Implement sampling (#55)

* Implement sampling

* Update sample projects

src/SkyWalking.Abstractions/Config/AgentConfig.cs

@@ -31,7 +31,9 @@ namespace SkyWalking.Config
         public static string ApplicationCode { get; set; }
 
         /// <summary>
-        /// Negative or zeor means off ,by default. Sample_N_Per_3_Secs means sampling N TraceSegment in 10 seconds tops.
+        /// The number of sampled traces per 3 seconds
+        /// Negative number means sample traces as many as possible, most likely 100% , by default
+        /// 
         /// </summary>
         public static int Sample_N_Per_3_Secs = -1;
 

src/SkyWalking.AspNetCore/SkyWalkingHostedService.cs

@@ -53,6 +53,7 @@ namespace SkyWalking.AspNetCore
             LogManager.SetLoggerFactory(loggerFactory);
             AgentConfig.ApplicationCode = options.Value.ApplicationCode;
             CollectorConfig.DirectServers = options.Value.DirectServers;
+            AgentConfig.Sample_N_Per_3_Secs = options.Value.SamplePer3Secs;
             _logger = LogManager.GetLogger<SkyWalkingHostedService>();
             _diagnosticObserver = diagnosticObserver;
         }

src/SkyWalking.AspNetCore/SkyWalkingOptions.cs

@@ -47,5 +47,16 @@ namespace SkyWalking.AspNetCore
             get;
             set;
         }
+
+        /// <summary>
+        /// The number of sampled traces per 3 seconds
+        /// Negative number means sample traces as many as possible, most likely 100% , by default
+        /// 
+        /// </summary>
+        public int SamplePer3Secs
+        {
+            get; 
+            set;
+        } = -1;
     }
-}
+}
\ No newline at end of file

src/SkyWalking.Core/Sampling/SamplingService.cs

@@ -16,31 +16,49 @@
  *
  */
 
+using System;
 using System.Threading;
 using System.Threading.Tasks;
+using SkyWalking.Boot;
+using SkyWalking.Config;
+using SkyWalking.Utils;
 
 namespace SkyWalking.Sampling
 {
-    public class SamplingService : ISampler
+    // ReSharper disable once ClassNeverInstantiated.Global
+    public class SamplingService :TimerService, ISampler
     {
+        private readonly AtomicInteger _atomicInteger = new AtomicInteger();
+        private readonly int _sample_N_Per_3_Secs = AgentConfig.Sample_N_Per_3_Secs;
+        private readonly bool _sample_on = AgentConfig.Sample_N_Per_3_Secs > 0;
 
         public bool TrySampling()
         {
-            return true;
+            if (!_sample_on)
+            {
+                return true;
+            }
+
+            return _atomicInteger.Increment() < _sample_N_Per_3_Secs;
         }
 
         public void ForceSampled()
         {
+            if (_sample_on)
+            {
+                _atomicInteger.Increment();
+            }
         }
 
-        public void Dispose()
+        protected override TimeSpan Interval { get; } = TimeSpan.FromSeconds(3);
+        
+        protected override Task Execute(CancellationToken token)
         {
-        }
+            if (_sample_on)
+            {
+                _atomicInteger.Value = 0;
+            }
 
-        public int Order { get; } = 1;
-
-        public Task Initialize(CancellationToken token)
-        {
             return Task.CompletedTask;
         }
     }

src/SkyWalking.Core/Utils/AtomicInteger.cs

@@ -24,7 +24,11 @@ namespace SkyWalking.Utils
     {
         private int _value;
 
-        public int Value => _value;
+        public int Value
+        {
+            get => _value;
+            set => Interlocked.Exchange(ref _value, value);
+        }
 
         public AtomicInteger()
             : this(0)

test/SkyWalking.Core.Tests/Utils/AtomicIntegerTests.cs

@@ -60,5 +60,13 @@ namespace SkyWalking.Core.Tests.Utils
             Assert.True(atomicInteger == 5);
             Assert.False(atomicInteger != 5);
         }
+
+        [Fact]
+        public void Set_Value()
+        {
+            AtomicInteger atomicInteger = 5;
+            atomicInteger.Value = 10;
+            Assert.Equal(10, atomicInteger.Value);
+        }
     }
 }
\ No newline at end of file