Spike: Fractional evaluation in clients

Author: dylanlerch

src/Octopus.OpenFeature.Provider/Octopus.OpenFeature.Provider.csproj

@@ -26,6 +26,7 @@
     <ItemGroup>
         <PackageReference Include="OpenFeature" Version="2.11.1" />
         <PackageReference Include="System.Text.Json" Version="9.0.12" />
+        <PackageReference Include="murmurhash" Version="1.0.3" />
     </ItemGroup>
 
     <ItemGroup>

src/Octopus.OpenFeature.Provider/OctopusFeatureClient.cs

@@ -12,15 +12,19 @@ public class FeatureToggles(FeatureToggleEvaluation[] evaluations, byte[] conten
     public byte[] ContentHash { get; } = contentHash;
 }
 
-public class FeatureToggleEvaluation(string name, string slug, bool isEnabled, KeyValuePair<string, string>[] segments)
+public class FeatureToggleEvaluation(string name, string slug, bool isEnabled, string evaluationKey, KeyValuePair<string, string>[] segments, int percentage)
 {
     public string Name { get; } = name;
 
     public string Slug { get; } = slug;
 
     public bool IsEnabled { get; } = isEnabled;
 
+    public string EvaluationKey { get; } = evaluationKey;
+
     public KeyValuePair<string, string>[] Segments { get; } = segments;
+
+    public int Percentage { get; } = percentage;
 }
 
 interface IOctopusFeatureClient

src/Octopus.OpenFeature.Provider/OctopusFeatureContext.cs

@@ -1,5 +1,7 @@
-using System.Text.RegularExpressions;
+using System.Text;
+using System.Text.RegularExpressions;
 using Microsoft.Extensions.Logging;
+using Murmur;
 using OpenFeature.Constant;
 using OpenFeature.Model;
 
@@ -64,6 +66,23 @@ bool MatchesSegment(EvaluationContext? context, IEnumerable<KeyValuePair<string,
     bool Evaluate(FeatureToggleEvaluation evaluation, EvaluationContext? context = null)
     {
         return evaluation.IsEnabled &&
+                (evaluation.Percentage == 100 || context?.TargetingKey is not null && GetNormalizedNumber(evaluation.EvaluationKey, context.TargetingKey) < evaluation.Percentage) &&
                (evaluation.Segments.Length == 0 || MatchesSegment(context, evaluation.Segments));
     }
+
+    /// <summary>
+    /// Produces a normalized number between 0 and 100 for a given TargetingKey, with less than 1% variance
+    /// </summary>
+    private int GetNormalizedNumber(string evaluationKey, string targetingKey)
+    {
+        const int one = 1;
+        const string separator = ":";
+
+        var bytes = Encoding.UTF8.GetBytes(string.Concat(evaluationKey, separator, targetingKey));
+
+        using var algorithm = MurmurHash.Create32();
+        var hash = algorithm.ComputeHash(bytes);
+        var value = BitConverter.ToUInt32(hash, 0);
+        return (int)(value % 100 + one);
+    }
 }