feat: connect Catalog to Basket with direct C# basket previews

src/Basket.API/Extensions/Extensions.cs

@@ -2,6 +2,7 @@
 using eShop.Basket.API.Repositories;
 using eShop.Basket.API.IntegrationEvents.EventHandling;
 using eShop.Basket.API.IntegrationEvents.EventHandling.Events;
+using eShop.Basket.API.Services;
 
 namespace eShop.Basket.API.Extensions;
 
@@ -14,6 +15,8 @@ public static void AddApplicationServices(this IHostApplicationBuilder builder)
         builder.AddRedisClient("redis");
 
         builder.Services.AddSingleton<IBasketRepository, RedisBasketRepository>();
+        builder.Services.AddHttpClient<CatalogClient>(client =>
+            client.BaseAddress = new("https+http://catalog-api"));
 
         builder.AddRabbitMqEventBus("eventbus")
                .AddSubscription<OrderStartedIntegrationEvent, OrderStartedIntegrationEventHandler>()

src/Basket.API/Grpc/BasketService.cs

@@ -2,12 +2,16 @@
 using eShop.Basket.API.Repositories;
 using eShop.Basket.API.Extensions;
 using eShop.Basket.API.Model;
+using eShop.Basket.API.Services;
+using Polly.CircuitBreaker;
+using Polly.Timeout;
 
 namespace eShop.Basket.API.Grpc;
 
 public class BasketService(
     IBasketRepository repository,
-    ILogger<BasketService> logger) : Basket.BasketBase
+    ILogger<BasketService> logger,
+    CatalogClient catalog) : Basket.BasketBase
 {
     [AllowAnonymous]
     public override async Task<CustomerBasketResponse> GetBasket(GetBasketRequest request, ServerCallContext context)
@@ -46,6 +50,40 @@ public override async Task<CustomerBasketResponse> UpdateBasket(UpdateBasketRequ
             logger.LogDebug("Begin UpdateBasket call from method {Method} for basket id {Id}", context.Method, userId);
         }
 
+        // Bound the batch lookup and reject ambiguous or invalid basket lines before any I/O.
+        var productIds = request.Items.Select(item => item.ProductId).ToArray();
+        if (request.Items.Count > 100 ||
+            request.Items.Any(item => item.ProductId <= 0 || item.Quantity <= 0) ||
+            productIds.Distinct().Count() != productIds.Length)
+        {
+            throw new RpcException(new Status(StatusCode.InvalidArgument,
+                "A basket supports up to 100 unique products with positive IDs and quantities."));
+        }
+
+        // Clearing a basket should remain possible even when Catalog is unavailable.
+        if (productIds.Length > 0)
+        {
+            HashSet<int> existingIds;
+            try
+            {
+                existingIds = await catalog.GetProductIdsAsync(productIds, context.CancellationToken);
+            }
+            catch (Exception exception) when (exception is HttpRequestException or JsonException or
+                TimeoutRejectedException or BrokenCircuitException ||
+                exception is OperationCanceledException && !context.CancellationToken.IsCancellationRequested)
+            {
+                logger.LogWarning(exception, "Catalog lookup failed while updating a basket");
+                throw new RpcException(new Status(StatusCode.Unavailable,
+                    "Catalog is temporarily unavailable. Your basket has not been changed. Please retry."));
+            }
+
+            if (productIds.Any(id => !existingIds.Contains(id)))
+            {
+                throw new RpcException(new Status(StatusCode.FailedPrecondition,
+                    "One or more products no longer exist in the catalog. Refresh your basket and try again."));
+            }
+        }
+
         var customerBasket = MapToCustomerBasket(userId, request);
         var response = await repository.UpdateBasketAsync(customerBasket);
         if (response is null)

