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Subject: RE: [sca-j] ISSUE 8: PROPOSED RESOLUTION: Concurrency model for Service Reference instances
- From: "Peshev, Peter" <peter.peshev@sap.com>
- To: "Simon Nash" <NASH@uk.ibm.com>, "OASIS Java" <sca-j@lists.oasis-open.org>
- Date: Thu, 13 Mar 2008 20:22:47 +0100
Hi,
At least to me a multi-threaded composite-scoped client
engaged in several conversations and desiring to set manually the conversationId
is not a convincing usecase that justifies the confusion of having
two methods in the API -- getServiceReference &
createServiceReference.
If somebody wishes to create such designs, than
he\she should be on his\her own to assure thread safety. In addition if a
real productively used component is designed to be multithreaded, than most
probably it already would need some synchronizations around its business
data and methods.
Best Regards
Peter
I would be OK with the semantics
proposed by Michael. I would prefer to keep "get" with these semantics
than remove it entirely. We would need to write slightly more formal words
for the spec to define exactly what the semantics are.
Simon
Simon C. Nash, IBM
Distinguished Engineer
Member of the IBM Academy of Technology
Tel.
+44-1962-815156 Fax +44-1962-818999
"Michael Rowley"
<mrowley@bea.com>
13/03/2008 18:34
|
To
| Simon Nash/UK/IBM@IBMGB, "OASIS
Java" <sca-j@lists.oasis-open.org>
|
cc
|
|
Subject
| RE: [sca-j] ISSUE 8: PROPOSED
RESOLUTION: Concurrency model for Service Reference
instances |
|
To
record my reaction to this on this thread...
An
explicit createServiceReference method is a good idea. Given this, I
think that getServiceReference should either be removed or its sematics should
be that it returns the same service references as was (or would be) injected for
the component. In other words, its result is not affected by calls to
createServiceReference().
I don’t feel
strongly about whether we remove it or keep it with those semantics. I do
not, however, think that “get” should be given the liberal semantics that Simon
suggested.
Michael
From: Simon Nash
[mailto:NASH@uk.ibm.com]
Sent: Thursday, March 13, 2008 11:04
AM
To: OASIS Java
Subject: [sca-j] ISSUE 8: PROPOSED
RESOLUTION: Concurrency model for Service Reference instances
I presume the reference is to a conceptual internal "set" that would
occur as part of rewiring, and not any "set" API call. With this
understanding, your model for "get" semantics makes sense.
In
our last discussion of the issue there seemed to be some support for introducing
a createServiceReference() method on ComponentContext to return a newly created
ServiceReference object. This solves the problem and is upward compatible
with the existing API. We would probably also need to add a
createServiceReferences() method with similar semantics.
The
question then arises whether the semantics of getServiceReference() or
getServiceReferences() need to be specified more tightly, for example to always
return the same instance that was returned by a previous call except in some
specified list of cases. I believe this isn't necessary or desirable, as
it would over-constrain implementation flexibility.
So my
proposal is to resolve this issue by adding the following methods to
ComponentContext():
<B> ServiceReference<B>
createServiceReference(Class<B> businessInterface, String referenceName) -
Returns a newly created typed service reference for a business interface type
and a reference name. This method MUST throw an IllegalArgumentException
if the reference has multiplicity greater than one.
<B> Collection<ServiceReference<B>>
createServiceReferences(Class<B> businessInterface, String referenceName)
- Returns a list of newly created typed service references for a business
interface type and a reference name.
Simon
Simon
C. Nash, IBM Distinguished Engineer
Member of the IBM Academy of
Technology
Tel. +44-1962-815156 Fax +44-1962-818999
"Michael Rowley"
<mrowley@bea.com>
28/02/2008 21:50
|
To
| Simon
Nash/UK/IBM@IBMGB, "OASIS Java"
<sca-j@lists.oasis-open.org>
|
cc
|
|
Subject
| RE: [sca-j] AW: ISSUE 8:
Concurrency model for Service Reference
instances |
|
Wiring being changed requires a call to
set(), so naturally, after a set() the value of get() will be different.
If someone accesses a reference twice, without the reference having been
modified between those two accesses, then one would expect to get the same
thing.
Michael
From: Simon Nash
[mailto:NASH@uk.ibm.com]
Sent: Thursday, February 28, 2008 7:35
AM
To: OASIS Java
Subject: RE: [sca-j] AW: ISSUE 8:
Concurrency model for Service Reference instances
The semantics of getServiceReference() require it to return a
newly created object in some cases. Specifically, this will happen when
wiring has been changed, as we agreed in the resolution to issue 4. For
consistency and simplicity, I think this API should always return a newly
created object. The business logic can cache the result of a previous call
if it doesn't want a new object to be created.
Simon
Simon C. Nash, IBM Distinguished
Engineer
Member of the IBM Academy of Technology
Tel. +44-1962-815156
Fax +44-1962-818999
"Michael Rowley"
<mrowley@bea.com>
23/02/2008 16:05
|
To
| "Barack, Ron"
<ron.barack@sap.com>, "OASIS Java"
<sca-j@lists.oasis-open.org>
|
cc
|
|
Subject
| RE: [sca-j] AW: ISSUE 8:
Concurrency model for Service Reference
instances |
|
I agree.
I guess my concern is that the scenario in which the P2 solution is
helpful is, IMO, unlikely. One of the problems is that I view threads as
being used for only a few seconds at a time, while I expect that conversations
will typically last much longer than that.
So, what is the
harm in P2? I suppose the main harm is in the name of the routine:
getServiceReference(). In my opinion, a “get” doesn’t sound like a create,
and so I would be surprised if I got a newly created object with each call to
get(). If the API to do create were called createServiceReference()
instead, I suppose I would be less uncomfortable about it. I suppose the
question would then be, how the client get the “current” conversational service
reference for the reference, as I believe that the most common case is that
there will only be one conversation going at a time, especially when the client
is conversation scoped.
Michael
From: Barack, Ron
[mailto:ron.barack@sap.com]
Sent: Friday, February 22, 2008 1:26
PM
To: Michael Rowley; OASIS Java
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]
Sent: Thursday, February 21, 2008 5:15
PM
To: OASIS Java
Subject: [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.
<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]
Gesendet: Donnerstag, 21. Februar 2008
20:45
An: OASIS Java; Barack, Ron
Betreff: 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;
sr.setConversationID(ticket);
srv.check(itemId);
}
}
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:
@Scope(“COMPOSITE”)
class BookBatch {
@Reference BookStore store;
void
buyBook(String ISBN) {
store.addToCart(ISBN);
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 {
@Reference ServiceReference<BookStore> storeRef;
void
buyBook(String ISBN) {
if (storeRef.getConversation()
== null)
storeRef.setConversationID(chooseID());
storeRef.getService().addToCart(ISBN);
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
Unless stated otherwise above:
IBM United
Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
3AU
Unless stated otherwise above:
IBM United
Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
3AU
Unless stated otherwise above:
IBM United
Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
3AU
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