Remove the role of the Sequence Size for the aggregators that are not relying on the default SequenceSizeReleaseStrategy [INT-2134]

[spring-operator]

Oleg Zhurakousky opened INT-2134 and commented

Currently, when user specifies custom ReleaseStrategy there is still a call to MessageGroup.isComplete which relies on 'sequenceSize' header. However this call is only relevant if the default SequenceSizeReleaseStrategy is used.

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Affects: 2.1 M1

This issue is a sub-task of #6135

Issue Links:

• Auto-completion of the Message group and race condition between the Reaper and discard-channel [INT-2014] #5999 Auto-completion of the Message group and race condition between the Reaper and discard-channel

[spring-operator]

Oleg Zhurakousky commented

While working on it it seems like since Completion and Release is the same thing when it comes to the aggregator, there is really no use case for when canRelease() may return false, but isComplete() may return true, therefore there seem to e no need for the discard channel.

[spring-operator]

Mark Fisher commented

We do still need the discard channel, but it should be used for a Message Group that has already completed (which is the same as has already released messages). As described in EIP, there are times when you want to be sure that a Message arriving later than its group has been "closed-out" does not create a new group. Therefore, to support those use-cases with Aggregator we need to have the capability of remembering the MessageGroup (keeping it in the store) after completion - until it is expired (either by the reaper or explicitly).

Two examples where we need to remember the group after completion (until expired):

1. completion condition: N messages have arrived and we do not want a new group created for that same correlation key after the initial one
2. completion condition: first message that meets a certain threshold/criteria (but ignore any that arrive later with the same correlation key)

There are two other categories of use case where we do not need to remember MessageGroups (i.e. they can be expired immediately upon completion):

1. those where no "late" messages are expected
2. those where a new MessageGroup should always be created when a "first" message arrives

The SequenceSizeReleaseStrategy is an example of #1 since it only "completes" when the entire sequence has arrived (even though it is possible to release the partial/incomplete group if it is expired BEFORE completion - that is a different behavior).

The cyclical batch case (aggregate every N messages) is an example of #2.