Wait for 2 secs before scaling out for SB ScaleMonitor (Azure#50923)

* Wait for 2 secs before scaling out

* Change to TimeSpan

* Update changlog

Author: larohra

sdk/servicebus/Microsoft.Azure.WebJobs.Extensions.ServiceBus/CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Bugs Fixed
 
+- Wait for 2 secs before scaling out for ScaleMonitor to avoid aggressive scaling (matches with ScaleControlelr V2) - AB#32302750
+
 ### Other Changes
 
 ## 5.17.0 (2025-06-20)

sdk/servicebus/Microsoft.Azure.WebJobs.Extensions.ServiceBus/src/Listeners/ServiceBusScaleMonitor.cs

@@ -23,6 +23,9 @@ internal class ServiceBusScaleMonitor : IScaleMonitor<ServiceBusTriggerMetrics>
         private readonly ILogger<ServiceBusScaleMonitor> _logger;
         private readonly ServiceBusMetricsProvider _serviceBusMetricsProvider;
 
+        // to avoid frequent scaling out when queue time is increasing, we wait for at least 2s before scaling out (backward compatibility with SCV2).
+        public static TimeSpan MinimumLastQueueMessageInSecondsThreshold = TimeSpan.FromSeconds(2.0);
+
         public ServiceBusScaleMonitor(
             string functionId,
             string entityPath,
@@ -137,7 +140,9 @@ private ScaleStatus GetScaleStatusCore(int workerCount, ServiceBusTriggerMetrics
                 }
             }
 
-            if (metrics[0].QueueTime > TimeSpan.Zero && metrics[0].QueueTime < metrics[NumberOfSamplesToConsider - 1].QueueTime)
+            var lastSampleQueueTime = metrics[NumberOfSamplesToConsider - 1].QueueTime;
+
+            if (metrics[0].QueueTime > TimeSpan.Zero && metrics[0].QueueTime < lastSampleQueueTime)
             {
                 bool queueTimeIncreasing =
                     IsTrueForLastN(
@@ -146,9 +151,19 @@ private ScaleStatus GetScaleStatusCore(int workerCount, ServiceBusTriggerMetrics
                         (prev, next) => prev.QueueTime <= next.QueueTime);
                 if (queueTimeIncreasing)
                 {
-                    status.Vote = ScaleVote.ScaleOut;
-                    _logger.LogInformation($"Queue time is increasing for '{_entityPath}'.");
-                    return status;
+                    // to avoid frequent scaling out when queue time is increasing, we wait for at least 2s (backward compatibility with SCV2).
+                    if (lastSampleQueueTime >= MinimumLastQueueMessageInSecondsThreshold)
+                    {
+                        status.Vote = ScaleVote.ScaleOut;
+                        _logger.LogInformation($"Queue time is increasing for '{_entityPath}'.");
+                        return status;
+                    }
+                    else
+                    {
+                        status.Vote = ScaleVote.None;
+                        _logger.LogInformation($"Queue time is increasing for '{_entityPath}' but we do not scale out unless queue latency is greater than {MinimumLastQueueMessageInSecondsThreshold.TotalSeconds}s. Current queue latency is {lastSampleQueueTime.TotalSeconds}s.");
+                        return status;
+                    }
                 }
             }
 

sdk/servicebus/Microsoft.Azure.WebJobs.Extensions.ServiceBus/tests/Listeners/ServiceBusScaleMonitorTests.cs

@@ -697,6 +697,33 @@ public void GetScaleStatus_QueueTimeIncreasing_ReturnsVote_ScaleOut()
             Assert.AreEqual($"Queue time is increasing for '{_entityPath}'.", log.FormattedMessage);
         }
 
+        [Test]
+        public void GetScaleStatus_QueueTimeIncreasingLessThanMinimumWaitTime_ReturnsScaleVoteNone()
+        {
+            var context = new ScaleStatusContext<ServiceBusTriggerMetrics>
+            {
+                WorkerCount = 1
+            };
+            var timestamp = DateTime.UtcNow;
+            var lastQueueTime = 0.5;
+            context.Metrics = new List<ServiceBusTriggerMetrics>
+                {
+                    new ServiceBusTriggerMetrics { MessageCount = 100, PartitionCount = 0, QueueTime = TimeSpan.FromSeconds(0.1), Timestamp = timestamp.AddSeconds(15) },
+                    new ServiceBusTriggerMetrics { MessageCount = 100, PartitionCount = 0, QueueTime = TimeSpan.FromSeconds(0.2), Timestamp = timestamp.AddSeconds(15) },
+                    new ServiceBusTriggerMetrics { MessageCount = 100, PartitionCount = 0, QueueTime = TimeSpan.FromSeconds(0.3), Timestamp = timestamp.AddSeconds(15) },
+                    new ServiceBusTriggerMetrics { MessageCount = 100, PartitionCount = 0, QueueTime = TimeSpan.FromSeconds(0.4), Timestamp = timestamp.AddSeconds(15) },
+                    new ServiceBusTriggerMetrics { MessageCount = 100, PartitionCount = 0, QueueTime = TimeSpan.FromSeconds(lastQueueTime), Timestamp = timestamp.AddSeconds(15) },
+                };
+
+            var status = _scaleMonitor.GetScaleStatus(context);
+            Assert.AreEqual(ScaleVote.None, status.Vote);
+
+            var logs = _loggerProvider.GetAllLogMessages().ToArray();
+            var log = logs[0];
+            Assert.AreEqual(LogLevel.Information, log.Level);
+            Assert.AreEqual($"Queue time is increasing for '{_entityPath}' but we do not scale out unless queue latency is greater than {ServiceBusScaleMonitor.MinimumLastQueueMessageInSecondsThreshold.TotalSeconds}s. Current queue latency is {lastQueueTime}s.", log.FormattedMessage);
+        }
+
         [Test]
         public void GetScaleStatus_QueueLengthDecreasing_ReturnsVote_ScaleIn()
         {