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https://issues.apache.org/jira/browse/SLING-8854?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16981430#comment-16981430
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Ashish Chopra commented on SLING-8854:
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bq. We may be able to do it without extending the {{DistributionQueue}} SPI.
How about keeping the status map in the {{SimpleDistributionQueueProvider}} and
pass the status map to {{SimpleDistributionQueue}} and
{{SimpleDistributionQueueProcessor}}.
Thanks for reviewing the approach [~marett]!
I agree that the mechanism would work for _current_ implementation of
{{SimpleDistributionQueue}} - currently, the status is stored in a transient
data-structure which isn't dependent on {{DistributionQueue}}'s implementation.
However, there may be a scenario where updating an item-dequeue-status depends
on queue-implementation (e.g., incrementing a column in the table-entry, if the
queue is implemented as a db-table). In such a case, the implementation detail
of the queue will have to be 'exposed' to a {{DistributionQueueProcessor}} so
it can update the status. (the same detail will need to be exposed to
{{DistributionQueueProvider}} as well, but exposing that to a queue-provider
may be OK).
I think the trouble arises from the fact that (unlike typical queue
interfaces), {{DistributionQueue}} doesn't have a notion of distinct operations
for "peek" and "dequeue". Instead, it has a {{getHead}} operation, which (based
on the intent of the caller) may not always map to "processing a queue entry".
IMHO either {{DistribitionQueue}} should provide distinct "peek"/"dequeue"
operations, or have a way for the caller to express the "intent" (i.e., what it
intends to do with the item obtained from the queue's head).
In my proposal, I went with the latter since that has the least chances of
breaking existing implementations. Do let me know your views on this.
> In-file and In-memory queue-providers for Forward Distribution report
> incorrect value for processing "Attempts"
> ---------------------------------------------------------------------------------------------------------------
>
> Key: SLING-8854
> URL: https://issues.apache.org/jira/browse/SLING-8854
> Project: Sling
> Issue Type: Bug
> Components: Content Distribution
> Affects Versions: Content Distribution Core 0.4.0
> Reporter: Ashish Chopra
> Assignee: Timothee Maret
> Priority: Major
> Attachments:
> 0001-SLING-8854-In-file-and-In-memory-queue-providers-for.patch
>
>
> {{ForwardDistributionAgentFactory}} allows specifying "In-Memory" and
> "In-File" queue-providers [0]. Both these implementations are provided by
> {{SimpleDistributionQueue}} [1].
> While responding to a queue-processing request, the head of the queue is
> fetched to create a {{DistributionQueueEntry}}, which encapsulates a
> {{DistributionQueueItemStatus}}. While retrieving the queue-head-entry, the
> number-of-attempts are also incremented by 1, implying _one more_
> processing-attempt to have happened on a given queue-item. [2]
> The trouble arises from the fact that {{SimpleDistributionQueue#getHead}}
> ends up being called from at least three other places:
> # Queue status Health Check [3], which is a periodic job
> # UI (via {{ExtendedDistributionServiceResourceProvider}}) [4]
> # Distribution Queue status MBean [5]
> Out of these, #1 is most troublesome - because of its periodic nature, the
> number of Attempts for a queue-item that's being processed (or, is blocked)
> will keep on increasing without corresponding dequeue-attempts.
> Due to #2, everytime the queue status is delivered to the UI, the attempt
> would again increase. Similar would be the case of MBean invocations.
> This needs to be addressed. Notably, "Sling Jobs" based queue doesn't suffer
> from this issue because it relies on retry-counts as maintained by Sling Job
> Execution Engine.
> [0]
> https://github.com/apache/sling-org-apache-sling-distribution-core/blob/deb3d2ae33c4f4678c8503091a9fffdbb141e569/src/main/java/org/apache/sling/distribution/agent/impl/ForwardDistributionAgentFactory.java#L258-L262
> [1]
> https://github.com/apache/sling-org-apache-sling-distribution-core/blob/deb3d2ae33c4f4678c8503091a9fffdbb141e569/src/main/java/org/apache/sling/distribution/queue/impl/simple/SimpleDistributionQueue.java
> [2]
> https://github.com/apache/sling-org-apache-sling-distribution-core/blob/deb3d2ae33c4f4678c8503091a9fffdbb141e569/src/main/java/org/apache/sling/distribution/queue/impl/simple/SimpleDistributionQueue.java#L110-L112
> [3]
> https://github.com/apache/sling-org-apache-sling-distribution-core/blob/deb3d2ae33c4f4678c8503091a9fffdbb141e569/src/main/java/org/apache/sling/distribution/monitor/DistributionQueueHealthCheck.java#L113
> [4]
> https://github.com/apache/sling-org-apache-sling-distribution-core/blob/deb3d2ae33c4f4678c8503091a9fffdbb141e569/src/main/java/org/apache/sling/distribution/resources/impl/ExtendedDistributionServiceResourceProvider.java#L158
> [5]
> https://github.com/apache/sling-org-apache-sling-distribution-core/blob/deb3d2ae33c4f4678c8503091a9fffdbb141e569/src/main/java/org/apache/sling/distribution/monitor/impl/DistributionQueueMBeanImpl.java#L62-L91
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