[#4942] Add random time to the scheduled task for pulling instances from serviceCenter (#4978)

Author: chengyouling

clients/service-center-client/src/main/java/org/apache/servicecomb/service/center/client/ServiceCenterDiscovery.java

@@ -23,6 +23,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Random;
 import java.util.concurrent.ConcurrentHashMap;
 
 import org.apache.servicecomb.http.client.task.AbstractTask;
@@ -101,6 +102,8 @@ public static class SubscriptionValue {
 
   private final Object lock = new Object();
 
+  private final Random random = new Random();
+
   public ServiceCenterDiscovery(ServiceCenterClient serviceCenterClient, EventBus eventBus) {
     super("service-center-discovery-task");
     this.serviceCenterClient = serviceCenterClient;
@@ -219,10 +222,17 @@ class PullInstanceTask implements Task {
     public void execute() {
       pullAllInstance();
 
-      startTask(new BackOffSleepTask(pollInterval, new PullInstanceTask()));
+      startTask(new BackOffSleepTask(buildPollIntervalWithSalt(), new PullInstanceTask()));
     }
   }
 
+  private long buildPollIntervalWithSalt() {
+    int positive = random.nextInt(5);
+    int sign = random.nextBoolean() ? 1 : -1;
+    long currentPollInterval = pollInterval + sign * positive * 1000;
+    return currentPollInterval > 0 ? currentPollInterval : pollInterval;
+  }
+
   class PullInstanceOnceTask implements Task {
     @Override
     public void execute() {