Title: RE: [wsrp-coord] Events vs StateChanges
I am
not sure I am following you.
I
think it is OK the say that a consumer (that does events) that receives an event
NS:X MUST forward it to all portlets (in the same context) that subscribe to
event NS:X.
Event
mapping only comes into play when the inbound and outbound events are
different.
The
number of subscribed portlets is really not important here. As I said,
event mapping should not be a part of WSRP, it is just that WSRP should
allow/help portals to do event mapping if they want to, with any mechanism they
choose.
When a
portlet fires an event, he has no knowledge which portlets will receive it. When
a portlet receives an event, it does not know where it came from (at least in
the strict sense, there may be a parameter indicating the source of the event).
The model should be decoupled.
Yossi.
-----Original Message-----
From: Andre Kramer
[mailto:]
Sent: Wednesday, August 06, 2003
2:02 PM
To:
Subject: RE:
[wsrp-coord] Events vs StateChanges
Your
example of a potlet taking in event A and raising event B and a consumer
transforming events may differ when more than one portlet subscribes to event
A. I.e. does a consumer filter all events before they are raised to portlets
or not? That's they type of question we need to address.
regards,
Andre
-----Original Message-----
From: Tamari, Yossi
[mailto:]
Sent: 06 August 2003
11:27
To:
Subject: RE:
[wsrp-coord] Events vs StateChanges
Hi
Andre,
I
am also less comfortable with the exposed state model, but the
"justification" from its proponents is that the portlet chooses which
property it wants to expose for reading or writing, so in effect it is
equivalent to the event model. (each inbound/outbound event can be modeled
as a readOnly/writeOnly property, and vice-versa.)
Event transformation example: Say the race car telecast portlet from
my previous examples exposes an event CarSelected, and passes as parameters
all its information regarding this car (driver info, car model, etc.), in
addition to the car number. On the other hand I have a portlet that displays
driver history, but it does so based on getting the driver name (through an
event). It does not expect all the extra info that the telecast portlet
exposes. Event Mapping enables the consumer (with the help of the page
designer) to extract the driver name from the published event, and send it
to the the driver history details.
Another more rudimentary example: An SAP R/3 sales orders
portlet contains an event CustomerSelected [outbound, CustomerIdType],
which is fired when the displayed sales order changes, so that another SAP R/3 customer
details portlet can synchronize with the displayed sales order. However, a
specific customer uses Siebel to hold its customer details, and has a Siebel
portlet that subscribes to the event DisplayCustomer [inbound,
CustomerIdType]. Because the names of the events are different, without some
kind of consumer mapping, the two portlets cannot coordinate. With a simple
event name mapping the problem is solved.
We
should remember that we cannot prevent event mapping, even if the spec will
says it is disallowed, since any one can create an invisible portlet that
subscribes to event A and sends event B. This portlet could then always send
event B when it receives event A with some parameter manipulation. So event
mapping will happen. Now we should try to make it work
well.
I
think we all agree on having the consumer as event broker for in-band
events. It is not as clear for OOB events, which is another reason to leave
it out of scope.
Yossi.
-----Original Message-----
From: Andre Kramer
[mailto:]
Sent: Wednesday, August 06,
2003 12:48 PM
To:
Subject: RE: [wsrp-coord] Events
vs StateChanges
Yossi,
Thanks for explaining the shared data model.
Still seems to break encapsulation, as the consumer is directly
involved in all interactions.
On event transformations - I'm not uncomfortable, just less
comfortable (would like to see examples).
Both seem to model the consumer as a broker (mediating events or
performing direct writes) so maybe that's our core
pattern?
regards,
Andre
-----Original Message-----
From: Tamari, Yossi
[mailto:]
Sent: 06 August 2003
10:30
To:
Subject:
RE: [wsrp-coord] Events vs StateChanges
Hi Andre,
What Mike is proposing is not that one portlet
can change the state of another, or send an event to another, but that
it can declare its own state has changed, and that enables consumers to
change the state of another portlet when the state of the first portlet
is changed.
(In other words, each portlet exposes a set of
properties, that can be readOnly\writeOnly\both, and the consumer reads
state changes from one portlet and writes them to
another).
Of course, this requires mapping, since there
is absolutely no way for two portlets to communicate without the
consumer knowing which of their properties need to be mapped.
This is another huge advantage of the events model, since if two
portlets agree on the signature of an event, no mapping is
needed.
