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https://issues.apache.org/jira/browse/SOLR-15549?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17383417#comment-17383417
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Houston Putman commented on SOLR-15549:
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I should note the difference between the PR for {{branch_8x}} and {{main}}.
h2. {{branch_8x}}
This changes the check in {{ZkStateReader}} from just checking for
{{/clusterstate.json}} to looking for either {{/clusterstate.json}} or
{{/collections}}. This will allow for users to use the 8.10 SolrJ APIs for 9.x
Clouds.
h2. {{main}}
This adds a better error message in {{ZkStateReader}}, that lets the user know
why the ZK Node is not hosting a SolrCloud, i.e. which ZNode is missing
(usually {{/aliases.json}}). The implementation here had already been improved
to not checking for individual paths, but capturing
{{KeeperException.NoNodeExceptions}} and turning them into more user-usuably
{{SolrExceptions}} . I also am using this PR to forward-port the ChangeLog
entry.
> 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: 20m
> 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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