From 5e1bf4b34b9d6825090f9fc32dd5d66928e6df9c Mon Sep 17 00:00:00 2001
From: Houston Putman <houston@apache.org>
Date: Thu, 13 Feb 2025 17:05:29 -0600
Subject: [PATCH] SOLR-17673: Disable multithreaded search at the node level by
 default

---
 solr/CHANGES.txt                                            | 4 ++++
 solr/core/src/java/org/apache/solr/core/NodeConfig.java     | 3 +--
 .../upgrade-notes/pages/major-changes-in-solr-9.adoc        | 6 ++++++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/solr/CHANGES.txt b/solr/CHANGES.txt
index b3184eb3740..359d7a460bd 100644
--- a/solr/CHANGES.txt
+++ b/solr/CHANGES.txt
@@ -190,6 +190,10 @@ Bug Fixes
 
 * SOLR-17652: Fix a bug that could cause long leader elections to leave PULL replicas in DOWN state forever. (hossman)
 
+* SOLR-17673: Disable multi-threaded search execution by default at the node-level.
+  Users can still opt-in by providing a "indexSearcherExecutorThreads" > 0.
+  (Houston Putman, Varun Thacker, David Smiley, Luke Kot-Zaniewski)
+
 Dependency Upgrades
 ---------------------
 * SOLR-17471: Upgrade Lucene to 9.12.1. (Pierre Salagnac, Christine Poerschke)
diff --git a/solr/core/src/java/org/apache/solr/core/NodeConfig.java b/solr/core/src/java/org/apache/solr/core/NodeConfig.java
index 72ec1bd5ee0..439a922d1df 100644
--- a/solr/core/src/java/org/apache/solr/core/NodeConfig.java
+++ b/solr/core/src/java/org/apache/solr/core/NodeConfig.java
@@ -627,8 +627,7 @@ public static class NodeConfigBuilder {
     // No:of core load threads in cloud mode is set to a default of 8
     public static final int DEFAULT_CORE_LOAD_THREADS_IN_CLOUD = 8;
 
-    public static final int DEFAULT_INDEX_SEARCHER_EXECUTOR_THREADS =
-        Runtime.getRuntime().availableProcessors();
+    public static final int DEFAULT_INDEX_SEARCHER_EXECUTOR_THREADS = 0;
 
     private static final String DEFAULT_CORESLOCATORCLASS =
         "org.apache.solr.core.CorePropertiesLocator";
diff --git a/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc b/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc
index fd15a943f5b..7fbb96353ed 100644
--- a/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc
+++ b/solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-9.adoc
@@ -67,6 +67,12 @@ It is always strongly recommended that you fully reindex your documents after a
 In Solr 8, it was possible to add docValues to a schema without re-indexing via `UninvertDocValuesMergePolicy`, an advanced/expert utility.
 Due to changes in Lucene 9, that isn't possible any more.
 
+== Solr 9.8.1
+=== Configuration
+In solrconfig.xml, the `indexSearcherExecutorThreads` now defaults to 0.
+Therefore, multi-threaded search execution is disabled at a node-level by default.
+It is currently recommended to only use an `indexSearcherExecutorThreads > 0` if all query requests sent to Solr opt-in to multiThreaded search.
+
 == Solr 9.8
 === Configuration
 In solrconfig.xml, the `numVersionBuckets` and `versionBucketLockTimeoutMs` settings are now obsolete and ignored; a warning will be logged if specified.