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Subject: RE: [ws-rx] PR001 - Feedback on MSFT's MC proposal
"Durand, Jacques R."
<JDurand@us.fujitsu.com>
11/03/2006 10:41 PM |
|
"Durand, Jacques R."
<JDurand@us.fujitsu.com>
11/02/2006 03:48 PM |
|
Marc Goodner <mgoodner@microsoft.com>
11/01/2006 02:28 PM |
|
Doug,
Thanks for the feedback. Detailed responses inline below.
The net is I don’t see anything below that can’t already be done using
the proposal I made. As I already mentioned in the response to Anish’s
message the only potential change to the proposal I made is adding a new
EPR specifically for MC.
From: Doug Davis [mailto:dug@us.ibm.com]
Sent: Thursday, October 26, 2006 8:57 AM
To: ws-rx@lists.oasis-open.org
Subject: [ws-rx] PR001 - Feedback on MSFT's MC proposal
Per my todo, here's feedback on the MSFT proposal for PR001 which
also has the various use-cases interspersed.
MG: My understanding is that the AI you took was to provide a set of enumerated
use cases so the two proposals could be compared. I do not believe that
this mail does that. However, I’m willing to see the AI closed if you
agree that you don’t have any additional use cases beyond this for MC.
Metacomments:
- Current solution in spec is not broken nor incompatible with WSA
MG: If this were true CR33 would not have been filed.
DUG: No matter how CR33 ends up (even close w/no action) the current RM
spec can compose with WSA - the only thing that needs to be made clear
is the value people should use for wsaw:Anonymous. No biggie. CR33
wasn't opened because RM couldn't work today but rather because of a problem
with WSA.
- This proposal will force a new PR review and Interop
MG: This is not a technical argument against the proposal. The TC always
planned on a second PR review and it’s up to the TC whether or not
we really need another round of interop.
- This proposal does not address any new features or use-cases
MG: I agree. It fixes a part of the spec that did not work as designed.
It also clearly describes the feature and what it is intended to. I see
that as an improvement over the current spec.
DUG: Which parts of the current spec do not work? Making wild claims
like this need to be proven.
Legend:
MC = MakeConnection
MP = MessagePending
CS = CreateSequence
Specific comments:
- Lack of support for multiple endpoints per Sequence.
When a MC is received the receiver can determine which Sequence
is being targeted by the ref-p's of the incoming message. However,
not all messages being sent on that Sequence can be received by
all anon endpoints. How can the receiver of MC ensure that the
proper message (e.g. response) is given to the proper anon endpoint?
Ability to share RM state does not equate to share of messages.
MG: I think you are mixing two different scenarios here.
1) Support for using a single sequence to and from multiple endpoints.
The proposal I made certainly supports this.
DUG: This isn't an answer. Please show how it works with your proposal.
I think concrete message flows are needed.
2) Support for querying a endpoint for specific messages. This is not needed
to support 1, nor is it needed for any RM scenario. Which unreachable endpoint
receives and processes a given message and when it does so are implementation
problems in the Client RMD and part of its contract with the applications
that use it.
DUG: Again, not an answer. Please show how two anon clients can be
assured that when they turn on RM and use your MC proposal they will each
receive the correct response. Saying that's an implementation problem
is not only not answer but leaves things wide open to interop problems.
Please show the message flows for how this would work.
- Restrictions on RM Processing model.
Base specifications should, within reason, try to avoid restricting the
usage pattens under which they are used. Since the RM spec is silent
on
algorithm used to determine when (and how many) Sequences are used we need
to be careful about adding things to the spec that would restrict the
impl's choices. Above we talk about a single Sequence spanning multiple
endpoints, but going the other direction there may be RM processors that
choose to group messages destined to the same endpoint under one Sequence.
In essence what we're talking about grouping based on wsa:To. This
proposal
would not allow for this type of processing model because all wsa:To values
would be the WSA Anon URI.
MG: This is an implementation style argument and I don’t see a scenario
here. However, this can be accomplished by the server returning a MP header
with an appropriate EPR and the unreachable endpoint occasionally sending
an empty MC to that EPR.
