Iwasa,
Good that you have started the discussion on this topic.
I was also planning to send a mail just before I saw yours.
So instead I'm replying to your email.
Let me express my concern and Oracle's position on this.
Unlike Bob's suggestion, we do want ExpiryTime for the
reasons you have mentioned too, i.e., for persistence
primarily and for DuplicateElimination as a fallout.
However, we wanted the semantics of ExpiryTime be
what it means, i.e., this message will expire after this
timestamp.
Just to recap, we have 3 options as Jacques stated on the call.
1) ExpiryTime will have the same semantics of TTL in
our previous version.
2) His current proposal on Rel 50 dated Fri, 7 Nov 2003
19:20:08 -0800
3) No ExpiryTime at all.
So, our preference is (1) and I see some discrepancy in the
usage of it
in different current proposals. Mainly, I see 2 inconsistencies:
1) The semantics of ExpiryTime in overloaded.
In Rel 50, we are saying that ExpiryTime is
nothing but
RejectMeIfYouReceiveMeAfterThisTime, i.e.,
this is to
reject it it was delayed at the transport
layer and nothing
to do with it's life span once received.
However, in Rel 52, for the termination conditions
t2 and t4,
we are using max of ExpiryTime of individual
messages to figure out
the GroupExpiryTime if the latter is not present.
So here we are using the
real semantics of ExpiryTime and not RejectMeIfYouReceiveMeAfterThisTime.
If it is for only to check the transport lag
only, why are we using here then?
This is my primary concern.
2) DuplicateElimination in Singleton/Group-Unordered is different
than DuplicateElimination in a Group-Ordered
case.
Here is the example:
Say you are in Group-Unordered case.
Msg1 with Group Id 1 and Seq No 0 is
sent at 12 noon with Msg ExpiryTime
at 4.00 pm and GroupExpiryTime at 10.00 pm.
Msg2 with Group Id 1 and Seq No 1 is
sent at 12.01 noon with Msg ExpiryTime
at 8.00 pm and GroupExpiryTime at 10.00 pm.
Msg3 with Group Id 1 and Seq No 2 is
sent at 12.02 noon with Msg ExpiryTime
at 4.00 pm and GroupExpiryTime at 10.00 pm.
Assume the Msg3 was received at 1.00 pm and
Msg 2 was not received. Since
this is a Group-UnOrdered case, the Msg will
be acked, persisted and made
available to the user.
At 4.01 pm, the Msg3 is removed from
the persistence store.
Assume somehow the Receiver receives the same
message (Msg3) at 5.00 pm
with a different ExpiryTime say 6.00 pm. Obviously
this won't be discarded, rather
accepted. [Note that we had Rel 38 that says
Tiemstamps have to be the
same during retries -First of all, Timestamp
is gone, and secondly, we don't say any
thing about ExpiryTime during Retries, so
Retries can have different ExpiryTimes].
Similar scenario will apply to Singleton case
too.
Take the same case except that assume it is
now ordered.
When Msg 3 arrives at 1..0 pm, it will be
acked, persisted and buffered (at the RMP
layer itself) since Msg 2 doesn't arrive.
At 4.01 pm, the Msg3 won't be deleted from
the persistence cache as per the new
proposal (since the GroupExpiryTime is 10.00
pm).
And when a retried Msg 3 arrives at 5.00 pm
with ExpiryTime, it will rejected as
a duplicate as the previous Msg3 still exists
in the persistence cache.
So essentially there is a difference in behavior
of duplicates depending upon whether
it is Singleton, Group-UnOrdered or Group
Ordered.
These inconsistencies that trouble me.
So lets take a step back and see what are the issues with the
actual TimeToLive semantics.
1) Periodic checking for Msg expires: As I said on the call,
we could solve this by having the
checks only twice. Once during "acceptance"
stage and other during "make it available" stage.
2) Sender won't know whether a Msg. was really made it available
to the User.
As we agreed at the F2F (on thursday),
the way to solve this is by saying that
Sender will only assume that a msg.
was really made it available is when
i) It's not expired
AND
ii) It was acked
AND
iii) For a Group ordered
or un-ordered case, the Group is not terminated
AND
iii) And in a group-ordered
case, all preceding messages (the ones with lower seq. nos.)
are acked.
