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From "Tim Owen (JIRA)" <>
Subject [jira] [Created] (SOLR-9503) NPE in Replica Placement Rules when using Overseer Role with other rules
Date Mon, 12 Sep 2016 13:11:20 GMT
Tim Owen created SOLR-9503:

             Summary: NPE in Replica Placement Rules when using Overseer Role with other rules
                 Key: SOLR-9503
             Project: Solr
          Issue Type: Bug
      Security Level: Public (Default Security Level. Issues are Public)
          Components: Rules, SolrCloud
    Affects Versions: 6.2, master (7.0)
            Reporter: Tim Owen

The overseer role introduced in SOLR-9251 works well if there's only a single Rule for replica
placement e.g. {code}rule=role:!overseer{code} but when combined with another rule, e.g. {code}rule=role:!overseer&rule=host:*,shard:*,replica:<2{code}
it can result in a NullPointerException (in Rule.tryAssignNodeToShard)

This happens because the code builds up a nodeVsTags map, but it only has entries for nodes
that have values for *all* tags used among the rules. This means not enough information is
available to other rules when they are being checked during replica assignment. In the example
rules above, if we have a cluster of 12 nodes and only 3 are given the Overseer role, the
others do not have any entry in the nodeVsTags map because they only have the host tag value
and not the role tag value.

Looking at the code in ReplicaAssigner.getTagsForNodes, it is explicitly only keeping entries
that fulfil the constraint of having values for all tags used in the rules. Possibly this
constraint was suitable when rules were originally introduced, but the Role tag (used for
Overseers) is unlikely to be present for all nodes in the cluster, and similarly for sysprop
tags which may or not be set for a node.

My patch removes this constraint, so the nodeVsTags map contains everything known about all
nodes, even if they have no value for a given tag. This allows the rule combination above
to work, and doesn't appear to cause any problems with the code paths that use the nodeVsTags
map. They handle null values quite well, and the tests pass.

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