fix hot node in weak read (#422)

Author: WeiXinChan

src/main/java/com/alipay/oceanbase/rpc/location/model/partition/ObPartitionLocation.java

@@ -23,6 +23,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.concurrent.ThreadLocalRandom;
 
 public class ObPartitionLocation {
     private ReplicaLocation       leader;
@@ -90,10 +91,9 @@ public ReplicaLocation getReplica(ObReadConsistency consistencyLevel,
     }
 
     private ReplicaLocation getReadReplicaNoLdc(ObRoutePolicy routePolicy) {
-        for (ReplicaLocation r : replicas) {
-            if (r.isValid() && !r.isLeader()) {
-                return r;
-            }
+        ReplicaLocation follower = getRandomFollowerFromList(replicas);
+        if (follower != null) {
+            return follower;
         }
         if (routePolicy == ObRoutePolicy.FOLLOWER_ONLY) {
             throw new IllegalArgumentException("No follower replica found for route policy: "
@@ -107,25 +107,22 @@ private ReplicaLocation getReadReplicaByRoutePolicy(ObRoutePolicy routePolicy)
         // 路由策略优先：FOLLOWER_FIRST 优先选择 follower，FOLLOWER_ONLY 只能选择 follower
         // 在满足路由策略的前提下，按就近原则选择（同机房 -> 同 region -> 其他 region）
 
-        // 优先在同机房找 follower
-        for (ReplicaLocation r : sameIdc) {
-            if (r.isValid() && !r.isLeader()) {
-                return r;
-            }
+        // 优先在同机房找 follower（随机选择以避免热点）
+        ReplicaLocation follower = getRandomFollowerFromList(sameIdc);
+        if (follower != null) {
+            return follower;
         }
 
-        // 如果同机房没有 follower，在同 region 找 follower
-        for (ReplicaLocation r : sameRegion) {
-            if (r.isValid() && !r.isLeader()) {
-                return r;
-            }
+        // 如果同机房没有 follower，在同 region 找 follower（随机选择以避免热点）
+        follower = getRandomFollowerFromList(sameRegion);
+        if (follower != null) {
+            return follower;
         }
 
-        // 如果同 region 没有 follower，在其他 region 找 follower
-        for (ReplicaLocation r : otherRegion) {
-            if (r.isValid() && !r.isLeader()) {
-                return r;
-            }
+        // 如果同 region 没有 follower，在其他 region 找 follower（随机选择以避免热点）
+        follower = getRandomFollowerFromList(otherRegion);
+        if (follower != null) {
+            return follower;
         }
 
         // 如果都没有找到 follower
@@ -139,6 +136,33 @@ private ReplicaLocation getReadReplicaByRoutePolicy(ObRoutePolicy routePolicy)
         return leader;
     }
 
+    /**
+     * 从列表中随机选择一个有效的 follower，避免热点问题
+     *
+     * @param locations replica 列表
+     * @return 随机选择的 follower，如果没有有效的 follower 则返回 null
+     */
+    private ReplicaLocation getRandomFollowerFromList(List<ReplicaLocation> locations) {
+        if (locations == null || locations.isEmpty()) {
+            return null;
+        }
+        
+        // 收集所有有效的 follower
+        List<ReplicaLocation> validFollowers = new ArrayList<>();
+        for (ReplicaLocation r : locations) {
+            if (r.isValid() && !r.isLeader()) {
+                validFollowers.add(r);
+            }
+        }
+        
+        if (validFollowers.isEmpty()) {
+            return null;
+        }
+        
+        // 随机选择一个 follower
+        return validFollowers.get(ThreadLocalRandom.current().nextInt(validFollowers.size()));
+    }
+
     /*
      * Classify Replica for weak read, according to Server LDC location.
      * Synchronized to avoid duplicate initialization.