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Subject: RE: [sca-j] ISSUE 4 - Dependency reinjection


We are all rightly worrying about complexity here.  On the other hand we are also thinking about genuinely
useful capabilities.

I think I'd like to separate this into two different problem areas, which I think are handled differently:

1) Change to or Removal of a Target service

2) Modification of wiring.

For the present, this can ONLY affect connections being made at the SCA domain level.  Anything lower
in the composition hierarchy requires a redeployment of some composite - and this involves stopping
and restarting something.

1) Change or Removal of a Service

Given the loose coupling implied by SOA, I assume that it is reasonable for a service component to
be removed or updated at any time.  The question is what happens to any clients of the service:

a) Require all clients to be stopped and restarted too.  Could be a heavy burden for a popular service.
b) Existing client instances continue to use a "preserved copy" of the old version of the service until all the
client instances have ceased to exist.  Could be a problem with long-lived clients (eg composite scoped)
since this would mean an indeterminate lifecycle for the preserved copy of the service.  Places a burden
of managing multiple versions of a service in parallel.
c) Existing client instances either get "service unavailable" faults or simply use the new version of  the service
on their next invocation (simply by means of calling the same endpoint address as previously).
Gives the deployer/operator of the system certainty at the expense of causing failures for the client applications.

I argue that reference reinjection should not take place for any of these cases.  It isn't going to help much.

2) Modification of Wiring.

Let's be clear that modifying wiring can be done in these ways:

a) Change the configuration of the reference
b) If the reference uses autowire, change the potential set of target services in the domain (through
deployment actions on the services)
c) Where the wiring is expressed through Wire elements deployed separately from the reference(s)
and those wires are changed or removed.

a) requires redeployment of the client components.  I argue that this cannot involve reinjection.

b) and c) originally required a decision on the part of the assembler/deployer that the wiring config would
be separate from the client components.  It is these cases which may involve reinjection, since the
targets can be changed without redeployment of the clients.  One thing to consider is what is the expectation
of the deployer when the changes are made - will it be too hard for the deployer to understand that a
change in the wiring configuration does not necessarily take effect for some (indeterminate) time after
it is made?

Peter's question of whether this type of rewiring should be supported at all is a good one.  The current
specification certainly allows it.  The capability of ESBs is also certainly along these lines (up to and including
dynamic selection of target services for each invocation).

If the client code cannot tolerate changing the target of a reference by rewiring during component execution,
I think we've identified some approaches by which this may be handled:

i) Simple use of standard coding techniques - ie a reference injected via constructor can't be reinjected.
Other methods allow for reinjection.  Choose the technique suited to your code.

ii) Use of an intent.  This would mark a reference as "non reinjectable" or rather "not changeable".  This
would instruct the runtime not to reinject.  Might this intent also limit the ways in which wiring can be done?
(eg force the use of wires expressed as part of the configuration of the reference, and prevent the use
of autowire?).

I note that an intent is more general than the coding techniques.  It can apply to components written in
any language.

iii) Special handling for wires involving conversational interactions?  Or those involving callbacks?
There have been suggestions that wires involving these types of interactions can't be changed
(reinjected) during the span of the conversation.  Callbacks which are not conversational are
troubling in that there is no indication of whether a callback is outstanding.  Perhaps this does not
matter in that the callback object will be held in some form by the target service and will be
unaffected by the change in wiring.  However this again implies that the "old wiring" continues to
have a life after the "new wiring" is deployed.

iv) Limitation of reinjection based on lifecycle scope of the client component.  
- No reinjection for stateless components (they are assumed to be "short lived").
- Reinjection for composite scoped components (they "live forever")
- Uncertainty about conversation scoped components (they "live for a while") -
reinjection may be desirable for these guys - assuming the preservation of any
references involving conversations (as discussed in iii).

- so lack of toleration of reinjection requires choosing an appropriate scope
We expect most components to be stateless, so things there are really simple.
Conversation scope is the next most common - "no reinjection" is simple to
understand but it has the downside of "old wiring" living for a potentially
considerable time after it has been replaced.

- composite scoped components are a really special case - we don't expect many of
them, and so special rules don't seem out of place - there are going to be quite
a few rules anyway, I suspect.

I'm wondering whether a combination of techniques is the way forward here?  Views?

Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com

"Peshev, Peter" <peter.peshev@sap.com>

06/12/2007 08:37

"OASIS Java" <sca-j@lists.oasis-open.org>
RE: [sca-j] ISSUE 4 - Dependency reinjection

First  +1 to Dave that reinjection could cause huge problems for unprepared component, so it makes sense to allow it only if there is some explicit "I can handle it" annotation
On a more general note, I think reinjection is a big issue, so it's good if we clarify first what usecase we are solving and what we want to achieve  with that mechanism.
At least to me reinjection becomes relevant, only when the administrator rewires \ unwires a  domain reference. If we want to address the general problem - administrator changes some of the wiring, why not simply restart the application (i.e. contribution) which is the source of the wire ? Everything will be initialized properly afterwards.
One quote, which is one of my favorites and IMO seems relevant
"This group of experts may be able to understand the subtle semantics of the more esoteric cases, but I worry that the average business programmer will find some of the advanced features  ...  more confusing than beneficial."
And now we are speaking even not about programmers, but about administrator, who would probably be even less familiar with specs. In addition we are expecting that this guy will have deep understanding of the application (what is setter\constructor based injection, whether the scope is composite, etc.) All that information is even not in the SCDL-s, but at the java source.
Btw, are there any reinjection equivalents in the current java world ? Some app. servers for example allow EJB reference resolving across two applications. Does anyone support dynamic rewiring of @EJB via some UI without redeployment ?  Does any vendor offer reinjection of Java EE resources  (env-entries) or some Spring resources in case administrator wants to change them ?