src/Basket.API/Model/BasketPreview.cs

@@ -0,0 +1,13 @@
+namespace eShop.Basket.API.Model;
+
+/// <summary>A read-only line estimate. Creating a preview never writes a basket or reserves stock.</summary>
+public record BasketPreview(int ProductId, string ProductName, int Quantity, decimal UnitPrice, decimal TotalPrice)
+{
+    public static BasketPreview Create(BasketItem item)
+    {
+        Validator.ValidateObject(item, new ValidationContext(item), validateAllProperties: true);
+
+        return new BasketPreview(item.ProductId, item.ProductName, item.Quantity,
+            item.UnitPrice, item.UnitPrice * item.Quantity);
+    }
+}

src/Basket.API/Program.cs

@@ -1,6 +1,6 @@
 var builder = WebApplication.CreateBuilder(args);
 
-builder.AddBasicServiceDefaults();
+builder.AddServiceDefaults();
 builder.AddApplicationServices();
 
 builder.Services.AddGrpc();

src/Basket.API/Services/CatalogClient.cs

@@ -0,0 +1,24 @@
+using System.Text.Json.Serialization;
+
+namespace eShop.Basket.API.Services;
+
+public class CatalogClient(HttpClient httpClient)
+{
+    public async Task<HashSet<int>> GetProductIdsAsync(IEnumerable<int> productIds, CancellationToken cancellationToken)
+    {
+        var query = string.Join("&", productIds.Select(id => $"ids={id}"));
+        var products = await httpClient.GetFromJsonAsync(
+            $"/api/catalog/items/by?api-version=2.0&{query}",
+            CatalogJsonContext.Default.CatalogProductArray,
+            cancellationToken);
+
+        return products?.Select(product => product.Id).ToHashSet()
+            ?? throw new JsonException("Catalog returned a null product list.");
+    }
+}
+
+public record CatalogProduct(int Id);
+
+[JsonSourceGenerationOptions(JsonSerializerDefaults.Web)]
+[JsonSerializable(typeof(CatalogProduct[]))]
+internal partial class CatalogJsonContext : JsonSerializerContext;

src/Catalog.API/Apis/CatalogApi.cs

@@ -1,5 +1,6 @@
 using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
+using eShop.Basket.API.Model;
 using Microsoft.AspNetCore.Http.HttpResults;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
@@ -38,6 +39,11 @@ public static IEndpointRouteBuilder MapCatalogApi(this IEndpointRouteBuilder app
             .WithSummary("Get catalog item")
             .WithDescription("Get an item from the catalog")
             .WithTags("Items");
+        api.MapGet("/items/{id:int}/basket-preview", GetBasketPreview)
+            .WithName("PreviewBasketItem")
+            .WithSummary("Preview a basket line")
+            .WithDescription("Calculate a line estimate using Basket-owned C# validation and pricing code in-process. Does not write a basket or reserve stock.")
+            .WithTags("Items");
         v1.MapGet("/items/by/{name:minlength(1)}", GetItemsByName)
             .WithName("GetItemsByName")
             .WithSummary("Get catalog items by name")
@@ -235,6 +241,39 @@ public static async Task<Results<Ok<CatalogItem>, NotFound, BadRequest<ProblemDe
         return TypedResults.Ok(item);
     }
 
+    public static async Task<Results<Ok<BasketPreview>, NotFound, BadRequest<ProblemDetails>>> GetBasketPreview(
+        [AsParameters] CatalogServices services,
+        int id,
+        int quantity = 1)
+    {
+        if (id <= 0)
+        {
+            return TypedResults.BadRequest(new ProblemDetails { Detail = "Id is not valid" });
+        }
+
+        var item = await services.Context.CatalogItems.AsNoTracking().SingleOrDefaultAsync(item => item.Id == id);
+        if (item is null)
+        {
+            return TypedResults.NotFound();
+        }
+
+        try
+        {
+            // Direct C# call into the Basket assembly, not an HTTP request or an integration event.
+            return TypedResults.Ok(BasketPreview.Create(new BasketItem
+            {
+                ProductId = item.Id,
+                ProductName = item.Name,
+                UnitPrice = item.Price,
+                Quantity = quantity
+            }));
+        }
+        catch (ValidationException exception)
+        {
+            return TypedResults.BadRequest(new ProblemDetails { Detail = exception.Message });
+        }
+    }
+
     [ProducesResponseType<ProblemDetails>(StatusCodes.Status400BadRequest, "application/problem+json")]
     public static async Task<Ok<PaginatedItems<CatalogItem>>> GetItemsByName(
         [AsParameters] PaginationRequest paginationRequest,

