case for user-level services (those things that actually
consume the context), because we would need to look for a mangled
form of their current context structure in the
WS-Context format. Is this is a big thing? Maybe it
makes sense to differentiate WS-Context aware user-level services
from those that aren't?
If we changed the structure to say that there wasn't an
all-encompassing context "wrapper", then it does allow non-Context aware services to be driven
within a Context activity transparently - they just can't tell the
difference since the context structure is identical. However, we
would lose relationship information and Greg put this quite
well:
"For one thing, I've found these kinds of non-nested
relationships in XML to be awkward to work with and somewhat
non-intuitive -- this is how the EJB spec structured that
transcation attributes, for example, and I must say as an
implementor I hated it."
If we were to change the structure of
context so drastically (and I'm not convinced that it's
required), I'd suggest we keep the basic context data element
from the original context, but have it as a specific element.
So:
<SOAP:header>
<security>
...
</security>
<transactions>
...
</transactions>
<activity-service>
...
</activity-service>
</SOAP:header>
A side-effect of such a change is
that we'd lose the automatic nesting structure of the context.
However, how services (ALSs) deal with nesting in their own
context element is up to them. The problem comes if there's a
required relationship between the various contexts (e.g.,
transactions only belong to the lowest-level child in a nested
activity.) I haven't had a chance to fully think the implications
of this change through yet, but I suspect we could either punt on
this to the respective specifications, or provide some related-to
schema element:
<SOAP:header>
<wsctx:related-to>
<security>
...
</security>
<transactions>
...
</transactions>
<wsctx:related-to>
</SOAP:header>
with attributes that allow you to
specify where within a nested hierarchy a specific context element
applies. But this looks complex.
Mark.
----
Mark Little,
Chief Architect,
Transactions,
Arjuna Technologies Ltd.
www.arjuna.com
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