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https://issues.apache.org/jira/browse/SOLR-13234?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17881123#comment-17881123
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David Smiley commented on SOLR-13234:
-------------------------------------
Years later, observing this PR creates an Http SolrClient per node. This isn't
necessary; one can be used for all the nodes.
> Prometheus Metric Exporter Not Threadsafe
> -----------------------------------------
>
> Key: SOLR-13234
> URL: https://issues.apache.org/jira/browse/SOLR-13234
> Project: Solr
> Issue Type: Bug
> Components: contrib - prometheus-exporter, metrics
> Affects Versions: 7.6, 8.0
> Reporter: Danyal Prout
> Assignee: Shalin Shekhar Mangar
> Priority: Minor
> Labels: metric-collector
> Fix For: 7.7.2, 8.1, 9.0
>
> Attachments: SOLR-13234-branch_7x.patch
>
> Time Spent: 40m
> Remaining Estimate: 0h
>
> The Solr Prometheus Exporter collects metrics when it receives a HTTP request
> from Prometheus. Prometheus sends this request, on its [scrape
> interval|https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config].
> When the time taken to collect the Solr metrics is greater than the scrape
> interval of the Prometheus server, this results in concurrent metric
> collection occurring in this
> [method|https://github.com/apache/lucene-solr/blob/master/solr/contrib/prometheus-exporter/src/java/org/apache/solr/prometheus/collector/SolrCollector.java#L86].
> This method doesn’t appear to be thread safe, for instance you could have
> concurrent modifications of a
> [map|https://github.com/apache/lucene-solr/blob/master/solr/contrib/prometheus-exporter/src/java/org/apache/solr/prometheus/collector/SolrCollector.java#L119].
> After a while the Solr Exporter processes becomes nondeterministic, we've
> observed NPE and loss of metrics.
> To address this, I'm proposing the following fixes:
> 1. Read/parse the configuration at startup and make it immutable.
> 2. Collect metrics from Solr on an interval which is controlled by the Solr
> Exporter and cache the metric samples to return during Prometheus scraping.
> Metric collection can be expensive, for example executing arbitrary Solr
> searches, it's not ideal to allow for concurrent metric collection and on an
> interval which is not defined by the Solr Exporter.
> There are also a few other performance improvements that we've made while
> fixing this, for example using the ClusterStateProvider instead of sending
> multiple HTTP requests to each Solr node to lookup all the cores.
> I'm currently finishing up these changes which I'll submit as a PR.
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