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Subject: AW: [sca-j] AW: ISSUE 8: Concurrency model for Service Reference instances
Hi Michael,
So my understanding of the scenario was wrong... the
component isn't a middleman, he's a funnel. All the requests that come in,
regardless of the client conversation go into the same conversation with
amazon.
In this case, the deeper problem is not the race condition
of setConversationID, but whether or not ServiceReference is thread safe, and in
particular if the service invocation must be implemented in a thread safe
manner. AFAIK, the spec currently makes no such statement. OTOH, do
we say anywhere that service reference are NOT thread safe. Should
we?
I believe your code sample to be correct, and believe
that it's the client's responsibility to handle synchronization in this
case. I'm not sure how to even express the behavior that the runtime would
need, in order to relieve the client of this responsiblity.
But that wasn't my understanding of Issue-8.
Issue-8 is, I thought, 2 threads, both of which want to participate in seperate
conversations, using the same service reference. Does everyone now agree
that they cannot?
In a way, your solution is consistent with P2...
which also does not proposal any new functionality, but says to use the tools
that already there to solve the problem. In your scenario, where the
reference is shared, the client must perform synchronization. In my
scenario, where different threads participate in different
conversations, then ComponentContext must be used as a ServiceReference
factory.
Ron
Von: Michael Rowley [mailto:mrowley@bea.com] Gesendet: Freitag, 22. Februar 2008 18:49 An: Barack, Ron; OASIS Java Betreff: RE: [sca-j] AW: ISSUE 8: Concurrency model for Service Reference instances Ron, Thanks for taking on my
challenge. Responses inline... From: Barack,
Ron [mailto:ron.barack@sap.com] 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. <MR>I’ve always
described conversational services as being designed to enable conversations
between components that can possibly take days (as would this book buying
example).</MR> 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. <MR>It doesn’t
sound like that would solve the problem that my hypothetical developer is trying
to solve. He is trying to maintain a single outstanding conversation with
Amazon, in order to batch up orders of books (possibly to qualify for free
shipping?). Your solution seems to introduce multiple simultaneous
conversations, which would defeat the purpose of this batching.
Also, RequestContext...getConversationID() called from within the
BookBatch component would not return anything, since the communication to
BookBatch would probably be non-conversational – after all, it is composite
scoped.</MR> What I
don't understand is howthe 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. <MR>I’m not
arguing for P1. I’m arguing that whatever solution we come up with should
solve the scenario that I laid forth, since it is, I believe, the most common
scenario where people will run into this problem. And, naturally, I’d like
for it to be fairly easy to use. I suspect that the solution will include
some kind of lock, from the time that the client decides to set the
ConversationID, until the business method is called. Perhaps like
this: void buyBook(String ISBN) {
if (storeRef.getConversation() != null) {
storeRef.getService().addToCart(ISBN);
} else {
synchronized(storeRef) {
if (storeRef.getConversation() ==
null) {
storeRef.setConversationID(chooseID());
}
storeRef.getService().addToCart(ISBN);
} // synchronized
}
if (isTimeToCheckOut())
checkOut(); }
In this solution, the
call first call in the conversation has to be in a mutex section with the code
that sets the conversation ID, so it will not be concurrent. However, all
subsequent calls on the conversation can be concurrent. Note that this
solution is neither P1 nor P2. It basically just says that Java
synchronization needs to be used. </MR> Michael Ron Von:
Michael Rowley [mailto:mrowley@bea.com] 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.
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: @Scope(“COMPOSITE”) class BookBatch {
if (isTimeToCheckOut())
checkOut(); } 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: @Scope(“COMPOSITE”) class BookBatch {
if (storeRef.getConversation() == null)
storeRef.setConversationID(chooseID());
if (isTimeToCheckOut())
checkOut(); } 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? Michael |
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