On 06/28/2011 09:57 AM, Marnie McCormack wrote:
In JMS, the sequence id would be used to maintain order, but the expectation
is that the group are processed as a whole on receipt of the final message
in the group rather than simply shared out between consumers and processed
in order.
I think that is a third variation (further restriction really) on the
use case. I.e. where message group is used to piece together a large
unit from discrete messages.
So I think the requirements are:
(1) message group must always be processed in order
(2) all messages in a group are processed by the same consumer
(3) the consumer will only complete processing having received the
'final' message in the group
Where (3) implies (2) and (2) implies (1).
The third requirement essentially rules out an implementation of the
first requirement that refuses to deliver the next message in the group
until the previous message is accepted.
The second requirement is important where there is a desire to contain
all the necessary state for processing a group of messages within a
single consumer. I agree that is important and should be supported.
However I think it is not always required and it should be possible to
drop that if not needed as that could lead to more adaptive
load-balancing while meeting the first requirement.
So I think the configuration involved is:
(a) identification of the header that signifies a group within a given
queue, which implicitly enables requirement (1)
(b) an indication that 'stickiness' of group to consumer is required
(beyond simply meeting the first requirement)
I think we can avoid the need for any explicit configuration of
requirement (3). Refusing to deliver the next message in the group until
the previous message is accepted is in my view unnecessary to meet (1)
and could be emulated more or less by controlling prefetch.
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