Houston Putman created SOLR-15549:
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Summary: 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
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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