From: Blohm, Henning [mailto:henning.blohm@sap.com]
Wednesday, 5. December 2007 17:05
Mike Edwards; OASIS Java
AW: [sca-j] ISSUE 4 - Dependency reinjection

Hi all,
  please, let's not go down that "smart proxies" road. There should be precise description what a service reference instance represents, since it might be kept, you could have more than one based on the same ref, and you might even pass it on to some other component.
  I believe the most commonly expected behavior is that the service reference instance represents a resolved client view to the wire target of the reference specified when retrieving the service reference instance. This way, a component has a chance of issueing several calls in succession to the same target (that, even if stateless, are correlated via some stored data) - unless that target was removed, taken out of service or went out of scope for other administrative reasons - in which case an exception should be thrown.
  Every call to ComponentContext.getServiceReference should return a new service reference instance.
  The "convenience feature" of dependency injection should be driven from a perspective of developer convenience. Since the most basic approach is to refer to the injected field rather than copying it over to other members, I believe conversational components should not be subject of re-injection, as their state will usually be a reflection of an invocation history with their reference targets (ignoring the additional information provided via setter injection for the moment).
  For the Component Context I find it harder to identify the best behavior: stable over the life cycle scope of the component or reflecting the latest wiring situation. Last week I was tending to the former, but in the meantime I have been convinced that the latter is more appropriate since it allows the component to chose whether it wants to get the latest state.
  I think... I just repeated the proposal made by Michael ;-)

Von: Mike Edwards [mailto:mike_edwards@uk.ibm.com]
Mittwoch, 5. Dezember 2007 14:57
RE: [sca-j] ISSUE 4 - Dependency reinjection


I don't think the proposal is a hack - it expresses some capability in a way very familiar to Java


I agree that there is still a bigger problem - see my subsequent posting which muses about the

bigger problem.

Allowing reference reinjection has only a small connection with the bigger issues.

So, to address the particular issue you raise here - if a component has a reference and if it

cannot tolerate the loss of that reference then:

a) you are then establishing a contract between the component and the wider SCA runtime along

the lines that the runtime cannot undeploy a target component instance that is "being used" by

this component. This may imply a process of quiescing the target component when a change is

required, or the ability to run old and new versions of the target in parallel.  This is part of the

"bigger picture" question.  It may be indicated by an intent of some kind, although the target of

the intent is a very curious one - it is the runtime container of the target service component.  We

haven't got one quite like that, although it is close to some of the transaction intents.....

b) how do you deal with cases where the loss is of a service that is external to the SCA domain

over which SCA has no control?  I suppose "serviceunavailable" fault is the likely response to

this occurrence.  If this is so, why would things be different for a target service WITHIN the SCA


Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com

David Booz <booz@us.ibm.com>

05/12/2007 13:29

RE: [sca-j] ISSUE 4 - Dependency reinjection

Seems like a hack to me.  And it doesn't address the larger problem where
the reference suddenly becomes unusable because the target was changed.
This is similar to a referential integrity problem.  The component wants to
state that it can't tolerate the loss of a valid reference part way through
it's processing.

Dave Booz
STSM, SCA and WebSphere Architecture
Co-Chair OASIS SCA-Policy TC
"Distributed objects first, then world hunger"
Poughkeepsie, NY (845)-435-6093  or  8-295-6093

           Mike Edwards                                                  
           ibm.com>                                                   To
           12/05/2007 04:28                                           cc
                                     RE: [sca-j] ISSUE 4 - Dependency    


Controlling whether reinjection is allowed AT ALL for a component is
relatively simple in
my opinion:

- if a component does not want reference reinjection to occur, ever, then
the implementation
simply declares the reference annotation on a constructor parameter and
does not
provide the reference via either a field or via a setter method.  If the
reference is only
injectable via the constructor then it can never be changed by the

This simple design choice allows complete control by the developer of the

Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431
Email:  mike_edwards@uk.ibm.com

David Booz <booz@us.ibm.com>                                              
05/12/2007 02:37                                                       To
                                                RE: [sca-j] ISSUE 4 -    
                                                Dependency reinjection  

It's simple, just not well expressed.  I'm wondering if there are cases
where a component implementation might want to delay or "opt out" of
re-injection either permanently or temporarily.  There might be cases where
a particular component implementation really can't tolerate a re-injection.
I was thinking out loud about using concurrency control as a temporary
means to delay re-injection. There are other ways.  Maybe a different
approach like an annotation @AllowsReinjection (or the opposite) is

I understand that what you are proposing is optional for a runtime to
support, but for those runtimes that do support it, each component
implementation might need to have a say in how it works.  Still thinking
out loud....

