IllegalArgumentException when scroll ID has a node no longer part of the Cluster

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ### Changed
 - Add CompletionStage variants to methods in the Client Interface and default to ActionListener impl ([#18998](https://github.com/opensearch-project/OpenSearch/pull/18998))
+- IllegalArgumentException when scroll ID references a node not found in Cluster ([#19031](https://github.com/opensearch-project/OpenSearch/pull/19031))
 
 ### Fixed
 - Fix unnecessary refreshes on update preparation failures ([#15261](https://github.com/opensearch-project/OpenSearch/issues/15261))

server/src/main/java/org/opensearch/action/search/SearchScrollAsyncAction.java

@@ -159,7 +159,9 @@ private void run(BiFunction<String, String, DiscoveryNode> clusterNodeLookup, fi
             try {
                 DiscoveryNode node = clusterNodeLookup.apply(target.getClusterAlias(), target.getNode());
                 if (node == null) {
-                    throw new IllegalStateException("node [" + target.getNode() + "] is not available");
+                    throw new IllegalArgumentException(
+                        "scroll_id references node [" + target.getNode() + "] which was not found in the cluster"
+                    );
                 }
                 connection = getConnection(target.getClusterAlias(), node);
             } catch (Exception ex) {

server/src/test/java/org/opensearch/action/search/SearchScrollAsyncActionTests.java

@@ -298,7 +298,10 @@ protected void onFirstPhaseResult(int shardId, SearchAsyncActionTests.TestSearch
         latch.await();
         ShardSearchFailure[] shardSearchFailures = action.buildShardFailures();
         assertEquals(1, shardSearchFailures.length);
-        assertEquals("IllegalStateException[node [node2] is not available]", shardSearchFailures[0].reason());
+        assertEquals(
+            "IllegalArgumentException[scroll_id references node [node2] which was not found in the cluster]",
+            shardSearchFailures[0].reason()
+        );
 
         SearchContextIdForNode[] context = scrollId.getContext();
         for (int i = 0; i < results.length(); i++) {