DUG: Please show how grouping by wsa:To is achieved because I just don't
see how a MP header+an EPR solves it. Please show concrete message
flows.
- How many MC messages are needed?
This proposal would require one MC per Sequence. If there are multiple
Sequences at play between the two endpoints the current spec allows
for a single MC to pull back messages from any Sequence. This could be
a significant performance issue.
MG: Both proposals require one MC per Sequence. The only difference
is that in this proposal one MC is for a specific Sequence; but the same
number of messages are exchanged. There is not an issue here.
DUG: Not true. This proposal requires one MC per Sequence - which means
if there are two active Sequences then there MUST always be two MC's sent
- in the current spec only one MC is needed. Yes one MC per message
is needed, but if there's nothing to pull back then this proposal is quite
a bit more chatty and far less optimal.
- Since MC is an optional feature, how does the receiver know it will be
used?
Here there are two reasons the receiver of the MC would like to know
whether or not MC will be used at all. First, the receiver may choose to
reject requests to use the anon EPR if MC will not be used - thus saving
resources. Second, in cases where RM is optional for messages targeted
to the Anon EPR, if the receiver knows that MC will not be used it may
choose to not turn on RM at all, thus allowing normal WSA rules to
apply to its delivery. This would still allow a non-MC enabled client
to function, where if the receiving end assumed that MC were always
an option it may never actually deliver any message until a MC is sent.
Thus never sending any messages.
MG: The receiver can fault on receipt of MC if it wished to, so the first
point above isn’t an issue. The second point does not have a clear use
case. Why would an endpoint turn off RM if it knew MC wasn’t going to
be used? That sounds like this is no longer an RM scenario. My proposal
does not cover using MC for non-RM messages. I don’t understand how the
current spec would address what you just described either.
DUG: This issue is about cases where MC is not used so I don't understand
your statements at all. How can it fault on a MC that is never sent?
The RMD may choose, at run-time by examining the replyTo of the request,
to decide whether or not to use RM at all for the responses. You keep forgetting
about cases where RM is available but optional. So, the RMD may turn
off RM and just use WSA rules because it just doesn't want to support MC
at all - would have thought you, of all people, would want this option
:-)
Also, if the receiver of the MC makes the wrong choice (meaning it thinks
MC will not be used) and MC's do flow then a new (unused) Sequence will
be created.
MG: If you are not expecting an MC why would you create a sequence instead
of just faulting?
DUG: Because your proposal says that when an MC flows to an unknown AcksTo
EPR or to the Server EPR it means the client wants a new Sequence - according
to your proposal what else can the RMD do but create a new Sequence?
- Requires special AcksTo EPR construction.
While obviously its possible to create AcksTo EPRs with distinguishing
ref-p's, w/o this proposal there is no need to. All RM protocol
messages have a wsrm:Identifier already that is used to identify which
Sequence is at play. This would require a secondary mechanism to
also
be used. While not a deal-breaker it is a duplication of function
and
forces exposure of the MC feature into the other RM aspects. Current
version has no impact the rest of the RM spec - thus when this RM
feature is turned on it doesn't require a change to the rest of the RM
stack.
MG: The current MC also has distinguishing information in it separate from
the wsrm:Identifier. I do not see an issue here.
DUG: Not true. Sequence based MC uses RM's SeqID. Since the
MC+URI variant isn't based on pulling by RM sequence it doesn't need to
worry about this. This proposal dups things unnecessarily and trying
to point the finger some place else doesn't change the issue.
- The AcksTo is not the same thing as the RMS.
Implicit in this proposal is that the AcksTo endpoint has access to all
of the same information as the RMS - which is not necessarily true. The
AcksTo EPR is not required to do anything more than be able to update the
state of the RM Sequences w.r.t. Acks - nothing more. This is one
of the
reasons we have a dedicated AcksTo EPR at all - otherwise we would have
just overloaded an existing EPR - like wsa:ReplyTo or wsa:From. Using the
AcksTo EPR for this purpose is forcing a certain implementation choice.
MG: There was a lot of discussion on last week’s call of sending protocol
faults to the AcksTo EPR as they were protocol control messages. So this
EPR is already overloaded. I think the MC is another type of protocol control
message so I don’t see a problem here either. That said, I’m open to
seeing a new MC EPR defined.
