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Subject: RE: [sca-j] AW: ISSUE 8: Concurrency model for Service Reference instances




I think I laid out a fairly believable example where a composite scoped client (BookBatch) would deal with a conversational service (Amazon) and then pointed out the place where threading problems might occur.  I don’t see, based on your email, how you would expect this example to be solved.


If you don’t like the example of setting the conversation ID, then setting a callback ID might be more to your liking.  It would run into the same race condition.




From: Mike Edwards [mailto:mike_edwards@uk.ibm.com]
Sent: Friday, February 22, 2008 6:14 AM
To: OASIS Java
Subject: Re: [sca-j] AW: ISSUE 8: Concurrency model for Service Reference instances



I wonder whether we're really getting to the heart of the problem here.

Let's step back and examine the whole picture for a moment.

At one level, we have a concern over the use of references by multiple threads.
At another level we have an interest in understanding how multiple conversations can be conducted by a client.

These interact, but not in the way that we have been discussing.

There is a third question lurking about how Callbacks work in a multi-threaded environment.  I'll get to that one later.

First, let's take a look at a simple non-conversational service and a client with a reference to that service.
I think that the scope of the client matters little, in fact.  It's the usage of the reference that matters.

I think that a reference to a stateless service can happily be invoked at any time by any thread belonging to
the client component.  If the client has multiple threads, there may be some concerns about the order of
requests that get sent, but that is a problem for the client to sort out.  It won't matter to the service.  There
are certainly no concerns that there will be any "technical" problems invoking the service operations.

Now, switch attention to a conversational service and the reference to that service held by a client.

It seems to me very simple to express a principle here:  a single reference instance object represents
at most a single conversation.  ( I say "at most" since I think that we have said that is possible for such
a reference to be in "conversation not started" state and for non conversational invocations to take
place in that state.)

Now the usage of that single reference object can be handled in one of two ways:
1) all uses of the reference, by any thread, are for one conversation and one conversation only

2) over time, the reference may be used for multiple conversations, but serially - again, only one
conversation at a time

1) requires relatively little management other than ensuring only one set-up of a conversation ID.
2) requires more control to ensure that each conversation is started and ended in a controlled
fashion (ie setting of a conversationID, calling of an endsConversation method etc, must be

Let's express another principle: if a client of a conversational service wants to conduct multiple
conversations at the same time, it must use multiple reference objects, one for each conversation.
(This implies necessary use of the context API to fetch new copies of reference objects)

I think that the main concern over multi-thread use of a conversational reference is thus solved
in a relatively simple way.  If you stick with the idea that one reference is only involved in one
conversation at a time, then things are relatively simple and only a small amount of mechanics
is required.

One comment I make is:  "why do we allow the client to set the conversationID?"  What's the
purpose of this?  The conversation is primarily about enabling the same service provider
data to be reached - the conversation ID matters far more to the provider than to the client.
I argue for removing the capability for a client to set the conversation ID.

Now, that has dealt with conversational references.  That leaves callbacks.  I'll leave those
for another email....

