[GitHub] [solr] noblepaul commented on a diff in pull request #1318: SOLR-16636: ZkStateReader.waitForState should not register a watch if state already matches predicate

["noblepaul (via GitHub)" <gi...@apache.org>]

noblepaul commented on code in PR #1318:
URL: https://github.com/apache/solr/pull/1318#discussion_r1091033787


##########
solr/solrj-zookeeper/src/java/org/apache/solr/common/cloud/ZkStateReader.java:
##########
@@ -1682,6 +1687,37 @@ public DocCollection getCollectionLive(String coll) {
     }
   }
 
+  /**
+   * fetch the collection that is already cached. This may return a null if it is not already cached
+   * This is an optimization to avoid fetching state if it is not modified. this is particularly
+   * true for PRS collections where state is rarely modified
+   */
+  private DocCollection fetchCachedCollection(String coll) {
+    String collectionPath = DocCollection.getCollectionPath(coll);
+    DocCollection c = null;
+    ClusterState.CollectionRef ref = clusterState.getCollectionRef(coll);
+    if (ref == null) return null;
+    c = ref.getOrNull();
+    if (c == null) return null;
+    Stat stat = null;
+    try {
+      stat = zkClient.exists(collectionPath, null, false);
+    } catch (Exception e) {
+      return null;
+    }
+    if (stat != null) {
+      if (!c.isModified(stat.getVersion(), stat.getCversion())) {
+        // we have the latest collection state
+        return c;
+      }
+      if (c.isPerReplicaState() && c.getChildNodesVersion() < stat.getCversion()) {
+        // only PRS is modified. just fetch it and return the new collection
+        return c.copyWith(PerReplicaStatesFetcher.fetch(collectionPath, zkClient, null));
+      }

Review Comment:
   actually, there is another PR that is waiting to be merged #1242 . that ensures that the this update is in-place. 
   
   I'm waiting for perf tests to come back to merge that



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