magibney commented on a change in pull request #677: SOLR-13257: support for
stable replica routing preferences
URL: https://github.com/apache/lucene-solr/pull/677#discussion_r303906601
##########
File path:
solr/core/src/java/org/apache/solr/handler/component/HttpShardHandlerFactory.java
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@@ -449,9 +556,83 @@ private static boolean hasReplicaType(Object o, String
preferred) {
}
}
+ private final ReplicaListTransformerFactory randomRltFactory = (String
configSpec, SolrQueryRequest request,
+ ReplicaListTransformerFactory fallback) ->
shufflingReplicaListTransformer;
+ private ReplicaListTransformerFactory stableRltFactory;
+ private ReplicaListTransformerFactory defaultRltFactory;
+
+ /**
+ * Private class responsible for applying pairwise sort based on inherent
replica attributes,
+ * and subsequently reordering any equivalent replica sets according to
behavior specified
+ * by the baseReplicaListTransformer.
+ */
+ private static final class TopLevelReplicaListTransformer implements
ReplicaListTransformer {
+
+ private final NodePreferenceRulesComparator replicaComp;
+ private final ReplicaListTransformer baseReplicaListTransformer;
+
+ public TopLevelReplicaListTransformer(NodePreferenceRulesComparator
replicaComp, ReplicaListTransformer baseReplicaListTransformer) {
+ this.replicaComp = replicaComp;
+ this.baseReplicaListTransformer = baseReplicaListTransformer;
+ }
+
+ @Override
+ public void transform(List<?> choices) {
Review comment:
True, random-with-seed should work, and I'm open to that. One use case that
would benefit from a list-rotation-based implementation (as opposed to
random-with-seed) would be if you wanted to set up a tiered or grouped routing
system.
For example, replication factor of 2, say you have two distinct types of
users with different access patterns (that would result in different cache
usage patterns). Rather than simply routing deterministically for a given
user/request, you could choose one of exactly 2 routing params, bifurcating
traffic depending on expected use. With list-rotation, you could specify
`routingParam=0` or `routingParam=1` and get the desired behavior;
random-with-seed might not work that way, depending on the exact seeds chosen.
Granted you could achieve this tiered/grouped routing in other ways, but
with a list-rotation-based implementation it would be trivial. The cost, as you
say, would be some additional complexity (code-wise, not performance-wise) in
`HttpShardHandlerFactory`.
In fact, it's this type of use case that I had in mind when providing
support for the ability to specify `dividend` directly (as opposed to always
hashing the routing param, which would be similarly opaque and thus
incompatible with a tiered/grouped routing strategy).
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