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https://issues.apache.org/jira/browse/SOLR-15549?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17384398#comment-17384398
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ASF subversion and git services commented on SOLR-15549:
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Commit 97ce67383d8d2b00c5ea4a0cdb7b7437c0470301 in lucene-solr's branch 
refs/heads/branch_8x from Houston Putman
[ https://gitbox.apache.org/repos/asf?p=lucene-solr.git;h=97ce673 ]

SOLR-15549: Add support for Solr 9.0 clouds in ZkStateReader (#2533)



> Old SolrJ implementations (8.x) are incompatible with 9.0 Clouds
> ----------------------------------------------------------------
>
>                 Key: SOLR-15549
>                 URL: https://issues.apache.org/jira/browse/SOLR-15549
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: SolrJ
>    Affects Versions: 8.0
>            Reporter: Houston Putman
>            Assignee: Houston Putman
>            Priority: Major
>          Time Spent: 0.5h
>  Remaining Estimate: 0h
>
> The {{ZkStateReader}} in 8.x (and previous versions) checks that a 
> {{/clusterstate.json}} node exists in the ZK ChRoot, to ensure that the 
> ChRoot hosts a Solr Cloud. However, starting in 9.0, {{/clusterstate.json}} 
> has been removed, and it is auto-deleted if a user tries to create one.
> That means that the ZkStateReader from SolrJ 8.x will error when trying to 
> connect with a Solr 9 cloud, with the message:
> {quote}Cannot connect to cluster at localhost:2181/: cluster not found/not 
> ready
> {quote}
> The solution, is to have the ZK State Reader check both 
> {{/clusterstate.json}} and {{/collections}} and only error if both are 
> missing.  {{/clusterstate.json}} is long-deprecated in 8.x anyways, so adding 
> this additional check is good practice in general.
> While it would be nice for every user to use the same SolrJ version as the 
> version they are running for Solr, it can be difficult in practice, 
> especially when upgrading major Solr versions. It would be preferable to 
> support at least version + 1 clouds in SolrJ, for the purpose of upgrades.



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