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

"Barack, Ron" <ron.barack@sap.com>

21/02/2008 22:15


"OASIS Java" <sca-j@lists.oasis-open.org>




[sca-j] AW: ISSUE 8: Concurrency model for Service Reference instances




Hi Michael,
Let me first make sure I understand your scenario.  The composite-scoped component is essentially a middle man, involved in 2 conversations, with amazon on one side, and the customer on the other.  And the problem is to make sure that requests coming in from the client conversation get passed to the correct amazon conversation.  Both conversations can be long running.  Does that fit?
I think the scenario we had in mind was much more oriented to short lived conversations.  That is, the case where the whole conversation with StoreRef takes place within a single call to buy books. In this case, you simply have to replace the injected field "storeRef" with an injected ComponentContext, and the implementation of buyBook would  call context.getService("storeRef").  Whether the component calls storeRef.setConversationID or not, you never have any race conditions.
The situation is more complex for these long-running conversations, and I think it's unsurprising that the code would be, too.  In this case, the code would need to map from the client conversation ID, to the ID of the amazon conversation.  That is, instead of checking if storeRef.getConversation() is null, the code would call something like lookupStoreRefId(context.getRequestContext().getServiceReference().getConversationID()).  If the value returned was non-null, the component it would set the storeRef.conversationID accordingly.  Otherwise, it sets the conversionID to chooseID().  The method lookupStoreRefId probably would use a DB, but could use an in memory map, or anything else.
What I don't understand is how the alternative proposal, "P1", would work.  Are you expecting the runtime in inject storeRef's conversationID into some thread local storage before invoking buyBooks?  In this case, isn't the implication that the runtime would be maintaining the map, just like proposal P2 demands that the client do?  Or are you assuming that the conversationID is already on the thread from previous calls to setConversationID?  In this case, it's true that the client remains very simple, but the solution requires
     a) that the calls in the conversation always occur in the same thread, and
     b) that the server will not be restarted during the lifetime of the conversation.


Von: Michael Rowley [mailto:mrowley@bea.com]
Donnerstag, 21. Februar 2008 20:45
OASIS Java; Barack, Ron
ISSUE 8: Concurrency model for Service Reference instances

Here is the description of the issue 8 problem (from the PPT on today’s call):
While the current text says that a service reference represents a single conversation, it is not clear how a multi-threaded client should protect a non-conversational service reference's configuration (conversation id, callback, etc) so that it stays unmodified by other threads until an actual invocation is executed.

Consider the following code snippet for example:

class AComponent {
 @Reference ItemCheckerService srv;

 void goCheckItem(long ticket, String itemId) {
       ServiceReference sr = (ServiceReference) srv;

A simple synchronization may lead to strict serialization of remote calls which is generally undesirable.

I think we should have a good idea of the likely scenarios in which this multi-threading will happen.  On today’s call, Simon suggested that code could start its own threads.  I agree this is true, but I don’t want to concentrate on that case, since I think people who go there are willing to be pretty sophisticated about the threading logic.  
I believe other cases are that the client could be conversation or composite scoped.  Stateless and request scoped components are only active for one thread at a time.  This is implied by the semantics of the @Init and @Destroy methods, which are called at the beginning and end of the scope lifetime.  For a stateless scope, that lifetime is one call.  For request scope, it is one remotable call (to be clarified based on one of our open issues).
The scenario where a conversation-scoped client could be active in two threads at once is possible, but unlikely, so I’ll concentrate on the case where the client is composite scoped.
Consider this scenario: a composite scoped component exists for the purpose of batching up book orders to Amazon.  When orders come in to the BookBatch component, it forwards them on to Amazon, using the shopping cart that is associated with the current conversation.  After a certain amount of time, or a certain number of books, the current batch is purchased, and the conversation is ended.  When the next book order comes in, a new batch (conversation) will be started.  How might this look:
class BookBatch {
 @Reference BookStore store;

 void buyBook(String ISBN) {

        if (isTimeToCheckOut())
boolean isTimeToCheckOut() {}
void checkOut() {}

This seems like a potentially common scenario where the client would be multi-threaded.  Now, to run into the problem, we have to imagine that the client wanted to choose its own conversation ID.  So, perhaps it would look like this:
class BookBatch {
 @Reference ServiceReference<BookStore> storeRef;

 void buyBook(String ISBN) {

        if (storeRef.getConversation() == null)


        if (isTimeToCheckOut())
boolean isTimeToCheckOut() {}
void checkOut() {}
String chooseID() {}  // Choose a conversation ID for the next bookstore conversation.

In this version, we pick a new conversation ID if a conversation isn’t already going and set it on the service reference.
This version has a race condition!  Multiple threads could have null returned from getConversation() and so multiple threads will attempt to choose the next conversation ID.  In this particular case, it probably doesn’t matter which one wins that race, but I suppose that in some cases it would matter.
Is this the problem we are trying to solve?  If so, I’m not sure how the proposal in the PPT presentation given today would help much.
Ron or Simon, would you be willing to modify this class so that it works correctly given the proposed resolution to issue 8?


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