Issue 002 - WS-C: Clarify subordinated CreateCoordinationContext behaviour

["Peter Furniss" <peter.furniss@choreology.com>]

This is hereby declared to be ws-tx Issue 002.

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Issue name -- WS-C: Clarify subordinated CreateCoordinationContext
behaviour

Owner: Alastair Green [mailto:alastair.green@choreology.com] 

Protocol:  Coord

Artifact:  spec

Draft:     Coord spec working draft uploaded 2005-12-02

Link to the document referenced:
 
 
http://www.oasis-open.org/committees/download.php/15738/WS-Coordination-
2005-11-22.pdf
 
Section and PDF line number:
 
 Section 3 "Coordination Service", ll. 181-209
 
Issue type: Design / editorial

Related issues:

None

Issue Description:

Precisely specify normative behaviour of CreateCoordinationContext
against existing (superior) context
 

Issue Details:
 
 [This issue stems from Choreology Contribution issue TX-17.]
 
There are two intertwined problems. One is presentational, the other
more substantive. The initial part of Section 3 (up to and inluding 
the diagram, Figure 2., on p. 10) uses an example to convey three
things:
 
    a) the basic interactions of a registering application service and
       a Registration Service, and their relationship to Coordinators
and Participants

    b) the notion of an interposed Coordinator, and

    c) some seemingly normative rules relating to how interposed
       coordination works.

It should be made clear what is exemplification, and what is
normative. It is possible that the exposition of basic (C-P) and 
interposed (C-P/C-P) behaviours may be easier to grasp
with two successive and related
examples. These are primarily editorial matters. In the proposed 
resolution I will call this sub-issue a).
 
The substantive problem concerns the implied or stated normative rules 
of interposed coordination. There are two statements, which are too 
restrictive -- or perhaps which seem too restrictive because they are 
actually intended only as examples of one possible use of the 
protocol, but appear to have normative weight because
there are no other statements at all in the specification about the 
behaviours being discussed.

These statements are contained within the following three call-outs
which refer to the diagram. They are  flagged with the inserted tags 
[Statement 1] and [Statement 2].
 
ll. 197-206
 
     "3. App2 prefers CoordinatorB, so it uses CreateCoordinationContext
with Ca as an input to interpose
     CoordinatorB. CoordinatorB creates its own CoordinationContext Cb
that [Statement 1] contains the same
     activity identifier and coordination type as Ca but with its own
Registration service RSb.

     "4. App2 determines the coordination protocols supported by the
coordination type Q and then Registers for
      a coordination protocol Y at CoordinatorB, exchanging Endpoint
References for App2 and the protocol
      service Yb. This forms a logical connection between these Endpoint
References that the protocol Y can use.

     "5. [Statement 2] This registration causes CoordinatorB to forward
the registration onto CoordinatorA's
     Registration service RSa, exchanging Endpoint References for Yb and
the protocol service Ya. This forms a
     logical connection between these Endpoint References that the
protocol Y can use."

 Dealing with Statement [1] first:
 
It is wrong to mandate, in the WS-Coordination specification, that the 
activity identifier of the newly created CoordinationContext should be 
the same as that of the pre-existing (superior) Coordinator's 
CoordinationContext. (Proposed resolution sub-issue b).)
 
For the purposes of correct operation of the interposed coordinator
per se, the identity of the top transaction and the identity of the 
bottom transaction are irrelevant.

For the purposes of data sharing, every data-accessing participant in
a tree or sub-tree that is known to have an atomic outcome (e.g. WS-AT 
and WS-BA AO) should have available an identical value which can be 
used as an accessor token. A single, tree-wide value for all 
propagated context's /Identifier is a convenient way of
achieving this. It would be appropriate to insert a rule in WS-AT and 
WS-BA to the effect that a new atomic
context which is registered against an existing /atomic/ outcome must 
inherit the /Identifier value of the
existing context. It is inappropriate to place this rule in WS-Coord.
 
(The above point implicitly rests on the assumption that, for atomic
outcomes, every participant which registers with an RS associated with 
a given /Identifier will receive the same outcome. Issues are required 
to explicitly state this for WS-AT and WS-BA.)

To create a mixed outcome, one must either send differently identified 
contexts, or send identically identified contexts with a flag 
indicating that they cannot be relied upon to receive the same outcome 
(the MixedOutcome flag in a WS-BA context permits the latter option).
 
In this case we cannot assume data sharing, but nor can we rule it out 
(it may still be useful to have a common accessor identity for more 
sophisticated forms of isolation control than outright blocking). At 
this point, we might conclude that the identifier-inheritance rule is 
strictly unnecessary but harmless.
 
Scenario: A BA tree which combines MixedOutcome with AtomicOutcome.
(Other cases can be created involving combining WS-AT and WS-BA, and 
we have no control (at the level of
WS-Coordination) on future interposition or
sub-coordination behaviours for entirely new coordination types.)
 
A consuming service CS-Buyer uses three provider services: PS-Goods,
PS-Shipping, PS-Insurance. There are two viable (successful) outcomes 
possible -- {G,S}, {G,S,I} -- so we use MixedOutcome to govern the 
relationship of the CS to the three PS. Inside each PS (and these are 
likely to be separate legal entities), there may be several services 
that are composed to offer a single transactional, contingent service. 
To take one example: PS-Goods uses internal services IS-Stock, 
IS-Credit, IS-Accounting to create a single atomic outcome
"OrderGoods" operation. PS-Goods creates a WS-BA AtomicOutcome 
sub-coordinator, and hooks it in to the CS-
propagated B2B transaction.  

Imagine that IS-Credit accesses an external bureau service.
PS-Insurance uses the same service. Both Goods and Insurance use the 
CS-Buyer transaction id to identify access to the credit system. If 
this has access control implications (causes data sharing) then the 
fact that Insurance may fail independently of Goods can cause
unexpected side effects, if the rule "common id = atomic outcome" is 
relied upon.
  
(Less strained examples can be created using trees which combine WS-BA 
MO and WS-AT in intra-enterprise environments where data sharing is 
more likely.)
 
WS-Coordination should say nothing about identifier inheritance. It is
a matter for coordination protocols, for transaction domain bridging 
software, and for applications (which may use identifiers to create 
groupings or dependencies which are far more subtle).
 
Much of the above discussion hinged on the business need to combine
coordination types in a transaction tree. But the second part of 
[Statement 1] implies that interposition can only occur between 
contexts of identical coordination types. As Tom Freund pointed out at 
the first F2F, the design intent hitherto has been to permit
combined-type tree creation. (Proposed resolution sub-issue c).)
 
Turning to Statement [2] (proposed resolution sub-issue d)):
 
The practice of having a subcoordinator delay registration with its
nominated superior coordinator until it first receives registration(s) 
from Participant(s) is viable, given certain rules. So is the practice 
of eagerly registering subcoordinators as they are created. The 
lazy/eager choice is not the property of WS-
Coordination, and the spec should make it clear that subcoordinator 
registration time is not defined.
 

Proposed Resolution:

 Sub-issue a). Editorial work required to clarify example, and
 accompanying figure, and to create a section that describes the model 
 and normative operation of sub-coordination.
 
 Sub-issue b). Remove any normative reference to subcoordinator
 identifier inheritance in WS-Coordination.
 
 Sub-issue c). Remove any normative statement relating to sameness of
 superior and sub-coordinator coordination types or protocols.
 
 Sub-issue d). Remove any normative statement relating to the timing of 
 sub-coordinator registration, other than to state that the 
 sub-coordinator must be registered to take part in activity 
 completion.