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https://issues.apache.org/jira/browse/STORM-1949?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15371055#comment-15371055
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Abhishek Agarwal commented on STORM-1949:
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ok. my understanding was that spout will continue emitting new tuples even if
consumer is not consuming fast enough. But that won't happen because spout
doesn't emit further if transfer queue is full -
https://github.com/apache/storm/blob/1.x-branch/storm-core/src/clj/org/apache/storm/daemon/executor.clj#L647
[~kabhwan] doesn't this address your concern?
> Backpressure can cause spout to stop emitting and stall topology
> ----------------------------------------------------------------
>
> Key: STORM-1949
> URL: https://issues.apache.org/jira/browse/STORM-1949
> Project: Apache Storm
> Issue Type: Bug
> Reporter: Roshan Naik
>
> Problem can be reproduced by this [Word count
> topology|https://github.com/hortonworks/storm/blob/perftopos1.x/examples/storm-starter/src/jvm/org/apache/storm/starter/perf/FileReadWordCountTopo.java]
> within a IDE.
> I ran it with 1 spout instance, 2 splitter bolt instances, 2 counter bolt
> instances.
> The problem is more easily reproduced with WC topology as it causes an
> explosion of tuples due to splitting a sentence tuple into word tuples. As
> the bolts have to process more tuples than the spout is producing, spout
> needs to operate slower.
> The amount of time it takes for the topology to stall can vary.. but
> typically under 10 mins.
> *My theory:* I suspect there is a race condition in the way ZK is being
> utilized to enable/disable back pressure. When congested (i.e pressure
> exceeds high water mark), the bolt's worker records this congested situation
> in ZK by creating a node. Once the congestion is reduced below the low water
> mark, it deletes this node.
> The spout's worker has setup a watch on the parent node, expecting a callback
> whenever there is change in the child nodes. On receiving the callback the
> spout's worker lists the parent node to check if there are 0 or more child
> nodes.... it is essentially trying to figure out the nature of state change
> in ZK to determine whether to throttle or not. Subsequently it setsup
> another watch in ZK to keep an eye on future changes.
> When there are multiple bolts, there can be rapid creation/deletion of these
> ZK nodes. Between the time the worker receives a callback and sets up the
> next watch.. many changes may have undergone in ZK which will go unnoticed by
> the spout.
> The condition that the bolts are no longer congested may not get noticed as a
> result of this.
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