DUG: Yet another change to the base RM protocol needed to support this
proposal.
- Orphan AcksTo
This proposal suggests that a MP header may contain an AcksTo EPR as a
signal that the receiver of the MP should then send a MC to that EPR
as a mechanism by which new Sequences can be created. This would
then
create an 'orphan' AcksTo EPR. This EPR is not associated with a
Sequence, thus its an orphan, and quite a change to how RM works.
Or if it is associated with a Sequence then what is the state of that
Sequence? This would imply that a Sequence is created before a CS
is
actually processed.
MG: Reusing the AcksTo EPR from the MP in the CS that flows back would
solve this. I don’t really see an issue here either.
DUG: Again, you're creating an AcksTo EPR w/o creating a Sequence - this
goes against the current RM model - of course there's an issue here.
- MP on nonexistent message.
This proposal talks about a MP being able to flow so that the sender of
message can signal to the anon endpoint that a MC needs to be sent so a
CS can flow. But in cases where this is the first message being sent
how can this work? There is no back-channel from the anon endpoint to
send it on. Also, even if there was some incoming message how did
the sender of the MP who which anon endpoint sent it? There is no identifying
info in the message to distinguish one endpoint from another.
MG: The unreachable endpoint can poll with empty MCs.
If the receiver of MCs receives an empty MC with no other identifying headers
(e.g. no previously communicated ref-params) then the sender of the
MC is truly anonymous and the response to the MC will include a CS and
information that will make the unreachable client identifiable in the future.
This is a non-issue.
DUG: I need a concreate message flow example here because this makes no
sense. If there is no message going back there's nothing to piggy-back
the MP onto. How does the client know to send a MC at all? If
there are multiple clients how does the RMD know which new/empty-MC coming
in is related at all to an existing request/response MEP. All of
these "this is a non-issue" type of responses makes it clear
to me that you haven't thought about this from an implementation point
of view because there are just too many unknowns or assumptions going on
here.
- Poor performing CreateSequence
When the sending endpoint needs to use MP to create new Sequence we see
the following message exchanges:
Client -> some msg -> Server
Client <- some msg+MP/AcksTo <- Server
Client -> MakeConnection -> Server/AcksTo
Client <- CreateSequence <- Server/AcksTo
vs current solution:
Client -> MakeConnection -> Server
Client <- CreateSequence <- Server
MG: This is not correct. My proposal allows for polling for the CS with
MC as well, so the minimal exchange in both cases is exactly the same.
I see the first form above as a distinct advantage of my proposal over
the current form in that it allows the server side to set an explicit relation
between previously received messages and messages over a newly established
sequence.
DUG: Please show how this works when there are multiple clients talking
to the same server. Above you said that the RMD can fault on the
receipt of a MC if it wants to - so relate that to this scenario. The
RMD gets new MC's from unknown/anon clientts how does it know which should
get back a CS and which should get a fault? There is no correlating
info at all. The only possible way I saw it working is how I showed
it above so I don't see how your claim holds up.
As stated in the previous bullet, there's no guarantee that the "some
msg"
data is actually available to do that msg exchange, but also this
proposed solution is quite a bit more verbose and non-optimized.
- Unreliable-in/Reliable-Out not supported.
Client sends a GetQuote, ReplyTo=anon. Server decides to use RM to
send the response. How can this work? The proposal implies
that
either a CS or a MP is sent back but how and on what socket?
MG: No, the proposal is clear. It sends a MP on the backchannel triggering
the client to respond with the MC to get the CS.
DUG: there is no back-channel to send it on. please show concrete message
flows to show how this works.
Anon replyTo means the only thing that can flow back on the current
sockets is the GetQuoteResponse so how does the CS flow?
Anon doesn't allow the GetQuoteResponse to flow back on any other
socket than the one carrying the request. Some people argue that
it
can if we're in the error case (broken socket) and using RM, but what
about the non-error case - socket is still there and waiting for a
response? WSA rules would still need to apply. Or are we suggesting
this proposal changes WSA processing rules?
