Implement multi-backend retrieval architecture with comprehensive tests

Co-authored-by: jongalloway <[email protected]>

Author: Copilot

src/NLWebNet/Extensions/ServiceCollectionExtensions.cs

@@ -1,5 +1,8 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Diagnostics.HealthChecks;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
 using NLWebNet.Models;
 using NLWebNet.Services;
 using NLWebNet.MCP;
@@ -94,4 +97,104 @@ public static IServiceCollection AddNLWebNet<TDataBackend>(this IServiceCollecti
 
         return services;
     }
+
+    /// <summary>
+    /// Adds NLWebNet services with multi-backend support
+    /// </summary>
+    /// <param name="services">The service collection</param>
+    /// <param name="configureOptions">Optional configuration callback</param>
+    /// <param name="configureMultiBackend">Multi-backend configuration callback</param>
+    /// <returns>The service collection for chaining</returns>
+    public static IServiceCollection AddNLWebNetMultiBackend(this IServiceCollection services, 
+        Action<NLWebOptions>? configureOptions = null,
+        Action<MultiBackendOptions>? configureMultiBackend = null)
+    {
+        // Configure options
+        if (configureOptions != null)
+        {
+            services.Configure(configureOptions);
+        }
+
+        // Configure multi-backend options
+        if (configureMultiBackend != null)
+        {
+            services.Configure<MultiBackendOptions>(configureMultiBackend);
+        }
+
+        // Add logging (required for the services)
+        services.AddLogging();
+
+        // Register core NLWebNet services
+        services.AddScoped<INLWebService>(provider =>
+        {
+            var options = provider.GetRequiredService<IOptions<NLWebOptions>>();
+            if (options.Value.MultiBackend.Enabled)
+            {
+                // Use multi-backend constructor
+                return new NLWebService(
+                    provider.GetRequiredService<IQueryProcessor>(),
+                    provider.GetRequiredService<IResultGenerator>(),
+                    provider.GetRequiredService<IBackendManager>(),
+                    provider.GetRequiredService<ILogger<NLWebService>>(),
+                    options);
+            }
+            else
+            {
+                // Use single backend constructor for backward compatibility
+                return new NLWebService(
+                    provider.GetRequiredService<IQueryProcessor>(),
+                    provider.GetRequiredService<IResultGenerator>(),
+                    provider.GetRequiredService<IDataBackend>(),
+                    provider.GetRequiredService<ILogger<NLWebService>>(),
+                    options);
+            }
+        });
+
+        services.AddScoped<IQueryProcessor, QueryProcessor>();
+        services.AddScoped<IResultGenerator>(provider =>
+        {
+            var options = provider.GetRequiredService<IOptions<NLWebOptions>>();
+            if (options.Value.MultiBackend.Enabled)
+            {
+                // Use multi-backend constructor
+                return new ResultGenerator(
+                    provider.GetRequiredService<IBackendManager>(),
+                    provider.GetRequiredService<ILogger<ResultGenerator>>(),
+                    options,
+                    provider.GetService<IChatClient>());
+            }
+            else
+            {
+                // Use single backend constructor for backward compatibility
+                return new ResultGenerator(
+                    provider.GetRequiredService<IDataBackend>(),
+                    provider.GetRequiredService<ILogger<ResultGenerator>>(),
+                    options,
+                    provider.GetService<IChatClient>());
+            }
+        });
+
+        // Register MCP services
+        services.AddScoped<IMcpService, McpService>();
+
+        // Register multi-backend services
+        services.AddScoped<IBackendManager, BackendManager>();
+
+        // Register default data backend (can be overridden)
+        services.AddScoped<IDataBackend, MockDataBackend>();
+
+        // Add health checks
+        services.AddHealthChecks()
+            .AddCheck<NLWebHealthCheck>("nlweb")
+            .AddCheck<DataBackendHealthCheck>("data-backend")
+            .AddCheck<AIServiceHealthCheck>("ai-service");
+
+        // Add metrics
+        services.AddMetrics();
+
+        // Add rate limiting
+        services.AddSingleton<IRateLimitingService, InMemoryRateLimitingService>();
+
+        return services;
+    }
 }

src/NLWebNet/Models/MultiBackendOptions.cs

@@ -0,0 +1,78 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace NLWebNet.Models;
+
+/// <summary>
+/// Configuration options for multi-backend retrieval architecture.
+/// </summary>
+public class MultiBackendOptions
+{
+    /// <summary>
+    /// The configuration section name for binding from appsettings.json.
+    /// </summary>
+    public const string SectionName = "NLWebNet:MultiBackend";
+
+    /// <summary>
+    /// Whether multi-backend mode is enabled. When false, uses single backend for backward compatibility.
+    /// </summary>
+    public bool Enabled { get; set; } = false;
+
+    /// <summary>
+    /// The identifier of the backend to use as the primary write endpoint.
+    /// </summary>
+    public string? WriteEndpoint { get; set; }
+
+    /// <summary>
+    /// Configuration for individual backend endpoints.
+    /// </summary>
+    public Dictionary<string, BackendEndpointOptions> Endpoints { get; set; } = new();
+
+    /// <summary>
+    /// Whether to enable parallel querying across backends.
+    /// </summary>
+    public bool EnableParallelQuerying { get; set; } = true;
+
+    /// <summary>
+    /// Whether to enable automatic result deduplication.
+    /// </summary>
+    public bool EnableResultDeduplication { get; set; } = true;
+
+    /// <summary>
+    /// Maximum number of concurrent backend queries.
+    /// </summary>
+    [Range(1, 10)]
+    public int MaxConcurrentQueries { get; set; } = 5;
+
+    /// <summary>
+    /// Timeout for individual backend queries in seconds.
+    /// </summary>
+    [Range(1, 120)]
+    public int BackendTimeoutSeconds { get; set; } = 30;
+}
+
+/// <summary>
+/// Configuration for an individual backend endpoint.
+/// </summary>
+public class BackendEndpointOptions
+{
+    /// <summary>
+    /// Whether this backend endpoint is enabled.
+    /// </summary>
+    public bool Enabled { get; set; } = true;
+
+    /// <summary>
+    /// The type of backend (e.g., "azure_ai_search", "mock", "custom").
+    /// </summary>
+    public string BackendType { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Priority for this backend (higher values = higher priority).
+    /// Used for ordering results when deduplication is disabled.
+    /// </summary>
+    public int Priority { get; set; } = 0;
+
+    /// <summary>
+    /// Backend-specific configuration properties.
+    /// </summary>
+    public Dictionary<string, object> Properties { get; set; } = new();
+}

src/NLWebNet/Models/NLWebOptions.cs

@@ -71,4 +71,9 @@ public class NLWebOptions
     /// Rate limiting configuration
     /// </summary>
     public RateLimitingOptions RateLimiting { get; set; } = new();
+
+    /// <summary>
+    /// Multi-backend configuration options. When enabled, overrides single backend behavior.
+    /// </summary>
+    public MultiBackendOptions MultiBackend { get; set; } = new();
 }