fix: newly added instance needs to create a connection pool immediately (#3334)

src/main/java/com/actiontech/dble/backend/datasource/PhysicalDbInstance.java

@@ -467,9 +467,11 @@ public abstract class PhysicalDbInstance implements ReadTimeStatusInstance {
 
     public void updatePoolCapacity() {
         //minCon/maxCon/numOfShardingNodes
-        checkPoolSize();
-        connectionPool.evictImmediately();
-        connectionPool.fillPool();
+        if ((dbGroupConfig.getRwSplitMode() != RW_SPLIT_OFF || dbGroup.getWriteDbInstance() == this) && !dbGroup.isUseless()) {
+            checkPoolSize();
+            connectionPool.evictImmediately();
+            connectionPool.fillPool();
+        }
     }
 
     public void closeAllConnection(String reason) {

src/main/java/com/actiontech/dble/backend/heartbeat/MySQLHeartbeat.java

@@ -227,6 +227,10 @@ public class MySQLHeartbeat {
                 Map<String, String> labels = AlertUtil.genSingleLabel("dbInstance", this.source.getDbGroupConfig().getName() + "-" + this.source.getConfig().getInstanceName());
                 AlertUtil.alertResolve(AlarmCode.HEARTBEAT_FAIL, Alert.AlertLevel.WARN, "mysql", this.source.getConfig().getId(), labels);
         }
+        //after the heartbeat changes from failure to success, it needs to be expanded immediately
+        if (source.getTotalConnections() == 0) {
+            source.updatePoolCapacity();
+        }
         if (isStop) {
             LOGGER.warn("heartbeat[{}] had been stop", source.getConfig().getUrl());
             detector.quit();