From 2b420a3877996b1c77c22e88cf0cb60228837d29 Mon Sep 17 00:00:00 2001
From: Meet Jain <56895638+meetjain74@users.noreply.github.com>
Date: Mon, 25 May 2026 14:45:54 +0530
Subject: [PATCH] Add secondary and tertiary sort for hosts in Isolation
 Scheduler

---
 .../storm/scheduler/IsolationScheduler.java   | 31 +++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/storm-server/src/main/java/org/apache/storm/scheduler/IsolationScheduler.java b/storm-server/src/main/java/org/apache/storm/scheduler/IsolationScheduler.java
index 2ebd4bd89c..4aad446771 100644
--- a/storm-server/src/main/java/org/apache/storm/scheduler/IsolationScheduler.java
+++ b/storm-server/src/main/java/org/apache/storm/scheduler/IsolationScheduler.java
@@ -303,7 +303,12 @@ private Map<String, Set<WorkerSlot>> hostToUsedSlots(Cluster cluster) {
         return hostUsedSlots;
     }
 
-    // returns list of list of slots, reverse sorted by number of slots
+    // Returns list of hosts with their assignable slots, sorted by:
+    //   1. (primary)   total assignable slots,    descending — same as before
+    //   2. (secondary) currently free slots,      descending — prefer hosts that
+    //                                                         need fewer evictions
+    //   3. (tertiary)  host name,                 ascending  — deterministic order
+    //                                                         for testability
     private LinkedList<HostAssignableSlots> hostAssignableSlots(Cluster cluster) {
         List<WorkerSlot> assignableSlots = cluster.getAssignableSlots();
         Map<String, List<WorkerSlot>> hostAssignableSlots = new HashMap<String, List<WorkerSlot>>();
@@ -316,6 +321,16 @@ private LinkedList<HostAssignableSlots> hostAssignableSlots(Cluster cluster) {
             }
             slots.add(slot);
         }
+
+        final Map<String, Integer> hostFreeSlotCount = new HashMap<String, Integer>();
+        for (WorkerSlot slot : cluster.getAvailableSlots()) {
+            String host = cluster.getHost(slot.getNodeId());
+            if (hostAssignableSlots.containsKey(host)) {
+                Integer count = hostFreeSlotCount.get(host);
+                hostFreeSlotCount.put(host, (count == null ? 0 : count) + 1);
+            }
+        }
+
         List<HostAssignableSlots> sortHostAssignSlots = new ArrayList<HostAssignableSlots>();
         for (Map.Entry<String, List<WorkerSlot>> entry : hostAssignableSlots.entrySet()) {
             sortHostAssignSlots.add(new HostAssignableSlots(entry.getKey(), entry.getValue()));
@@ -323,7 +338,19 @@ private LinkedList<HostAssignableSlots> hostAssignableSlots(Cluster cluster) {
         Collections.sort(sortHostAssignSlots, new Comparator<HostAssignableSlots>() {
             @Override
             public int compare(HostAssignableSlots o1, HostAssignableSlots o2) {
-                return o2.getWorkerSlots().size() - o1.getWorkerSlots().size();
+                int bySlots = o2.getWorkerSlots().size() - o1.getWorkerSlots().size();
+                if (bySlots != 0) {
+                    return bySlots;
+                }
+                int free1 = hostFreeSlotCount.containsKey(o1.getHostName())
+                            ? hostFreeSlotCount.get(o1.getHostName()) : 0;
+                int free2 = hostFreeSlotCount.containsKey(o2.getHostName())
+                            ? hostFreeSlotCount.get(o2.getHostName()) : 0;
+                int byFree = free2 - free1;
+                if (byFree != 0) {
+                    return byFree;
+                }
+                return o1.getHostName().compareTo(o2.getHostName());
             }
         });