Simon,
I like the idea of combining the
callbackID and the conversation ID, since in our proposal only one of them ever
made sense at a time. I also like the way you described it: “thinking of
callbacks as a type of conversation that starts with a forward call and ends
when the last callback has been made by the service provider”.
You objected to our proposal 4, which we
titled: “Removing the distinction between stateless and conversational callback
interfaces.” However, you then proposed exactly the semantics I was
thinking of (although I hadn’t thought of merging the two kinds of IDs).
Your proposal (1) says: “A
bidirectional interface MUST have a conversational callback interface.” This
is exactly what I wanted. There is no choice to be made. However, since there
is no choice, I was not going to require that the callback interface be marked
as “conversational”. Its semantics are the semantics of a
callback. The “conversational” marking doesn’t help.
One question that hasn’t been answered
in either of our proposals is what instance is a callback delivered to? I’m
thinking that for stateless-scoped clients, it goes to a new (or pooled)
instance. For every other scope, it goes to the instance that made the
outgoing call, if it still exists. If it doesn’t exist, it means that
the client no longer cares about the callback and so it is dropped (and
probably logged).
Michael
From: Simon
Nash [mailto:]
Sent: Thursday, February 14, 2008
5:02 AM
To: OASIS Java
Subject: RE: [sca-j] Java Callback
Simplification Proposal
I need to make a small change to my proposal so that it
covers another important case.
If
the forward interface is always conversational, it is difficult to handle the
case of multiple forward calls each with its own separate series of callbacks,
because the same conversation ID would be used for all the callbacks and it
would be hard to tie the callbacks to their corresponding forward call.
The
solution is to allow the forward interface to be non-conversational even though
the callback interface is conversational (as is possible at present). Each
forward call would start a new callback conversation (and allocate a new
conversation ID) but there is no conversational relationship between calls in
the forward direction.
Here's
an example of how this works (extending the previous example):
a)
Stateless client A makes a forward call F to stateless service B over
bidirectional non-conversational interface X (with callback conversational
interface Y).
b)
Call F causes the infrastructure to start a callback conversation and allocate
a conversationID C for this, which is passed on call F.
c) Stateless client A makes a second forward call F to stateless service B over
the same interface X
d) The second call F causes the infrastructure to start a callback conversation
and allocate a conversationID D for this, which is passed on the second call F.
e) The first invocation of F on B makes callbacks G and H. These are
routed to new stateless instances of A. These callbacks carry
conversationID C, which can be used by A to perform correlation with the first
call F (in place of callbackID).
f) Call H ends the conversation that was started by the first call F. (If
this isn't done, the conversation will eventually time out.)
g) The second invocation of F on B also makes callbacks G and H. These
are routed to new stateless instances of A. These callbacks carry
conversationID D, which can be used by A to perform correlation with the second
call F (in place of callbackID).
h) Call H ends the conversation that was started by the second call F. (If
this isn't done, the conversation will eventually time out.)
Steps
e) and g) can be interleaved without any confusion, because the callbacks are
distinguished by the different conversation IDs C and D.
Here
is a summary of the revised proposal (with RFC2119 keywords this time):
1.
A bidirectional interface MUST have a conversational callback interface. The
forward interface MAY be non-conversational or conversational. The client
and service implementations MAY have any scope.
2. A forward call over a bidirectional conversational forward interface MUST
start a forward conversation and a callback conversation unless a forward
conversation is already active. The same conversation ID MUST be used for
both the forward and callback conversations.
3. A forward call over a bidirectional non-conversational interface MUST start
a callback conversation and MUST NOT start a forward conversation..
4. All callbacks made through a callback reference created by a forward call
MUST be treated as part of the callback conversation that was started or
continued by the forward call.
5. Normal rules for ending the callback conversation apply. The final
callback MAY end the conversation, or the conversation MAY expire through
timing out..
6. If the service implementation is conversation-scoped, the infrastructure
MUST perform instance routing based on the conversation ID passed on the
forward call. Otherwise, the conversation ID is available to the service
implementation to perform its own correlation.
