sca-j message

Subject: RE: [sca-j] ISSUE 25 - Callback Simplification

From: Simon Nash <NASH@uk.ibm.com>
To: sca-j@lists.oasis-open.org
Date: Thu, 17 Apr 2008 05:10:40 +0100

Michael,
Sorry for the delay in responding. I have been on vacation in a primitive and beautiful part of the USA (Southern Utah) where internet connectivity is a very scarce resource, and not possessed by the current and previous places where I have been staying.

I'll start by giving the code for this example using my proposal. There's a slight inconsistency in your code, with orderData being passed on the call to orderProcessor.placeOrder() where I think you meant to pass order. I've made this change in the code below. This makes the Client class rather trivial, as it seems to add no value at all but simply forwards the order passed to its buy() method straight through to orderProcessor. A more realistic scenario would be where different items in the order were ordered from different suppliers.

@Callback(OrderCallback.class)
public interface OrderService {
void placeOrder(Order order);
}

public interface OrderCallback {
void confirm(Order order);
}

public class Client implements OrderCallback {
@Reference
OrderService orderService;

void buy(Order order) {
orderProcessor.placeOrder(order);
}

void confirm(Order order) {
order.setConfirmed(true);
}
}

This contains less code than even your conversational client case, and it works for all client scopes. It handles the stateless scope case with no need for a ServiceReference, the calls to getCallbackID(), or a PersistenceHelper (although the more realistic scenario that I described above would need a PersistenceHelper). It handles multiple outstanding callbacks (for multiple orders in progress at any one time, which is certainly plausible). Despite the extra parameter on the callback, I'd say that this is simpler than your version.

Some more comments on your proposed approach:
1. Your assertion that service providers will design services only for conversational clients and then get caught out when clients have other scopes doesn't makes sense to me. One of the basic principles of SOA is that service providers should design a service and its interface without knowing what kind of client will invoke the service. We should not be optimizing SCA around use cases that violate SOA principles.
2. There's a timing window in your stateless scoped example. It doesn't work if the callback occurs before the client has called persistenceHelper.persist(), To get around this, you'ld need to first create your own unique callbackID (an orderID), then use this orderID to make the persistenceHelper.persist() call, then call setCallbackID() on the ServiceReference, and finally call orderProcessor.placeOrder(). The need to create this orderID in business logic means that your code would become closer to my equivalent, in which an orderID is created in business logic, passed to the service, and returned on the callback.
3. Your reference to WCF duplex contracts is interesting, and I'll check it out when I get back to civilization. I'll be interested to see how it handles the issues I have raised above.

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>

10/04/2008 21:37

To	"Michael Rowley" <mrowley@bea.com>, Simon Nash/UK/IBM@IBMGB, <sca-j@lists.oasis-open.org>
cc
Subject	RE: [sca-j] ISSUE 25 - Callback Simplification

Note that BEA is not the only company with experience treating service callbacks in this way. Take a look at an example “duplex contract” in WCF. Note the lack of any correlation data in the callback parameters.

Michael

From: Michael Rowley [mailto:mrowley@bea.com]
Sent: Thursday, April 10, 2008 4:26 PM
To: Simon Nash; sca-j@lists.oasis-open.org
Subject: RE: [sca-j] ISSUE 25 - Callback Simplification

Simon,

Looking at code is indeed an excellent way to handle this. Here is a bidirectional interface pair that I think represents the most common case:

@Callback(OrderCallback.class)
public interface OrderService {
void placeOrder(Order order);
}
public interface OrderCallback {
void confirm();
}

Here is a conversational client. I believe this client can be written using either of our proposals:

@Scope(CONVERSATIONAL)
public class Client implements OrderCallback {

Order myOrder;

@Reference
OrderService orderService;

void buy(Order order) {
myOrder = order;
orderProcessor.placeOrder(orderData);
}

void confirm() {
myOrder.setConfirmed(true);
}
}

The above client is, I believe, the simplest and most common case. I think it is much less common that there will be multiple outstanding callbacks, but if there were, the client might need to maintain a Map of, for example, items ordered and confirmations for each item (assuming there is a callback for each). However, it would still be in the context of the original purchase order that had been made.

