Tim Owen created SOLR-9503:
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Summary: NPE in Replica Placement Rules when using Overseer Role
with other rules
Key: SOLR-9503
URL: https://issues.apache.org/jira/browse/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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