7. If the client implementation is conversation-scoped, the infrastructure MUST
perform instance routing based on the conversation ID passed on the callback. Otherwise,
the conversation ID is available to the client implementation to perform its
own correlation.
8. Any correlation that is more complex than the above mechanisms MAY be
performed by means of business data passed over forward and callback messages.
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
13/02/2008 22:25
To
"OASIS Java"
<>
cc
Subject
RE: [sca-j] Java Callback Simplification
Proposal
I'll resist the temptation to interleave more deeply.
I'm OK with proposals 1, 2 and 3. For proposal 2, I didn't fully understand
the use case involving third parties and mapping callbacks IDs to callback
endpoints, but I'm OK with this on simplification grounds.
I'd like to propose a small change to proposal 4. For the
conversational-both-ways case, I think it's better to still mark the callback
interface as conversational rather than having this implied from the
conversational marking on the forward interface. I think this is clearer
and would allow the callback interface to be used as a regular forward
conversational interface, which could somettimes be useful.
I would like to go a step further and eliminate callback IDs in all cases. I
believe this can be done by bringing callbacks and conversations more closely
together and using the conversation ID for client-side correlation in cases
that are currently treated as non-conversational and where the callback ID is
being used.
This approach involves thinking of callbacks as a type of conversation that
starts with a forward call and ends when the last callback has been made by the
service provider. This means that all callback interactions would in
effect be conversational in both directions (from an interface perspective). If
the client's implementation isn't conversation-scoped, the conversation ID from
the callback would be available to the client to use to perform its own
correlation, replacing the current use of callbackID for this purpose.
WIth this view of the world, a callback would observe the following pattern:
1. Bidirectional interfaces must be marked as conversational for both the
forward and callback interfaces. The client and service implementations
can have any scope.
2. A forward call over a bidirectional interface starts a conversation if it's
not already started (this is just the current conversational semantics).
3. All callbacks are part of the same conversation that was started or
continued by the forward call.
4. Normal rules for ending this conversation apply. Typically, the final
callback would end the conversation.
5. If the service implementation is conversation-scoped, the infrastructure
will automatically perform routing based on the conversation ID passed on the
forward call. Otherwise, the conversation ID is available to the service
implementation to perform its own correlation.
6. If the client implementation is conversation-scoped, the infrastructure will
automatically perform routing based on the conversation ID passed on the
callback. Otherwise, the conversation ID is available to the client
implementation to perform its own correlation.
7. Any correlation that is more complex than the above mechanisms provide must
be performed by means of business data passed over forward and callback
messages.
Here's an example of how this would work for the simplest "stateless
callback" case:
a. Stateless client A makes a forward call F to stateless service B over
bidirectional conversational interface X (with callback conversational
interface Y).
b. Call F causes the infrastructure to start a conversation and allocate a
conversationID C, which is passed on call F.
c. B makes callbacks G and H. These are routed to new stateless instances
of A. These callbacks carry conversationID C, which can be used by A to
perform correlation (in place of callbackID).
d. Call H ends the conversation that was started by call F. (If this
isn't done, the conversation will eventually time out.)
Simon
Simon C. Nash, IBM Distinguished Engineer
Member of the IBM
Academy of Technology
Tel. +44-1962-815156 Fax +44-1962-818999
"Michael Rowley"
<>
13/02/2008 14:58
To
Mike Edwards/UK/IBM@IBMGB, "OASIS
Java" <>
cc
Subject
RE: [sca-j] Java Callback Simplification
Proposal
Mike,
I’m glad you liked 3 of the 4 proposed simplifications. Regarding
the one you didn’t like:
The proposal:
3. Making ServiceReference.setCallbackID(Object id) illegal and
ServiceReference.getCallbackID() (assuming #2) return null for
conversational interactions.
Then part of the rationale, and your response to it, were:
...conversations by nature correlate
invocations between a client and provider instance. If finer-grained
correlation is needed such as between multiple invocations in the
same conversation, correlation data can be included as part of the
service operation data (e.g. “line item number” can be passed as a
parameter in the example above).
<mje>THIS is where I hear the baby screaming as it flies out with the
bathwater.