Nonetheless, it should be possible (although not necessarily easy, given its less frequent use) that someone be able to create a client for this service that is stateless. This stateless client would be responsible for the storage and correlation work that a conversational component would have done for him. Thus the stateless client would look like this (using the current API, so as not to confuse things too much):

@Scope(STATELESS)
public class Client implements OrderCallback {

@Reference
ServiceReference<OrderService> orderService;

@Context
RequestContext requestContext;

PersistenceHelper persistenceHelper; // a way of skipping junk I don’t want to show.

void buy(Order order) {
orderService.getService().placeOrder(order);
Object orderID = orderProcessor.getCallbackID();
persistenceHelper.persist(orderID, order);
}

void confirm() {
Object orderID = requestContext.getCallbackID();
Order order = persistenceHelper.find(orderID);
order.setConfirmed(true);
persistenceHelper.store(orderID, order);
}
}

This is leaving out all the technical junk that would also need to be done in order to set up the connection to the persistence context, dealing with transactions (even if declaratively), etc. All of the stuff that is done for you when you have a conversational client.

SUMMARY:
In both of our approaches people can, and therefore would, create the simple code in the conversational-scoped client of the first example. However, in your approach, it would then be impossible to use that service from the stateless client, such as I show in the second example. Because of this, people would unwittingly write services that could only be used by conversational clients.

I do understand that you would advise them _not_ to create interfaces such as the one I have here, and would tell them that they _should_ always pass around enough information so that they could create a stateless client even without help from a system generated callbackID. However, since in the conversational-scoped case that extra information would not be needed, I don’t think people would do it.

Michael

From: Simon Nash [mailto:NASH@uk.ibm.com]
Sent: Thursday, April 10, 2008 10:42 AM
To: sca-j@lists.oasis-open.org
Subject: RE: [sca-j] ISSUE 25 - Callback Simplification

Michael,
I think we're talking about example 2 here (stateless client and stateful callback).

The bidirectional interface provided by C should be usable by clients of all different scopes. For a stateless scoped client, the business data must be present or else the client cannot identify the order being confirmed. I'm not sure how your proposal would address this requirement. The automatic correlation works only in very limited cases (conversation scoped client with a single outbound request in progress at any one time). In all other cases, business data must be used for the correlation. It's far simpler to pass explicit business data on the service call in every case, with a consistent programming model, than to have an automated "convenience" feature for a single relatively uncommon case and a complex programming model that uses special APIs to handle all other cases.

Can you illustrate the code needed on the client and provider sides to implement example 2 with your proposal for a) a stateless client and b) a conversational client? The interface and provider code should be the same in both cases. I think this will make the complexity issues clear.

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>
08/04/2008 21:38

To
Simon Nash/UK/IBM@IBMGB

cc
<sca-j@lists.oasis-open.org>

Subject
RE: [sca-j] ISSUE 25 - Callback Simplification

Simon,

I am certainly willing to give up any automatic correlation when there may be multiple simultaneous outstanding callbacks. In that case, use business data.

However, for the common case where there is only one outstanding callback for a specific client, then your approach already allows the client to know what the callback is talking about without passing any correlation information in the business data. This is because your approach routes the callback to the right client instance.

I think this is good. We should encourage it. However, in the unfortunate event that someone can’t use a conversational-scoped client, they should still be able to use the bi-directional service without having to ask the service provider to change its interface. Admittedly, the stateless client will have to implement many of the steps that you list below, but that is the price for being non-conversational.

I also believe that if we followed your suggestion, then no matter what we say, people just won’t send unneeded correlation information as business data when they know that the callback is going to be sent to the right place. They won’t think: “Oh, just in case I ever have a stateless client, I better put this extra data into the business data.” Instead, they will just leave out the data, with the unintended consequence being that they’ve limited their clients to being only those that use a conversational scope. That would be bad.

Michael

From: Simon Nash [mailto:NASH@uk.ibm.com]
Sent: Thursday, April 03, 2008 12:13 PM
To: Michael Rowley
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

To
sca-j@lists.oasis-open.org

cc

Subject
[sca-j] ISSUE 25 - Callback Simplification

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

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References:
- RE: [sca-j] ISSUE 25 - Callback Simplification
  - From: "Michael Rowley" <mrowley@bea.com>