🏗️ Implement Multi-Backend Retrieval Architecture

doc/multi-backend-configuration.md

@@ -0,0 +1,147 @@
+# Multi-Backend Configuration Example
+
+This example demonstrates how to configure and use the multi-backend retrieval architecture in NLWebNet.
+
+## Single Backend (Default/Legacy)
+
+```csharp
+// Traditional single backend setup - still works
+services.AddNLWebNet<MockDataBackend>(options =>
+{
+    options.DefaultMode = QueryMode.List;
+    options.MaxResultsPerQuery = 20;
+});
+```
+
+## Multi-Backend Configuration
+
+```csharp
+// New multi-backend setup
+services.AddNLWebNetMultiBackend(
+    options =>
+    {
+        options.DefaultMode = QueryMode.List;
+        options.MaxResultsPerQuery = 50;
+    },
+    multiBackendOptions =>
+    {
+        multiBackendOptions.Enabled = true;
+        multiBackendOptions.EnableParallelQuerying = true;
+        multiBackendOptions.EnableResultDeduplication = true;
+        multiBackendOptions.MaxConcurrentQueries = 3;
+        multiBackendOptions.BackendTimeoutSeconds = 30;
+        multiBackendOptions.WriteEndpoint = "primary_backend";
+    });
+```
+
+## Configuration via appsettings.json
+
+```json
+{
+  "NLWebNet": {
+    "DefaultMode": "List",
+    "MaxResultsPerQuery": 50,
+    "MultiBackend": {
+      "Enabled": true,
+      "EnableParallelQuerying": true,
+      "EnableResultDeduplication": true,
+      "MaxConcurrentQueries": 3,
+      "BackendTimeoutSeconds": 30,
+      "WriteEndpoint": "primary_backend",
+      "Endpoints": {
+        "primary_backend": {
+          "Enabled": true,
+          "BackendType": "azure_ai_search",
+          "Priority": 10,
+          "Properties": {
+            "ConnectionString": "your-connection-string",
+            "IndexName": "your-index"
+          }
+        },
+        "secondary_backend": {
+          "Enabled": true,
+          "BackendType": "mock",
+          "Priority": 5,
+          "Properties": {}
+        }
+      }
+    }
+  }
+}
+```
+
+## Usage Example
+
+```csharp
+public class ExampleController : ControllerBase
+{
+    private readonly INLWebService _nlWebService;
+    private readonly IBackendManager _backendManager;
+
+    public ExampleController(INLWebService nlWebService, IBackendManager backendManager)
+    {
+        _nlWebService = nlWebService;
+        _backendManager = backendManager;
+    }
+
+    [HttpPost("search")]
+    public async Task<IActionResult> Search([FromBody] NLWebRequest request)
+    {
+        // Multi-backend search automatically handled
+        var response = await _nlWebService.ProcessRequestAsync(request);
+        return Ok(response);
+    }
+
+    [HttpGet("backend-info")]
+    public IActionResult GetBackendInfo()
+    {
+        // Get information about configured backends
+        var backendInfo = _backendManager.GetBackendInfo();
+        return Ok(backendInfo);
+    }
+
+    [HttpGet("write-backend-capabilities")]
+    public IActionResult GetWriteBackendCapabilities()
+    {
+        // Access the designated write backend
+        var writeBackend = _backendManager.GetWriteBackend();
+        if (writeBackend == null)
+        {
+            return NotFound("No write backend configured");
+        }
+
+        var capabilities = writeBackend.GetCapabilities();
+        return Ok(capabilities);
+    }
+}
+```
+
+## Key Features
+
+### Parallel Querying
+- Queries execute simultaneously across all enabled backends
+- Configurable concurrency limits and timeouts
+- Graceful handling of backend failures
+
+### Result Deduplication
+- Automatic deduplication based on URL
+- Higher scoring results from different backends take precedence
+- Can be disabled for scenarios requiring all results
+
+### Write Endpoint
+- Designate one backend as the primary write endpoint
+- Other backends remain read-only for queries
+- Useful for hybrid architectures
+
+### Backward Compatibility
+- Existing single-backend configurations continue to work
+- No breaking changes to existing APIs
+- Gradual migration path available
+
+## Migration from Single Backend
+
+1. Replace `AddNLWebNet<T>()` with `AddNLWebNetMultiBackend()`
+2. Set `MultiBackend.Enabled = false` initially to maintain existing behavior
+3. Configure additional backends in the `Endpoints` section
+4. Enable multi-backend mode by setting `MultiBackend.Enabled = true`
+5. Test and adjust concurrency and timeout settings as needed

samples/AspireDemo/AspireHost/Program.cs

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     options.AddFilter("Aspire.Hosting.ApplicationModel", LogLevel.Information);
     options.AddFilter("Aspire.Hosting", LogLevel.Information);
     options.AddFilter("Aspire", LogLevel.Warning);
-    
+
     // Reduce OpenTelemetry noise
     options.AddFilter("OpenTelemetry", LogLevel.Warning);
-    
+
     // Keep basic hosting messages
     options.AddFilter("Microsoft.Extensions.Hosting.Internal.Host", LogLevel.Information);
     options.AddFilter("Microsoft.Extensions.Hosting", LogLevel.Warning);
-    
+
     // Reduce ASP.NET Core noise but keep startup messages
     options.AddFilter("Microsoft.AspNetCore.Hosting.Diagnostics", LogLevel.Information);
     options.AddFilter("Microsoft.AspNetCore", LogLevel.Warning);
-    
+
     // Reduce DI and HTTP noise
     options.AddFilter("Microsoft.Extensions.DependencyInjection", LogLevel.Warning);
     options.AddFilter("System.Net.Http", LogLevel.Warning);