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Subject: RE: [wsn] Subscribe and GetCurrentMessage
I think this is a good approach in general.
However if we start down this route then I think there is a need for a
more general 'notification style' request. A subscriber might want
snapshot + update another before and after looks etc?
There are also policy implications as to which styles an event source
can support / offer?
I worry a bit that this is starting to overlap into event message design
which should be left to the service?
Chris Hipson
Web Service Technology Consultant
-----Original Message-----
From: Peter Niblett [mailto:peter_niblett@uk.ibm.com]
Sent: 06 December 2004 15:15
To: 'wsn-oasis'
Subject: RE: [wsn] Subscribe and GetCurrentMessage
I think that we should avoid solutions that rely on sequence numbers.
They
come under the heading of Reliable Messaging, and as such are out of
scope.
A "policy" option in the Subscribe Request that says "send me the
snapshot
first before any updates" seems by far the cleanest way of avoiding the
race condition
Some questions for us to discuss on today's call:
1. Is this a good approach?
2. Should we standardise this policy, or leave it to be a vendor
extension?
Leaving it as an extension simplifies our work.. but is this something
that
will be needed by WSRF RP?
3. Who decides which messages are snapshots and which are events? I
assume
that in the direct notification case, this is the job of the NP. However
in
Brokered Notification, how does a broker know which messages need to be
retained as snapshots? Is this another vendor extension?
4. If we have this policy option, do we still need getCurrentMessage?
Two
possible reasons for it
a) If a consumer crashes before it had a chance to save its update, it
can retrieve the latest update, without having to create a new
subscription
b) It allows entities other than the NC to receive an update (if they
have access to the SubManager EPR)
Peter Niblett
"Martin Chapman"
<martin.chapman@o
racle.com>
To
"'Lily Liu'"
03/12/2004 20:28 <lily.liu@webmethods.com>,
"'David
Hull'" <dmh@tibco.com>
cc
"'wsn-oasis'"
<wsn@lists.oasis-open.org>
Subject
RE: [wsn] Subscribe and
GetCurrentMessage
+1
-----Original Message-----
From: Lily Liu [mailto:lily.liu@webmethods.com]
Sent: 03 December 2004 20:11
To: 'David Hull'
Cc: wsn-oasis
Subject: RE: [wsn] Subscribe and GetCurrentMessage
SubscribePolicy seems to be the cleanest approach to me also. Although
the
snapshot/update case is a very valid use scenario, I think we should
leave
that policy definition to vendor extensions instead of covering it in
the
WSN policy spec.
Lily
-----Original Message-----
From: David Hull [mailto:dmh@tibco.com]
Sent: Wednesday, December 01, 2004 9:59 AM
Cc: wsn-oasis
Subject: Re: [wsn] Subscribe and GetCurrentMessage
I would say that the problem can be solved by telling the NP directly
to
send the appropriate form of [snapshot][update*] directly to the
consumer,
and this can be done within the existing framework several ways:
Some utterance in SubscribePolicy
Some special flag on the topic (which might break commutativity)
Some special element in <filter> (ditto)
New attributes on SubscribeRequest (which is disruptive to the
current spec and may raise issues with WSN)
Magically named topics (yuck)
Further, it can't be easily solved without telling the NP directly what
to
send. The best we can do involves sequence numbers and queuing (and
the
NP must keep track of the sequence numbers itself).
Of the choices above, SubscribePolicy seems the cleanest approach. If
so,
I would like to see something on it in the Policy document.
Samuel Meder wrote:
On Tue, 2004-11-30 at 16:15 -0500, David Hull wrote:
I think part of the confusion is that we've managed to
solve
part of
the problem over the course of the discussion.
There is a very basic race condition to avoid. We have to
be
sure
that the NP doesn't get a snapshot request until after the
subscription for updates has arrived. We can get around
that
by
having the Subscriber subscribe first, and only make the
snapshot
request after it receives a response from the subscription.
We then
only have to assume that the NP will not send back a
subscription
response until after it has established the subscription.
This seems
reasonable, but we should talk about it explicitly -- a
brokered or
indirect implementation may not be able to make quite that
guarantee.
Given this, we have to make sure the consumer knows which
updates are
applicable. This is solved by putting a sequence number in
each
update, and making sure that each snapshot contains the
sequence
number of the update the most current update. The consumer
then
queues updates (per topic) until it receives a snapshot,
and
discards
those that predate the snapshot. We assume that the
consumer
knows to
do this, presumably by some out-of-band communication
between
it and
the Subscriber.
The scenario I described, of not being able to tell late
updates from
no updates, is only relevant if the Consumer doesn't trust
the
Subscriber to do things in the right order. If the
Consumer
knows
that the Subscriber doesn't ask for the snapshot until it
gets
a valid
SubscribeResponse, then the arrival of a snapshot
guarantees
that any
relevant updates will arrive (possibly preceded by some
irrelevant
ones).