<MR>I don’t know... Claiming we are baby-killers seems a bit severe
doesn’t it :-) </MR>
IF you really believe that, then there is no need for a callback ID ever. All
you
do is use correlation data in the messages themselves. I suggested this
to
Michael Rowley not so long ago, with the model of BPEL correlation in mind. He
was of the opinion that the BPEL correlation approach was a failure and not
to be used as a model for SCA.
There is a good argument to be made for a system managed correlation
mechanism in that it simplifies the life of the programmer. Forcing the
use of
correlation data in the message may well have the consequence of distorting
the form of the messages, since they now *HAVE* to contain data fields that
are "uniquely identifying" - and this must be done for all callback
messages.
</mje>
<MR>Actually my point on BPEL correlation is a little more subtle.
Having to use correlation to get to the right process instance is indeed
too much of a pain. The system should be able to do that. However,
when you have a complex process and you need to get to the right place within
that process, then correlation is OK, and in fact the SCA BPEL spec makes it
clear that BPEL correlations can and should continue to be used for this.
Because this is a less-common complex case, it is OK to require some
complication in the solution.
The situation is analogous for Java. The infrastructure should be able to
route to the right client instance, based on the conversation that is taking
place. If you need finer-grained correlation, that is up to you.
Also, it would be difficult or impossible for the system to make any useful
guarantees about which callback ID will be received for a conversational
interaction. Imagine this case (pseudocode):
{
setCallbackID(“1”)
callService()
setCallbackID(“2”)
callService()
}
receiveCallback() {
getCallbackID(); -- what does this return?
}
When multiple callback IDs are set within a single conversation, there is
nothing to guarantee that the service provider will keep track of all of the
outstanding callback IDs (“1” and “2” in this case) and
use the most appropriate one when it is invoking a callback. Creating
such an ability to automatically maintain a set of outstanding callback IDs would
be too complex for the value it would provide.
Michael
From: Mike Edwards [mailto:]
Sent: Wednesday, February 13, 2008 5:24 AM
To: OASIS Java
Subject: Re: [sca-j] Java Callback Simplification Proposal
Jim,
Thanks for this detailed proposal.
I've got a few concerns, that I can summarize simply as "the baby being
thrown out with the bathwater"
- ie some capabilities that are actually quite important seem to get chopped in
the name of simplicity.
I'm left feeling that callbacks in particular are no longer of any use once the
proposals are accepted.
Let me try to express what I see as the problems in inline comments as <mje>...</mje>
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:
Jim Marino <>
12/02/2008 18:23
To
OASIS Java <>
cc
Subject
[sca-j] Java Callback Simplification Proposal
All,
Bellow is a proposal from BEA to simplify callback mechanisms in the
Java C&I specifications. I will work on the appropriate steps of
getting this into a JIRA so that it can be tracked.
Thanks,
Jim
This proposal aims to simplify using asynchronous communication in
the SCA Java programming model by:
1. Eliminating callback objects.
- Removing ServiceReference.get/setCallback()
<mje>So this implies the client MUST implement the callback
I/F</mje>
2. Removing the ability for service providers to get callback IDs.
- Moving CallableReference getCallbackID() to ServiceReference)
<mje>.+1 </mje>
<mje>There is one interesting argument to follow from this. It is
possible to
take the view that callbackID is only of interest to the client, while
conversationID
is only really of use to the provider. Each is being used to pick out
state of
some kind, but at opposite ends of the communication. So perhaps we can
eliminate the client having access to the conversation ID?
It would be nice to coalesce the 2 IDs, The problem is that they have
completely
different lifecycles. The conversationID has some fixed span dictated by
the
form of the forward call interface, with specific starting and ending
operations.
The callback ID has an indeterminate span, always starting with one forward
call, but with an undetermined number and type of callback operations
being included in its span. Logically it needs to change for each subsequent
forward call.
I'm now really heading off into the weeds, but it occurs to me that there is a
relationship of the callback ID to the concept of "Future" that is
used in the Java
concurrency packages. There, the Future represents the asychronous
operation
that is taking place, and can be used to feed back results. The Future is
handed
back to the "client" application on the invocation of the forward
"service" invocation.
