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https://issues.apache.org/jira/browse/NIFI-3452?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Koji Kawamura updated NIFI-3452:
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Attachment: wait-for-a-par-of-flow-to-finish.png
> Add Wait processor Wait Mode property
> -------------------------------------
>
> Key: NIFI-3452
> URL: https://issues.apache.org/jira/browse/NIFI-3452
> Project: Apache NiFi
> Issue Type: Improvement
> Components: Extensions
> Affects Versions: 1.2.0
> Reporter: Koji Kawamura
> Assignee: Koji Kawamura
> Attachments: wait-for-a-par-of-flow-to-finish.png
>
>
> NiFi back pressure is handled per relationship and as long as a relationship
> has room to receive more flow files, source processor is scheduled to run.
> However, this behavior is not ideal in some cases. For example, when there is
> very computationally expensive task and user wants to limit the number of
> FlowFiles can be processed at a given time, it's not always possible to limit
> the rate by existing RateControl nor back-pressure mechanism.
> As a more practical example, in the following flow, it's expected the GetSQS
> is triggered only when the previous FlowFile has been processed completely.
> Node 1 is parsing a flow file (indicated by the X in the connection between
> FetchS3Object and Parse). Both connections have a back-pressure threshold of
> 1, but because the object is already fetched, the first connection is empty
> and can thus be filled. This means that, if a new item becomes available in
> the queue, both of the following cases can happen with equal probability:
> {code}
> Case 1:
> ---------- ----------------- ---------
> Node 1: | GetSQS | -X-> | FetchS3Object | -X-> | Parse |
> ---------- ----------------- ---------
> ---------- ----------------- ---------
> Node 2: | GetSQS | ---> | FetchS3Object | ---> | Parse |
> ---------- ----------------- ---------
> Case 2:
> ---------- ----------------- ---------
> Node 1: | GetSQS | ---> | FetchS3Object | -X-> | Parse |
> ---------- ----------------- ---------
> ---------- ----------------- ---------
> Node 2: | GetSQS | -X-> | FetchS3Object | ---> | Parse |
> ---------- ----------------- ---------
> {code}
> To achieve that, we could improve Wait processor as follows.
> NiFi scheduler checks downstream relationship availability, when it's full,
> the processor won't be scheduled to run. In case a source processor has
> multiple outgoing relationships, and if ANY of those is full, the processor
> won't be scheduled.
> (This is how processor scheduling works with back-pressure, but can
> alter with @TriggerWhenAnyDestinationAvailable annotation. DistributeLoad is
> the only processor annotated with this)
> We could use this mechanism to keep the source processor waiting to be
> scheduled, by following flow:
> {code}
> GetSQS
> -- success --> FetchS3Object --> Parse --> Notify
> -- success --> Wait
> {code}
> To make it work as expected, we need to improve Wait so that user can choose
> how waiting FlowFile is handled, from either:
> "Route to 'wait' relationship" or "Keep in the Upstream connection".
> Currently it has only option to route to 'wait'.
> Use "Keep in the Upstream connection" Wait mode with the flow above,
> the incoming flow file in GetSQS -> Wait connection stays there until actual
> data processing finishes and Notify sends a notification signal.
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