Note that just saying getting an
ack doesn't always mean that the msg was made
available as they are cases such
as Group being expired, Persistence store failure etc..
Since we don't have async fault
notifications, we either assume rules such as above
or use Poll for all the reply
pattern cases as Tom suggested.
As I said on the call the new proposal does make implementations
simpler, but I'm just
"little" bit worried about the inconsistencies.
-Sunil
Iwasa wrote:
I
like this proposal. I
would like to reply some questions raised in thetelecon
today, which is "What is the fundamentalfunction
for ExpiryTime?". The
fundamental function for ExpiryTime, I believe, isto
enable receiver check duplicate message(s) even ifit
removes old message information. When
we don't want to enforce receiver keepall
previously received message inforamtion forever,we
need ExpiryTime. And it should not be changedwhen
the message is re-sent. In
this case, receiver have to keep old messageinformation
until at least the ExpiryTime.So
it is possible to check duplicate when theduplicate
message arrived before it's ExpiryTime. And
after the ExpiryTime has passed, receivermay
remove the message information.In
that case, receiver do not have to do duplicatecheck
for the message, since receiver just needto
send back Fault message to let the senderknow
about expiration of the message. That
would be a critical reason we need ExpiryTimeas
mandatory element, And
I see Sunil's concern, which is meaningful for brokensequence
use case. Let me clarify the use case thatSunil
mentioned. Use
case1:
Sender sent four messages with ordered delivery:
GroupId=x.y.z SequeceNumber=0
GroupId=x.y.z SequeceNumber=1
GroupId=x.y.z SequeceNumber=2
GroupId=x.y.z SequeceNumber=3
Receiver received three messages:
GroupId=x.y.z SequeceNumber=0
GroupId=x.y.z SequeceNumber=1
GroupId=x.y.z SequeceNumber=3
When receiver did not received SeqNum=2 message
forever, receiver have to keep SeqNum=3 message
forever, even if the ExpiryTime is expired,
if the message arrived at receiver by the ExpiryTime. I
believe this is Sunil's concern, isn't it? How
bout the following resolution: Resolution
1:- Adopt Jacques's
proposal for semantics of ExpiryTime as is.-
We clarify special error case only for OrderedDelivery like:
In case of one or more message(s) was/were missing in a
sequence, and the next message of the missing message
was expired, the receiver may remove all the following
messages. This
resolution doesn't change the semantics of ExpiryTimethat
Jacques proposed, but solves problem Sunil raised. I
do see problems if we change the sematics of ExpiryTimeitself,
since it would be affect to non ordered messages.The
use case is: Use
case 2:
1. Sender sent one message without ordered delivery,
and the ExpiryTime = 12.00.00
2. Receiver received the message at 11:59:59.
And sent back Ack to the sender.
3. Before the receiver give it to the application,
time was changed to 12.00.01. And receiver
considered it was expired, and removed it.I
don't believe this makes sence, and it would behappen
when we allow to remove any messagewhich
was expired even after the receiver receivedit
in time, and sent back Ack. In
conclution, I believe resolution1 abovesolves
all problems mentioned. Thanks, Iwasa
----- Original Message -----
From:
Jacques
Durand
To: ''
Sent: Saturday, November 08,
2003 12:20 PM
Subject: [wsrm] Proposed resolution
for Rel 50
Rel 50: Semantics of ExpiryTime:
-------------------------------
Proposal:
ExpiryTime indicates the ultimate
date after which the receiver RMP will
not accept a message. In order
to be accepted by an RMP, a message
must have schema-valid RM headers,
and must not have expired.
Only accepted messages are subject
to further processing by the RMP.
When receiving a message after
its ExpiryTime date, an RMP MUST NOT accept the message.
Once a message has been accepted
by the RMP, the fact that the message expires while
being processed by the RMP MUST
NOT affect its processing, nor its final availability
status to the receiver application.
Such expired messages must still be delivered to
the application, assuming they
satisfy other reliability requirements.
NOTE:
Although ExpiryTime will not
affect further the processing of accepted messages,
it is useful for an RMP to remember
it in order to avoid unnecessary duplicate checks
with future messages, when "at
most once" delivery is required.
Given the above definition of
ExpiryTime, it is impossible that a received message M
accepted by the RMP, is a duplicate
of past messages that are expired at the time M
was accepted.
