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https://issues.apache.org/jira/browse/SOLR-4449?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15053665#comment-15053665
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Jeff Wartes commented on SOLR-4449:
-----------------------------------
I looked around a bit, and unless I'm missing something, it looks like
solr-core doesn't really use metrics-core. At the end of SOLR-1972, the
necessary classes were just copy/pasted into the solr codeline. It sounds like
this was mostly due to being nervous after encountering some problems in the
metrics-core version at the time, and an aversion to a global registry
approach.
Unfortunately, this means that although requesthandlers have statistics, they
cannot be attached to a metrics Reporter, and instead you have to develop
something to interrogate JMX or some such.
Solr does include metrics-core 3.0.1, but there's only a few places it actually
gets used, and only in contrib modules.
I didn't have the negative experience with metrics-core. In fact, my
experiences with 3.1.2 over the last year and a half has been universally
positive. So when I added backup-percentile support to this issue I relied
heavily on the global SharedMetricsRegistry and the assumption that the library
was threadsafe in general. My scattershot code reviews of the metrics library
have generally enforced my opinion that this is ok, and I'm using my version of
this issue in production now. Initializing a well-known-named shared registry
with an attached reporter in jetty.xml has yielded all kinds of great
performance data.
This might be a useful point of information if anyone gets back to SOLR-4735.
I'll mention here if I do encounter any metrics-core related issues in the
future.
> Enable backup requests for the internal solr load balancer
> ----------------------------------------------------------
>
> Key: SOLR-4449
> URL: https://issues.apache.org/jira/browse/SOLR-4449
> Project: Solr
> Issue Type: New Feature
> Components: SolrCloud
> Reporter: philip hoy
> Priority: Minor
> Attachments: SOLR-4449.patch, SOLR-4449.patch, SOLR-4449.patch,
> patch-4449.txt, solr-back-request-lb-plugin.jar
>
>
> Add the ability to configure the built-in solr load balancer such that it
> submits a backup request to the next server in the list if the initial
> request takes too long. Employing such an algorithm could improve the latency
> of the 9xth percentile albeit at the expense of increasing overall load due
> to additional requests.
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