However, there's still at least one remaining question: How
does the
subscriber know the snapshot arrived? The NP doesn't
necessarily
know, so there would have to be some positive
acknowledgment
from the
Consumer to the Subscriber. Otherwise, the Subscriber
might
have to
retry, and the Consumer would have to ignore duplicate
snapshots.
I think you'd have the same problem with any of the update
messages
as
well, although I agree that sending the snapshots and the updates
over
the same channel makes things easier.
So to make this all work we need to:
* Introduce special "snapshot" topics.
* Require sequence numbers in data to be handled this
way
* Require the consumer to queue updates
* Require the consumer and subscriber to coordinate
to
make sure
everything went smoothly.
* Require the subscriber to follow a strict order in
establishing subscriptions.
Or, we could just tell the NP in one operation to send
snapshot
followed by updates. Then we just need a reliable
connection
between
the NP and the Consumer. This seems simpler.
Ok, I agree. I think we agree that this can be accomplished with
the
specifications as they are (by either associating special
semantics
with
certain topics or asking for the semantics in the subscribe
policy)?
/Sam
I have no doubt that a sequence number/queuing pattern
would
be needed
behinds the scenes in some brokered or indirect
implementations, but I
don't believe that it needs to become visible to
garden-variety
Subscribers and Consumers.
Samuel Meder wrote:
On Tue, 2004-11-30 at 14:28 -0500, David Hull wrote:
How does the consumer tell the difference
between
* Snapshot arrived ahead of first update.
* There is no first update because
there's
no traffic.
Thinking about it, I'm pretty sure we still have a
misunderstanding/unstated assumption somewhere
(either
that or I am
being particularly dense). In any case here is what I
was going to reply
with:
There is no way to tell the difference, but is the
difference really
detectable in any scenario? There is always going to
be
some time
between the state change on the service side and the
consumer being
aware of that state change, correct? This basically
means that a
consumer can never tell whether the state it is
currently aware of is
really up to date, ie there is no way of telling
whether
a state change
notification has happened and the notification is in
transit or if no
state change has occurred.
I'm guessing that this is not what you were trying to
get at though?
/Sam
Samuel Meder wrote:
On Tue, 2004-11-30 at 13:37 -0500, David
Hull wrote:
Comments in-line.
Samuel Meder wrote:
On Mon, 2004-11-29 at 11:35
-0500, David Hull wrote:
Could you work through
in
detail how this would
happen?
I believe we're up
against
an egg-unscrambling
problem here. On the
one hand, there will be
cases when the NP knows
exactly which updates
came after which
snapshot.
A common case would be
an
NP which locks
the database for
updates
during a snapshot. It
will be able to tell
which updates came in
before the lock and
which
came after. It can
therefore easily send
the
snapshot to the
consumer,
followed by
exactly the right
updates.
Here are the problems I
see under the current
setup:
* There is no way
to
get a snapshot, per se.
If the updates are
incremental,
which
they may legitimately
be,
then "last
message" and
"current state" are two
different things. If I
understand your
proposal, you're saying
that there could be
parallel sets
of
"current state" and
"update" topics. Fair
enough.
* The result of
getCurrentMessage goes
to
the subscriber, not to
the consumer.
The
behavior we want is for
the consumer to get
the snapshot
followed by (all and
only)
the relevant updates.
Under the
current
setup, either the
subscriber would have
to
forward the
result
to the subscriber, or
the
consumer would
have to make
the
getCurrentMessage call
directly (perhaps
having learned
the
NPs address from a
previous update,
assuming one
has
come in).
As far as I can tell there
are
multiple ways of doing this:
1) Make use of the
WS-Addressing
Reply-To feature to redirect
the
result
of a getCurrentMessage to the
consumer
This does not eliminate the race
condition. At the very least, both
update and snapshot must be
sequence
numbered, the consumer must be
prepared to queue updates until the
snapshot arrives, and the consumer
does not know for sure that the
snapshot is valid with respect to
the
updates until the first update
arrives.
In short, the subscriber and
consumer
have to collaborate to recover
information that the NP knows, but
cannot give out directly under the
current protocol.
2) As you mention, the
consumer
could obtain the NP address
via
some
(out-of-band?) means
This only solves part of the
problem.
The comparison is between:
* Subscriber tells NP: Send
this
consumer a snapshot of the
current state we're
interested
in, followed by all and only
those updates after the
snapshot. We're done.
and multi-part solutions in which
the
Subscriber and Consumer must
cooperate to figure out which
updates
to discard and to verify that
the whole process worked correctly
at
all. There is a qualitative
difference between these two
approaches.
