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https://issues.apache.org/jira/browse/STORM-1949?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15368451#comment-15368451
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Roshan Naik commented on STORM-1949:
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[~revans2] Not sure what you mean by "back on the write" .. are u saying have
background thread that simply polls ZK every so often ? That might fix this
issue.
However, there is one basic issue with this BP mechanism in general. Its can
put too much load on ZK. For each enable/disable throttle signal raised by any
worker we have all this interaction going on with ZK..
- Some worker adds/deletes ZK node
- ZK issues callbacks to all workers with watches setup
- All those workers will list the parent node in ZK to count the number of
children (expensive?)
- All those workers will setup another watch in ZK
Given that PaceMaker was introduced to take load off of ZK... this approach
feels like a regression. There are some other issues as well but thats for a
different JIRA.
After reviewing BP, I feel it is not mature enough to be considered stable and
ready for production.
IMO Until we have a more solid BP mechanism we should disable it by default as
soon as possible. I can open another jira for that.
> 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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