[jira] [Commented] (SOLR-12413) Solr ignores aliases.json from ZooKeeper at startup

["David Smiley (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-12413?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16511457#comment-16511457 ] 

David Smiley commented on SOLR-12413:
-------------------------------------

Perhaps the solution is as simple as setting Aliases.EMPTY's zkNodeVersion to -1?

> Solr ignores aliases.json from ZooKeeper at startup
> ---------------------------------------------------
>
>                 Key: SOLR-12413
>                 URL: https://issues.apache.org/jira/browse/SOLR-12413
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: SolrCloud
>    Affects Versions: 7.2.1
>         Environment: A SolrCloud cluster with ZooKeeper (one node is enough to reproduce).
> Solr 7.2.1.
> ZooKeeper 3.4.6.
>            Reporter: Gaël Jourdan
>            Priority: Major
>
> Since upgrading to 7.2.1, we ran into an issue where Solr ignores _aliases.json_ file stored in ZooKeeper.
>  
> +Steps to reproduce the problem:+
>  # SolrCloud cluster is down
>  # Direct update of _aliases.json_ file in ZooKeeper with Solr ZkCLI *without using Collections API* :
>  ** {{java ... org.apache.solr.cloud.ZkCLI -zkhost ... -cmd clear /aliases.json}}
>  ** {{java ... org.apache.solr.cloud.ZkCLI -zkhost ... -cmd put /aliases.json "new content"}}
>  # SolrCloud cluster is started => _aliases.json_ not taken into account
>  
> +Analysis:+ 
> Digging a bit in the code, what is actually causing the issue is that, when starting, Solr now checks for the metadata of the _aliases.json_ file and if the version metadata from ZooKeeper is lower or equal to local version, it keeps the local version.
> When it starts, Solr has a local version of 0 for the aliases but ZooKeeper also has a version of 0 of the file because we just recreated it. So Solr ignores ZooKeeper configuration and never has a chance to load aliases.
>  
> Relevant parts of Solr code are:
>  * [https://github.com/apache/lucene-solr/blob/branch_7_2/solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java] : line 1562 : method setIfNewer
> {code:java}
> /**
> * Update the internal aliases reference with a new one, provided that its ZK version has increased.
> *
> * @param newAliases the potentially newer version of Aliases
> */
> private boolean setIfNewer(Aliases newAliases) {
>   synchronized (this) {
>     int cmp = Integer.compare(aliases.getZNodeVersion(), newAliases.getZNodeVersion());
>     if (cmp < 0) {
>       LOG.debug("Aliases: cmp={}, new definition is: {}", cmp, newAliases);
>       aliases = newAliases;
>       this.notifyAll();
>       return true;
>     } else {
>       LOG.debug("Aliases: cmp={}, not overwriting ZK version.", cmp);
>       assert cmp != 0 || Arrays.equals(aliases.toJSON(), newAliases.toJSON()) : aliases + " != " + newAliases;
>     return false;
>     }
>   }
> }{code}
>  * [https://github.com/apache/lucene-solr/blob/branch_7_2/solr/solrj/src/java/org/apache/solr/common/cloud/Aliases.java] : line 45 : the "empty" Aliases object with default version 0
> {code:java}
> /**
> * An empty, minimal Aliases primarily used to support the non-cloud solr use cases. Not normally useful
> * in cloud situations where the version of the node needs to be tracked even if all aliases are removed.
> * A version of 0 is provided rather than -1 to minimize the possibility that if this is used in a cloud
> * instance data is written without version checking.
> */
> public static final Aliases EMPTY = new Aliases(Collections.emptyMap(), Collections.emptyMap(), 0);{code}
>  
> Note that a workaround is to force ZooKeeper to always have a version greater than 0 for _aliases.json_ file (for instance by not clearing the file and just overwriting it again and again).



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