I do not see why you are uncomfortable with
event mapping. What is wrong in a situation where a consumer decides
that that portlet A exposes an event that is logically the same as an
event that portlet B subscribes to, and decides to do some match-making?
This would need tailoring at the page level by the page designer at the
consumer, and certainly not every portal would support this, and the
spec does not need to specify how it should be done. The model should
simply allow events to have enough metadata to enable
this.
Yossi.
-----Original Message-----
From: Andre Kramer
[mailto:]
Sent: Wednesday, August
06, 2003 11:10 AM
To:
Subject: RE: [wsrp-coord]
Events vs StateChanges
I've got some background in CCS & CSP so am used
to pure signals and very
comfortable with
extending such a (message based) model to carry data
(events (c) in Mike's email). Another common extension is to
add a broker
or event mediation layer
(brokered events - i.e. the consumer routes events).
I'm less comfortable with arbitrary consumer
transformations of events
and would like to
see a proposal. Maybe supporting event sub-typing
to allow a portlet to register for generic sets of events would
cover
most use cases?
I'm very uncomfortable with any direct state based
approach (Portlet A
can update a shared
variable on Portlet B) as this seems to break
encapsulation even more than consumer event transformation.
Allowing
portlet A to force a state transition
on another portlet has similar
encapsulation
problems.
Mike - for (d,e) are you proposing that portletA can
do
portletB.write("variableName",
"value")
or
portletB.goto("stateName", "withData")
or ...?
regards,
Andre
-----Original Message-----
From: Michael Freedman [mailto:]
Sent: 06 August 2003 00:58
To:
Subject: [wsrp-coord] Events vs StateChanges
Last week we discussed the value of defining an
in-band event model vs.
a state changed
model. Here are some further thoughts:
I see the following types of
events that could be dealt with:
a) WSRP events -- these
are well known events that WSRP defines
--
currently these "events" are implicit in our protocol being
explicitly represented as lifecycle calls [and
their
parameters/returns]: i.e. perform
action, render. Examples of such
events we might consider adding are Begin/EndTransaction.
I.e. placing
a series of calls between
portlets within a consistent transaction.
[Important once we start propagating state changes/data across
portlets?]
b) Mappable events --
these are events typically defined by the
producer [though also can be done by the consumer] that the
receiving
portlet is not aware of.
Consumers provide a facility to declare
mappings from one portlets event namespace to anothers.
Yossi indicated
he will provide some examples
of this.
c) Known events -- these are events typically defined by the
producer [though also can be done by the
consumer] that the receiving
portlet is aware
of. No mapping is necessary, consumer merely route the
events to interested parties. Commonly
used within a producer to build
cooperation
within a known set or portlets or by producers building
building-block portlets that it hopes other
producers will include in a
cooperating
set.
d)
Mappable state changes -- probably the most common form of
mappable event where the consumer acts as a
mediator for data exchange
between two
portlets. I.e. an action results in state changes, a
portion of which a portlet wishes to publish
[e.g. bug report number].
The consumer
understands that the portlet "publishes" this data and
supports mapping it to similarly published
state in another portlet.
e) Known state changes -- like
known events, the consumer doesn't
provide any
mapping -- the sending/receiving portlets understand/agree
on the data being exchanged via prior
knowledge/agreement.
I agree with Yossi that Mappable events are a
generalization of mappable
state
changes. However, I am not yet convinced that our in-band
mechansim needs this generalization. For
me there is a developer and
user
cost/complexity to introducing MappableEvents here. As I don't
yet
have an example of a pure event -- one
that indicates no state
change/has no
associated state -- A mappable event leaves the developer
in the quandry of whether to define a new
event for every state change
or whether to
expose the state changes via a common event [e.g.
UpdateModel]. The later has the benefit of having a
single form
allowing the consumer to present a
simplified user interaction -- map
from this
portlets published data to the other ones. When individually
named events are involved the user must first
choose the event they want
to map and then map
the data between the events. For me, this is an
unnecessary complication to the user interaction. Until I
see/understand
the use cases for in-band
mappable events that aren't satisfied by
mappable state changes my preference is to limit our in-band
mechanism
just to mappable state
changes.
As for (c) and (e) -- known events/state changes --
should we care and
if so what is its
priority? Mappable events/state changes is a superset
of the known case -- should we care and if so
is it a priority to
support Consumers who
merely want to deal with distpaching/managing
known events/state changes but not mappable ones?
-Mike-
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