Today, Futures only handle one-shot pieces of asychronous work, with a single
"response" message. However, you could conceive of stretching
this, to permit
something you might call a "CallbackFuture" - which would represent
an asynchronous
process with a continuing life and an arbitrary number of "response"
messages.
The nasty thing about this is that with our current callback design there is no
obvious end to the life of a CallbackFuture - in other words, the callback interface
does not define the equivalent of an "endsConversation" which
indicates completion
of the work of the asynchronous service. Today, the "end" of
the callbacks must
be defined in metadata outside the interface. Perhaps we need to fix
that,
but it will not be easy.</mje>
3. Saying callback IDs are irrelevant in conversations.
- Making ServiceReference.setCallbackID(Object id) illegal and
ServiceReference.getCallbackID() (assuming #2) return null for
conversational interactions.
<mje>Why does having a conversation enable this to be done?
The callback ID I thought allowed a client to mark a service invocation in
a way that callbacks made to the client carried the same ID and that
this enables the client to establish which original request caused a
given response.
Conversation ID does not do this at all, unless I am mistaken, since
while callback ID can change from one invocation to the next, conversation ID
stays the same for the whole of a conversation. Thus conversation ID is
useless for distinguishing one callback from another.</mje>
4. Removing the distinction between stateless and conversational
callback interfaces.
- Removing the ability to specify conversational on callback
Interfaces.
<mje>+1</mje>
By simplification, we mean as a primarily reducing the number of
concepts and APIs a developer must understand to program
asynchronous services. This will entail some reduction in
capability. However, this reduction, we argue, affects edge-cases
that can be accommodated through alternative techniques. A secondary
aspect of simplification concerns implementation. By removing the
requirement for runtimes to provide infrastructure for handling edge
cases that can be accommodated through relatively simple application
programming techniques,implementing callback capabilities becomes
fairly straightforward.
<mje>Unfortunately, unless I'm mistaken, some rather central use cases
have been
trashed too.</mje>
[RATIONAL]
The rational for proposed items is dealt with individually in this
section.
1. ServiceReference.setCallback(Object object) and
ServiceReference.getCallback()
------------------------------------------------------------------------
---------------------------------------------
<mje>+1</mje>
This API exists as a mechanism to route callbacks to instances other
than an instance of the client component making a forward invocation.
This provides two capabilities. Clients do not have to implement a
callback interface. The API also allows clients to instruct the
runtime to perform callback dispatching for stateless interactions
that execute concurrently. A common example of this is inventory
check. A client component needs to perform an inventory check for
multiple line items. As an efficiency, it written to invoke the same
inventory service using a non-blocking call multiple times. For each
invocation, it wishes to dispatch the associated callback to a
different handler instance it creates. For example:
ServiceReference<InventoryService> serviceReference = ..
for (LineItem item: purchaseOrder.getLineItems()) {
CallbackService handler = new CallbackServiceImpl();
serviceReference.setCallback(handler)
serviceReference.getService().checkInventory(item);
}
This can be done using the CallableReference.getCallbackID():
// code that conducts the forward invoke:
ServiceReference<InventoryService> serviceReference = ..
for (LineItem item: purchaseOrder.getLineItems()) {
String id = // generate or assign the callback ID
// store the callback id and whatever information the callback
needs
cache.put(id, handler);
serviceReference.getService().checkInventory(item);
}
Callback processing can either be done directly by a component
implementation instance or delegated to CallbackServiceImpl as shown
below:
// code in the same class that handles the callback:
public void onCallback(InventoryStatus status) {
String id = requestContext.getServiceReference().getCallbackID();
CallbackInfo info = .. // retrieve the stored callback
information by using the callback id
CallbackService handler = new CallbackServiceImpl(info);
handler.onCallback(status);
}
Some advantages to eliminating get/set/Callback() are:
a. Reduces the number of concepts and APIs a developer will be
presented with without affecting common functionality. In other
words, this API is primarily used for edge-cases and removing it
simplifies the common case.
