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Re: [ws-caf] context block or one element per type
Title: Message
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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