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https://issues.apache.org/jira/browse/SOLR-8696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15157991#comment-15157991
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Mark Miller commented on SOLR-8696:
-----------------------------------
The history of legacy mode:
Initially, we had no collections API. Collections were created by a SolrCore
registering with ZK and doing state updates. This allowed you to pre-configure
SolrCores in solr.xml as you could with non SolrCloud. It was basically a way
to quickly get going. But it has many problems.
Eventually we got the collections API and for back compat reasons added the
legacyMode property. When not in legacy mode, state updates will not created
collections - you must explicitly use the collections API. It's the start of
what we call "zookeeper=truth" though currently only a small part of that is
implemented.
> Optimize overseer + startup
> ---------------------------
>
> Key: SOLR-8696
> URL: https://issues.apache.org/jira/browse/SOLR-8696
> Project: Solr
> Issue Type: Improvement
> Components: SolrCloud
> Affects Versions: 5.4.1
> Reporter: Scott Blum
> Labels: patch, performance, solrcloud, startup
> Attachments: SOLR-8696.patch
>
>
> ZkController.publishAndWaitForDownStates() occurs before overseer election.
> That means if there is currently no overseer, there is ironically no one to
> actually service the down state changes it's waiting on. This particularly
> affects a single-node cluster such as you might run locally for development.
> Additionally, we're doing an unnecessary ZkStateReader forced refresh on all
> Overseer operations. This isn't necessary because ZkStateReader keeps itself
> up to date.
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