sca-j message
[Date Prev]
| [Thread Prev]
| [Thread Next]
| [Date Next]
--
[Date Index]
| [Thread Index]
| [List Home]
Subject: RE: [sca-j] ISSUE 25 - Callback Simplification
- From: Simon Nash <NASH@uk.ibm.com>
- To: "OASIS Java" <sca-j@lists.oasis-open.org>
- Date: Fri, 4 Apr 2008 12:56:30 +0100
Mike,
At first sight this idea seems appealing.
On closer scrutiny, I don't think it fits with the normal semantics
of Futures. When the Future receives a callback, the only thing it
can do is pass the callback on to the calling component (B). It can't
support polling or blocking because the very nature of a callback is to
do something rather than just returning a result as is the case for a normal
Future. Given this, what's the value of putting an intermediary (a
Future object) between the callback initiator C and the callback recipient
B?
If the Future is to do more than simply
forward callback messages from C to B, then it will need to have business
methods to handle the callback semantics. This is not how Futures
currently work. Mixing infrastructure behaviour and business logic
on the Future object doesn't seem like a good idea to me.
Alternatively we could limit callbacks
so that they follow a simple setter pattern, and have the Future provide
those methods together with corresponding getter methods for the client
to use to support polling and blocking. This is very different from
the callback concept that we have in SCA today. It would require
extra mechanisms to create the appropriate setter and getter method implementations
on the Future object. This mixture of limited functionality and extra
complexity is not very appealing.
Could you use Example 2 in my proposal
to show how the Future approach would work, listing all the flows and objects
involved?
Simon
Simon C. Nash, IBM Distinguished Engineer
Member of the IBM Academy of Technology
Tel. +44-1962-815156 Fax +44-1962-818999
Mike Edwards/UK/IBM@IBMGB
04/04/2008 12:03
|
To
| "OASIS Java" <sca-j@lists.oasis-open.org>
|
cc
|
|
Subject
| RE: [sca-j] ISSUE 25 - Callback Simplification |
|
Folks,
I agree with Simon's analysis.
For callbacks, it should in general be possible to make multiple "overlapping"
forward calls from a client to a given service.
If such overlapping calls are made, then it is important for the client
to be able to distinguish the asynchronous response messages
resulting from each forward call.
This must either be done in business data, as shown in Simon's example
forward & callback interfaces, or it must be done through
infrastructure - equivalent to the "callbackID" that was part
of our model.
I would like to point out a "simpler approach" to the case that
Simon describes below for "callbackID". I've talked about
this before
to some of you but I've never written it down and shared it with everyone
on the list.
An alternative and simpler approach to functionality equivalent to "callbackID"
is to use the design of the "JAX-WS" asynchronous
client API - ie have a form of the forward API call that returns a Future<?>
- the Future represents the communication from the client to
the service. The client invokes this async form of the forward API
and gets the Future. When asynchronous responses are
received from the service, they flow not to the client itself, but instead
flow to the Future object (which implements the callback
interface). The Future object reports these responses back to the
client by the usual means for a Future (polling, blocking or
callback method invocation).
This dispenses with the need for callbackIDs. What flows with the
forward messages is simply an EPR for the Future object - and
this is used on each callback (under the covers, by the infrastructure).
When the response message arrives back at the client,
it is the Future object that gets the reply message. The Future object
then does what it does.
This approach has the virtue of marrying together both forms of client
asychronous API - that for async client to a synchronous
service (ie the standard JAX-WS async client to a sync service) and that
for an async client to an async service.
One beauty of this approach I note is that it works even where the originating
forward call is itself call-and-return. In that case,
the API for that forward call simply follows the form dictated by the JAX-WS
async rules. Where the service also has callbacks,
the only difference is that the returned Future is a more complex one -
one that will report on all the responses from the service.
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
Simon Nash/UK/IBM@IBMGB
03/04/2008 17:13
|
To
| mrowley@bea.com
|
cc
| sca-j@lists.oasis-open.org
|
Subject
| RE: [sca-j] ISSUE 25 - Callback Simplification |
|
There's a basic problem with this approach. Whether or not B has
conversational scope depends on its interactions with A. In a single
A to B conversation, the same conversational instance of B might place
5 orders with C. Using the A to B conversation ID to identify the
order that was placed is therefore never safe, even if B is conversational.
The only way this approach could work is if B were to generate a new "callback
conversation ID" for every request that it sends to C. This
would be different from any regular conversation ID that B may be sending
to C as part of a B to C conversation. Here are the steps that would
need to be followed.
1. B generates a new "callback conversation ID" and associates
it with its reference for C. This needs to happen before any calls
to C are made.