MG: If this is a true req/resp WSDL then how does the current spec address
this? That the server violate its own published contract at its whim? I
would argue that if you are expecting a reliable response you need to establish
the inbound sequence to handle it before you invoke that operation. There
isn’t a good way to handle the error case otherwise.
DUG: current spec works just fine because we don't use the WSA anon URI
and we clearly define the semantics of the new URI. This proposal
violates WSA's definition of anonymous. Concrete message flows are
needed to prove that this works in your proposal.
- Places burden of creating new sequences on the wrong side.
There are scenarios mentioned in this proposal where the RMD sends a
MC to, in essence, ask for a CS to flow. This is quite a change to
the current RM processing model where its the RMS that is always in
control over when to create new Sequence. Also, this then leads to
the question of how did the RMD know a new Sequence is needed?
MG: In both proposals you have to poll with MC.
DUG: This isn't an answer. Your proposal says there are times when
the client wants a new Sequence for the inbound messages - how does it
know this?
- Client Identification
Following on the previous comment, the proposal says:
When an unreachable client requires a new inbound sequence it MAY
send the MakeConnection header independently to RM service endpoint.
Upon receipt of a MakeConnection header block that the RM Source
cannot relate to an existing sequence it MUST respond with either
a CreateSequence message on the protocol specific back channel
of the request, or with a MakeConnectionRefused fault.
How does the service know if a Sequence already exists? What unique
info is there to compare?
MG: If the MC previously returned identifying ref params it can identify
the sender. There is not an issue here.
DUG: This doesn't solve it. How does the MC return identifying ref-p's
that are specific to a sender when it has no idea if its seen a particular
sender before? There is nothing to distinguish one anon client from
another.
Since these MC's can be delayed in the
network, its possible that many may be sent and thus lots of
CreateSequences may flow - meaning lots of unused Sequences may be
created. See next point.
- Unnecessary Sequences/Message flows.
Since we're talking about RM here we need to think about what happens
in cases where messages are late to the party. In this proposal if
a
rogue MC is received and the AcksTo EPR it pointed to is for a terminated
Sequence then a CS will flow back. This will cause an unused Sequence
on both endpoints and cause the generation of unused MC's that will
continually flow until the Sequence expires. And worse, what if the
CS had an Offer - we then double the number of unused Sequences.
MG: If the AcksTo is for a terminated seq then a fault can always be returned.
This is an implementation detail.
DUG: "Can"? Once a seq is terminate the state data may
be gone - per your proposal the RMD has no choice but to create a new sequence.
Again, this means that lots of unused sequences will be created unless
there's some special magic around that allows the RMD to know more than
what you're specified in your proposal.
- MP sent to unidentified client
The proposal says that a MP can be sent on inbound traffic to the
unreachable endpoint, but it doesn't link the MP header to the
client or to the Sequence so how does the sender of the MP know
whether or not any particular message can carry the MP header.
MG: The MP can be related to the sequence of the RM message it is piggy-backed
on.
DUG: This doens't answer my question so again please show concrete message
flows to prove this works.
- MP & client id again
The proposal says:
In the circumstance where an RM Source wishes to initiate a sequence
with an anonymous client, the RM Source MAY return a MessagePending
header over an existing transport backchannel. The MessagePending
header MAY contain an AcksTo EPR...
How does the RMS know which anon client its talking to?
MG: The MP can be related to the sequence of the RM message it is piggy-backed
on.
DUG: there is no sequence so this non-answer makes no sense.
Misc:
Some pointers to old discussions:
Discussion if i089 at Raleigh f2f:
http://www.oasis-open.org/apps/org/workgroup/ws-rx/download.php/17492/MinutesWSRX-f2f-032306.html#_Toc131830356
Old proposal for i089 but the word doc contains a lot of the rationale
behind the thinking:
http://www.oasis-open.org/apps/org/workgroup/ws-rx/email/archives/200605/msg00123.html
Pointer to i010, which dealt with Sequences spanning multiple endpoints:
http://docs.oasis-open.org/ws-rx/issues/ReliableMessagingIssues.xml#i010
thanks
-Doug
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