With all due respect, I have not
yet
heard from anyone a detailed and
provably correct procedure for
eliminating this race condition,
using
any combination of the existing
BaseN
primitives and any composable
mechanism for transactionality or
such. I hear "you could do it
with . . .", but I don't hear how
you
could do it. I believe I have
given detailed reasons why such a
solution probably doesn't exist.
Even if it did exist, it would
suffer
from too many connections
between too many moving parts.
Maybe I'm just not seeing the problem.
Assume both the snapshots and the
updates contain a related sequence
number,
what I mean by that is that
the sequence number for the latest
snapshot
is equal to the sequence
number in the latest update. This also
assumes that snapshots are always
up to date with respect to updates.
Where is the race condition in the
following
sequence:
1) subscriber subscribes consumer to
update
topic
2) subscriber does a getCurrentMessage
with
a redirect to consumer (for
example, any other methods for getting
the
current state to the consumer
should work).
The consumer should buffer any updates
until
it gets the current state
and can then discard any updates with
sequence numbers < sequence number
in current state.
It's quite possible that I will soon have
a
embarrassing a-ha
experience, but I currently don't see
anything racy in that.
What we need is a way to just ask
the
NP for what we want up front.
One way would be to bake "snapshot"
and "update" modifiers directly
into the Subscribe request, but
this
is not the only way.
3) You could make the
semantics
of the "current state" topic
such that a
subscription to it will
trigger
a single notification of the
current
state (ok, I admit that this
is
really stretching it).
That would provide for a snapshot
(as
opposed to current message as
such). It doesn't solve the race
condition directly, but a variant
would. We could always allow for
modifiers in the topic expression.
That is, you could subscribe to
"Foo",
or to "Foo(snapshot)" or to
"Foo(snapshot-and-updates)". If we
go
that way, we should consider
having the modifiers be an explicit
part of the topic expression, and
not baked into topic names in some
arbitrary way.
Another option would be to put
snapshot/update modifiers in the
"subscribe policy" open content.
Right, as a sort of QoS qualifier for the
subscription. That would make
sense to me as well.
/Sam
In all three cases, the NP should
be
able to advertise what it
supports, and MUST fault on a
request
for something it doesn't
support.
As long as all the
* It is decidedly
non-trivial to handle
races between the
snapshot and
the
update stream. At the
minimum, both need to
be tagged with
timestamps (or better,
sequence numbers).
But
even this
doesn't
seem sufficient,
particularly since I
don't
know whether
the
snapshot is in sync
with
the updates until I
actually get an
update, which could be
arbitrarily long.
One way to deal with this is
to
subscribe for updates and
then
get the
snapshot (I am assuming that
the
snapshot kept by the NP is
always up to
date wrt to updates,
otherwise
this whole thing would not be
workable
unless you had some way to
pull
past updates as well).
In short, we need a way
of
explicitly associating
the
snapshot request
with the request for
updates and making sure
they both go to the
same
endpoint, in the right
order. We would like
to
do this in a way that
requires no processing
by
the consumer. One of
the
key features of
WSN is that consumers
can
be dumb. The consumer
should not have to
buffer updates and
check
sequence numbers.
I think that consumers at
some
level (and this may be at the
framework
level) will have to worry
about
sequencing in any scenario
that
involves
updates and asynchronous
messaging.
/Sam
It should not have to
make the update
subscription itself.
And,
as
far as I can tell,
there
is no need for it to do
so, at least in the
plausible case that the
NP
already knows how to
provide exactly the
right information.
Samuel Meder wrote:
Another way of
doing
this that does
not
require WSRF-RP
is
to model your
topics in a way
that
allows for this:
A
top level topic
for
the whole
document (which
you
can call
getCurrentMessage()
on) and sub-topic
for
fields in the
document (which
you
would subscribe
to),
or even just a
single "update"
sub-topic. That
in
combination with
appropriate use
of
timestamps should
be
able to address
your
problem.
/Sam
On Tue,
2004-11-23
at 16:43 -0500,
David Hull wrote:
Steve
Graham
wrote:
David
Hull
<dmh@tibco.com>
wrote
on
11/22/2004
04:48:34 PM:
One
useful pub/sub paradigm involves the concept of notifications
as
updates
to
some
collection of state.
Indeed,
this
is
what
WSRF-Resource
Properties suggests.
In
such
cases, it is useful to be able to take a
snapshot
of the state, then be notified of updates to that state.
Agreed.
GetResourcePropertyDocument,
followed by a subscribe
operation
specifying
which
Resource
properties
(bits of state) the consumer should
receive
value
change
notifications.
Minus
whatever
changes
happened
between the
get and the
subscribe.
Or
plus
whatever
changes
happened
between the
subscribe
and
the get.
In
any case,
we
want to be
able to
cover
snapshot/update
scenarios where
the state
is
not
presented
as a
WS-Resource.
Or at least
I
would like
that.
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