b. Application migrations and versioning are easier using the
alternative approach. By using an object instance, runtime
implementations would be responsible for handling class versioning
for in-process interactions. By avoiding serialization and the use of
a specific classes, and instead storing callback data,
CallbackServiceImpl can be versioned and substituted for callbacks
that have not yet occurred.
c. Runtime performance characteristics are likely to improve. For
cases where the client is stateless, routing to the specific instance
set through setCallback() requires that the callback be routed to the
specific JVM where the instance is hosted. Eliminating routing to a
particular instance allows the callback to potentially be sent to any
JVM where the client component is hosted.
d. Eliminates the need for runtime implementations to manage garbage
collection of callback objects. For example, if a callback is never
made, the runtime at some point will need to remove the object from
storage or memory.
2. Moving CallableReference.getCallbackID() to ServiceReference
------------------------------------------------------------------------
--------------------
<mje>+1</mje>
We propose removing getCallbackID() from access by service providers
as it is cannot be used for correlation in third-party components
except when the third-party is configured with a callback to the same
client component. For example in the stateless case:
A---------B--------C
The only way C can callback to A is either by calling back B or
having B pass a CallableReference to C. The callback ID is not
sufficient, in itself, to be used by arbitrary code to send a message
that qualifies as a callback in this bidirectional exchange. For
example, if B has clients other than A, the callback ID doesn’t have
the information to tell which client it is for.
Moving CallableReference.getCallbackID() to ServiceReference
simplifies the Java programming model by:
a. Eliminating an API that can only be used in very particular
circumstances.
b. Removing the need for a runtime implementation to create and
maintain a distributed domain-wide mapping of callback ids to
callback endpoints. Note that routing information cannot be encoded
in the callback id as it can be set by the application.
3. Making ServiceReference.setCallbackID(Object id) illegal and
ServiceReference.getCallbackID() (assuming #2) return null for
conversational interactions.
------------------------------------------------------------------------
------------------------------------------------------------------------
----------------------------------------------------------------------
Having both callback ids and conversation ids for bi-directional
conversational wires is confusing and not needed as the conversation
id will be used for correlation.
<mje>This is what I challenge - it isn't true if you need to correlate
callback responses
to individual requests. Conversation ID is necessarily the same for all
the forward
invocations in a conversation. This then is useless for distinguishing
between
callback messages resulting from different forward invocations.
In fact, I argue that conversationID is almost useless for the client - it's
really a
provider-defined thing useful for the provider to access its state
data.</mje>
Sometimes a client may wish to
correlate callback invocations using the callback id. For
conversational callbacks, correlation using the callback id is
limited. Since conversations are serial between a particular client
instance and a provider instance, the additional correlation that may
be needed is between different invocations to the provider within the
same conversation. In other words, conversations by nature correlate
invocations between a client and provider instance. If finer-grained
correlation is needed such as between multiple invocations in the
same conversation, correlation data can be included as part of the
service operation data (e.g. “line item number” can be passed as a
parameter in the example above).
<mje>THIS is where I hear the baby screaming as it flies out with the
bathwater.
IF you really believe that, then there is no need for a callback ID ever. All
you
do is use correlation data in the messages themselves. I suggested this
to
Michael Rowley not so long ago, with the model of BPEL correlation in mind. He
was of the opinion that the BPEL correlation approach was a failure and not
to be used as a model for SCA.
There is a good argument to be made for a system managed correlation
mechanism in that it simplifies the life of the programmer. Forcing the
use of
correlation data in the message may well have the consequence of distorting
the form of the messages, since they now *HAVE* to contain data fields that
are "uniquely identifying" - and this must be done for all callback
messages.
</mje>
This simplifies the Java programming model by clarifying the
relationship between callback and conversation ids.
4. Removing the ability to specify stateless or conversational
callback service contracts
------------------------------------------------------------------------
-----------------------------------------------------
<mje>+1</mje>
We propose wires be either conversational or stateless in both
directions and that it be specified as part of the forward service
contract. In other words, @Conversational cannot be specified on the
callback interface definition. We don't believe having a stateless
callback for a stateful forward invocation and vice versa makes much
sense.
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Registered office: PO Box 41,
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