Dave Booz
STSM, SCA and WebSphere Architecture
Co-Chair OASIS SCA-Policy TC
"Distributed objects first, then world hunger"
Poughkeepsie, NY (845)-435-6093  or  8-295-6093

          "Michael Rowley"
          12/04/2007 08:11          David Booz/Poughkeepsie/IBM@IBMUS,
          PM                        <sca-j@lists.oasis-open.org>

                                    RE: [sca-j] ISSUE 4 - Dependency

   -----Original Message-----
   From: David Booz [mailto:booz@us.ibm.com]
   Sent: Tuesday, December 04, 2007 3:06 PM
   To: sca-j@lists.oasis-open.org
   Subject: RE: [sca-j] ISSUE 4 - Dependency reinjection


   And tagging on a few more questions:

   1) I presume that the use of setter based injection would allow for]
   concurrency serialization by the component implementation so that it
   stabilize it's use of references.  This brings up an interesting
   is there (or should there be) some linkage between the lifecycle of
   target service and these references?  Just because a ref target has
   altered, does not mean that the target service is gone.  It's the
   of that target service which will determine how long a reference
   usable after a wiring change occurs.  Just raising the question for
   because it will affect all the component's whose references can't be

[MR: I’m sorry, but I don’t follow you.]

   2) I'm curious about your introduction of InvalidServiceException.
   already have ServiceUnavailableException.  I think there's room to
   the wording of SUE to make room for ISE..

[MR: I think that a service that has been correctly identified, but isn’t
currently available is quite different from a service that is incorrectly
identified (it isn’t in the logical domain).  As such, I think it deserves
a different exception.]

   3) +1 to Mike E.

[MR: No objection from me.]


   Dave Booz
   STSM, SCA and WebSphere Architecture
   Co-Chair OASIS SCA-Policy TC
   "Distributed objects first, then world hunger"
   Poughkeepsie, NY (845)-435-6093  or  8-295-6093

                Mike Edwards
                                          "OASIS Java"
                12/04/2007 10:22          <sca-j@lists.oasis-open.org>

                                          RE: [sca-j] ISSUE 4 -


   Thanks for the proposal.  It is good to have something concrete as it
   crystallise the

   Please help me understand the rationale for treating Composite-scoped
   differently from Conversation scoped components.

   Both types of component have an extended lifecycle.  Both may easily
   have a
   that spans changes in configuration that affects their references,
   where those
   references are not conversational and do not in themselves involve
   lifetime.  Why is it justified to change references in the one case
   and not
   allow changes
   in the other case?

   Yours,  Mike.

   Strategist - Emerging Technologies, SCA & SDO.
   Co Chair OASIS SCA Assembly TC.
   IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great
   Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431
   Email:  mike_edwards@uk.ibm.com

   "Michael Rowley" <mrowley@bea.com> wrote on 29/11/2007 20:19:28:

   > I took an action item to make a more specific proposal for:
   > dependency reinjection.  Here it is:
   > Reinjection
   > -----------
   > References MAY be reinjected after the initial creation of a
   > component due to a change in wiring that has occurred since the
   > component was initialized.  In order for reinjection to occur, the
   > following MUST be true:
   > - The component MUST be composite-scoped.
   > - The reference MUST use either field-based injection or setter
   > injection.  References that are injected through constructor
   > injection MUST NOT be changed.
   > - If the reference has a conversational interface, then a
   > conversation MUST NOT be active at the time of the reinjection.
   > If processing in reaction to a change in a reference is necessary,
   > then setter injection should be used, with code in the setter
   > that does the proper processing in reaction to a change.
   > Components with any scope other than the composite scope MUST NOT
   > have references reinjected.  If an operation is called on a
   > reference where the target of that reference is no longer valid,
   > then InvalidServiceException MUST be thrown.
   > In cases where changes to a reference are not valid, the reference
   > as accessed through the component context also MUST NOT change.
   > More precisely, the ComponentContext.getService() and
   > getServiceReference() methods MUST return the same reference target
   > as would be accessed through injection.  However, the
   > ServiceReference that is returned by getServiceReference() never
   > changes its target.  If the wiring of a composite component causes
   > reference to be reinjected, any ServiceReference object that was
   > acquired before the reinjection will still correspond to the target
   > prior to the change.  If the target service for a ServiceReference
   > ever becomes invalid, then attempts to call business methods
   > that ServiceReference MUST throw InvalidServiceException.
   > The rules for reference reinjection also apply to references with a
   > 0..N or 1..N.  This means that in the cases listed above where
   > reference reinjection is not allowed, the array or Collection for
   > the reference MUST NOT change their contents.  In cases where the
   > contents of a reference collection MAY change, then for references
   > that use setter injection, the setter method MUST be called for any
   > change to the contents.  The injected collection MAY be the same
   > collection object as is currently used by the component, but with
   > some change to its contents.
   > Michael

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