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https://issues.apache.org/jira/browse/SOLR-14192?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17022705#comment-17022705
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David Smiley commented on SOLR-14192:
-------------------------------------
IMO we should remove the underlying complexity and not have the schema file
change on the fly. Certainly out of scope here but this bug reveals it's
tricky. Also I encountered this as a limitation when doing schema sharing in
SOLR-14040
> Race condition between SchemaManager and ZkIndexSchemaReader
> ------------------------------------------------------------
>
> Key: SOLR-14192
> URL: https://issues.apache.org/jira/browse/SOLR-14192
> Project: Solr
> Issue Type: Bug
> Affects Versions: 8.4
> Reporter: Andrzej Bialecki
> Assignee: Andrzej Bialecki
> Priority: Major
> Fix For: 8.5
>
> Attachments: SOLR-14192.patch
>
>
> Spin-off from SOLR-14128 and SOLR-13368.
> In SolrCloud when a SolrCore is created and it uses managed schema then its
> {{ManagedIndexSchemaFactory}} performs an automatic upgrade of the initial
> {{schema.xml}} to {{managed-schema}}. This includes removing the original
> {{schema.xml}} file.
> SOLR-13368 added some locking to make sure the changed resource name (i.e.
> {{managed-schema}}) becomes visible only when this process is complete, and
> that in-flight requests to /admin/schema block until this process is
> complete, to avoid returning inconsistent data. This locking mechanism uses
> simple Object monitors.
> However, if there's more than 1 node in the cluster the subsequent request to
> retrieve schema may execute on a core that still hasn't reloaded its schema
> ({{ZkIndexSchemaReader}} uses a ZK watcher, which may take some time to
> trigger), and the resource name in that stale schema still points to
> {{schema.xml}}, which by this time no longer exists because it was removed by
> {{ManagedIndexSchemaFactory}} in the first core.
> As I see it there are two bugs here:
> # there's no distributed locking when this upgrade is performed, so it's
> natural that there are multiple cores racing against each other to perform
> this upgrade.
> # the upgrade process removes {{schema.xml}} too early - it triggers all
> other cores by creating the {{managed-schema}} file, and then other cores
> reload from the new managed schema - but it should wait until this reload is
> complete on all cores because only then it's safe to delete the non-managed
> resource as it's no longer in use by any core.
> Issue 1. can be solved by adding an ephemeral znode lock so that only one
> core can perform the upgrade. Issue 2. can be solved by using
> {{ManagedIndexSchema.waitForSchemaZkVersionAgreement}} after upgrade, and
> deleting {{schema.xml}} only after it's done.
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