2. An API needs to be defined to allow B to get this "callback conversation
ID" before it makes the placeOrder() call to C. B stores the
"callback conversation ID" and associates it with the order it
is placing.
3. B makes a placeOrder() call to C.
4. The SCA infrastructure transmits the "callback conversation ID"
from B to C with the placeOrder() call.
5. C stores the "callback conversation ID" in the callback reference
so that it can be returned to B in any callbacks.
6. C makes a callback to B.
7. The SCA infrastructure transmits the "callback conversation ID"
from C to B with the callback.
8. An API needs to be defined to allow B to get this "callback conversation
ID" when the callback is executing. This ID is different from
the conversation ID that's active for the A to B conversation.
All of the above are not needed if the correlation information is passed
as business data, which is a much simpler approach.
I considered this approach when I was trying to come up with a good definition
of conversational callback interfaces between the 3/20 and 3/27 calls,
but I rejected it because I did not believe that the value justified the
complexity.
Simon
Simon C. Nash, IBM Distinguished Engineer
Member of the IBM Academy of Technology
Tel. +44-1962-815156 Fax +44-1962-818999
"Michael Rowley"
<mnrowley@gmail.com>
03/04/2008 15:40
Please respond to
mrowley@bea.com |
|
To
| Simon Nash/UK/IBM@IBMGB, sca-j@lists.oasis-open.org
|
cc
|
|
Subject
| RE: [sca-j] ISSUE 25 - Callback Simplification |
|
I like the fact that, in Simon's
model, callbacks always go back to the instance that made the outbound
call. However, it looks like he does not take advantage of that fact
in the interfaces of the examples that he created. For example, he
has the following:
@Callback(OrderCallback.class)
public interface Order {
void placeOrder(String
orderID, String orderData);
}
public interface OrderCallback {
void confirm(String
orderID);
void update(String
orderID, String status);
}
However, since you know that
the order callback goes back to the right instance, you should be able
to define the callback interface as follows:
public interface OrderCallback {
void confirm();
void update(String
status);
}
In Simon's model this should
be possible, but there is a catch. If the callback interface is defined
this way, the client has to use a conversational scope. If the client
is stateless or composite scoped, then the routing of the callback to the
right instance doesn't say anything useful about what is being confirmed.
I would like to enable this
style of callback interface irrespective of the scope of the client. As
with Simon's approach, if the client is conversational scoped, then the
client is especially simple and does not have to do any correlation at
all. However, if the client needs to use some other scope, for whatever
reason, then the service with the callback can still be used. However,
it just needs to get the callback ID (or conversation ID) that is associated
with the callback in order to determine what the callback is talking about.
Michael
From: Simon Nash [mailto:NASH@uk.ibm.com]
Sent: Thursday, March 27, 2008 11:47 AM
To: sca-j@lists.oasis-open.org
Subject: Re: [sca-j] ISSUE 25 - Callback Simplification
There's an update to this proposal at
http://www.oasis-open.org/apps/org/workgroup/sca-j/download.php/27733/JAVA-25-Proposal-2.doc
This version removes conversational callbacks as all my attempts to define
these involve more complexity than is justified by the functional value
of this capability.
Simon
Simon C. Nash, IBM Distinguished Engineer
Member of the IBM Academy of Technology
Tel. +44-1962-815156 Fax +44-1962-818999
Simon Nash/UK/IBM@IBMGB
20/03/2008 11:01
|
|
Last week I took an action to produce a written up proposal for callback
simplification by today. I have uploaded this to the document repository
as
http://www.oasis-open.org/apps/org/workgroup/sca-j/download.php/27649/JAVA-25-Proposal.doc.
The content of this proposal corresponds to the "Callback Simplification"
section of the proposal sent out by Michael R, and does not address the
changes proposed in the "API Simplification" section. I
think it is best to have these discussions separately.
This proposal corresponds to what I was describing at the sca-j F2F with
two changes / additions:
1. Outstanding callbacks don't pin conversational objects after the conversation's
creator has ended the conversation.
2. In all cases, the caller of a bidirectional interface creates the callback
EPR that will be used (including any reference parameters needed).
I believe the issues that still need to be discussed and resolved are as
follows:
a) Should all 4 combinations of conversational and non-conversational forward
and callback interfaces be allowed, or only 2 of these?
b) Should the callback interface used to make callbacks to a conversation-scoped
component be marked as conversational?
c) Should each forward request within a conversation generate a unique
ID that is not part of business data and is returned with the callback?
Simon
Simon C. Nash, IBM Distinguished Engineer
Member of the IBM Academy of Technology
Tel. +44-1962-815156 Fax +44-1962-818999
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
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
[Date Prev]
| [Thread Prev]
| [Thread Next]
| [Date Next]
--
[Date Index]
| [Thread Index]
| [List Home]