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From "Karan Mehta (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11583) Parent spans not initialized to NullScope for every DFSPacket
Date Mon, 27 Mar 2017 20:24:41 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11583?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15943979#comment-15943979

Karan Mehta commented on HDFS-11583:

Since a single {{DFSQueue}} of a {{DataStreamer}} thread can encounter packets that may or
may not have tracing enabled, a simple fix would be to initialize the tracing scope to {{NullScope.INSTANCE}}
for every loop iteration. The scope can start a new {{dataStreamer}} scope when it finds the
{{parents}} field of {{DFSPacket}} to be not empty.

> Parent spans not initialized to NullScope for every DFSPacket
> -------------------------------------------------------------
>                 Key: HDFS-11583
>                 URL: https://issues.apache.org/jira/browse/HDFS-11583
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: tracing
>            Reporter: Karan Mehta
> The issue was found while working with PHOENIX-3752.
> Each packet received by the {{run()}} method of {{DataStreamer}} class, uses the {{parents}}
field of the {{DFSPacket}} to create a new {{dataStreamer}} span, which in turn creates a
{{writeTo}} span as its child span. The parents field is initialized when the packet is added
to the {{dataQueue}} and the value is initialized from the {{ThreadLocal}}. This is how HTrace
handles spans. 
> A {{TraceScope}} is created and initialized to {{NullScope}} before the loop which runs
till the point when the stream is closed. 
> Consider the following scenario, when the {{dataQueue}} contains multiple packets, only
the first of which has a tracing enabled. The scope is initialized to the {{dataStreamer}}
scope and a {{writeTo}} span is created as its child, which gets closed once the packet is
sent out to a remote datanode. Before {{writeTo}} span is started, the {{dataStreamer}} scope
is detached. So calling the close method on it doesn't do anything at the end of loop. 
> The second iteration will be using the stale value of the {{scope}} variable with a DFSPacket
on which tracing is not enabled. This results in generation of an orphan {{writeTo}} spans
which are being delivered to the {{SpanReceiver}} as registered in the TraceFramework. This
may result in unlimited number of spans being generated and sent out to receiver. 

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