[GRAAP-WG] Modification to the wiki Page on Renegotiating an established Agreement

[Karl Czajkowski]

On Aug 23, Toshiyuki Nakata modulated:
> Hi Dominic:
> > So, yes, there are problems but it looks like a lot of people want to
> > have this extra commit message. Are there any written records on the
> > pros and cons of having the 2PC?
> 
> I think this info. is very important.
> Unfortunately, I am not able to address this issue.
> 
> Karl or others, please respond..

I definitely don't want to repeat the debate which is in the GRAAP-WG
email archives, but let me just summarize what I think were the final
observations and conclusions.

WS-Agreement is aimed at a distributed, soft real-time (one might say
isochronous) environment with limited trust between peers with bounded
rationality.  In other words, it is for establishing automated
agreement between programmatic systems in an Internet-type environment
where the agreement terms may involve wall-clock obligations which are
at the root of the problem.

When you boil it all down, there is always going to be a
synchronization hazard where due to communication delays or faults, or
even intentional decision delays, one party does not know whether the
other has accepted the agreement and therefore is at risk to perform
according to the terms of agreement when not necessary, or ignore the
terms of agreement when bound by them.

WS-Agreement formalizes this hazard to always place the burden on the
agreement initiator who makes an offer and is bound by it if the
responder accepts.  Early drafts, derived from our SNAP work, had an
extra signalling step to allow a sort of "pre-initiator" called an
invitation.  The group felt that this was too complicated to pursue,
particularly because it could be approximated via advertisement.

An invitation or advertisement would be a non-binding expression of
interest in receiving agreement offers, so that the typical
client-server roles could be reversed somewhat.  The only difference
is that an invitation is seen as usually a 1:1 relationship while an
advertisement is 1:many. However, due to the non-binding nature of
these signals and the assumption of bounded rationality (we can ignore
the possibility of bluffing and other psychological strategies), I am
not sure these differences are significant.  For privacy, one could
imagine secure advertisement channels (with authorization) to further
blur the boundaries.

Furthermore, 2PC can be synthesized with two WS-Agreement round-trips
and an appropriate domain-specific agreement semantics. So, either
advance reservation, combined with the right cost/penalty model, or
the underlying invitation system can both look like 2PC in practice:

 1.a.  initiator offers advance reservation, OR
 1.b.  advertiser publishes interest

after this first stage, both parties are aware of interest and primed
to negotiate...

 2.a.  responder accepts (or rejects) reservation offer, OR
 2.b.  initiator offers against advertisement (or ignores it)

after this stage, one party is obligated to satisfy and the other has
a choice to make... (For the advance reservation, the initiator has a
choice because the reservation includes a cost model with cheap
cancellation under reasonable deadlines).

 3.a.  initiator offers claiming agreement (or cancels reservation), OR
 3.b.  responder accepts offer (or rejects it)

after this stage, both parties are obligated to satisfy...

 4.a.  responder accepts claiming agreement (or acknowledges cancellation), OR
 4.b.  initiator acknowledges receipt (e.g. transport-level ACK)

after this stage, both parties actually know what the other knows...

In case it isn't obvious, advertisement is done via WS-Agreement
templates and advance reservation followed by claiming is anticipated
in the usage scenario examples as well.


> Best Regards
> Toshi
> -----
> Toshiyuki Nakata 中田　登志之
> Executive Chief Engineer, Central Research Lab. NEC
> 1753, Shimonumabe, Nakahara-Ku,
> Kawasaki,Kanagawa 211-8666,Japan
> Tel +81-44-431-7653 (NEC Internal 22-60035)
> Fax +81-44-431-7609 (NEC Internal 22-60509)
>  


karl

p.s. I will add that the fundamental challenge I see for GRAAP-WG and
WS-Agreement is that everyone approaches it with the same natural (but
incorrect) intuitions about trust, signalling, and consensus.  I think
there is still a large social experiment to see who can deploy systems
which are actually performing distributed agreement without a central
authority... economics and our real-life world says it should work,
most of the time at least. :-)

-- 
Karl Czajkowski
Software Architect

Univa Corporation
1001 Warrenville Road, Suite 550
Lisle, IL 60532

karlcz at univa.com
www.univa.com
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