src/Catalog.API/Catalog.API.csproj

@@ -37,6 +37,8 @@
   </ItemGroup>
 
   <ItemGroup>
+    <!-- Deliberate demo coupling: Catalog calls Basket's preview code in-process. -->
+    <ProjectReference Include="..\Basket.API\Basket.API.csproj" />
     <ProjectReference Include="..\EventBusRabbitMQ\EventBusRabbitMQ.csproj" />
     <ProjectReference Include="..\IntegrationEventLogEF\IntegrationEventLogEF.csproj" />
     <ProjectReference Include="..\eShop.ServiceDefaults\eShop.ServiceDefaults.csproj" />
@@ -50,4 +52,11 @@
     <InternalsVisibleTo Include="Catalog.FunctionalTests" />
   </ItemGroup>
 
+  <!-- Reuse Basket's assembly, not the configuration of its independently hosted service. -->
+  <Target Name="ExcludeBasketHostSettingsFromPublish" BeforeTargets="_HandleFileConflictsForPublish">
+    <ItemGroup>
+      <ResolvedFileToPublish Remove="$([MSBuild]::NormalizeDirectory('$(MSBuildProjectDirectory)', '..', 'Basket.API'))appsettings*.json" />
+    </ItemGroup>
+  </Target>
+
 </Project>

src/Catalog.API/Catalog.API.json

@@ -167,6 +167,76 @@
         }
       }
     },
+    "/api/catalog/items/{id}/basket-preview": {
+      "get": {
+        "tags": [
+          "Items"
+        ],
+        "summary": "Preview a basket line",
+        "description": "Calculate a line estimate using Basket-owned C# validation and pricing code in-process. Does not write a basket or reserve stock.",
+        "operationId": "PreviewBasketItem",
+        "parameters": [
+          {
+            "name": "id",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "pattern": "^-?(?:0|[1-9]\\d*)$",
+              "type": "integer",
+              "format": "int32"
+            }
+          },
+          {
+            "name": "quantity",
+            "in": "query",
+            "schema": {
+              "pattern": "^-?(?:0|[1-9]\\d*)$",
+              "type": [
+                "integer",
+                "string"
+              ],
+              "format": "int32",
+              "default": 1
+            }
+          },
+          {
+            "name": "api-version",
+            "in": "query",
+            "description": "The API version, in the format 'major.minor'.",
+            "required": true,
+            "schema": {
+              "type": "string",
+              "example": "1.0"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/BasketPreview"
+                }
+              }
+            }
+          },
+          "404": {
+            "description": "Not Found"
+          },
+          "400": {
+            "description": "Bad Request",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/catalog/items/{id}/pic": {
       "get": {
         "tags": [
@@ -955,6 +1025,56 @@
   },
   "components": {
     "schemas": {
+      "BasketPreview": {
+        "required": [
+          "productId",
+          "productName",
+          "quantity",
+          "unitPrice",
+          "totalPrice"
+        ],
+        "type": "object",
+        "properties": {
+          "productId": {
+            "pattern": "^-?(?:0|[1-9]\\d*)$",
+            "type": [
+              "integer",
+              "string"
+            ],
+            "format": "int32"
+          },
+          "productName": {
+            "type": [
+              "null",
+              "string"
+            ]
+          },
+          "quantity": {
+            "pattern": "^-?(?:0|[1-9]\\d*)$",
+            "type": [
+              "integer",
+              "string"
+            ],
+            "format": "int32"
+          },
+          "unitPrice": {
+            "pattern": "^-?(?:0|[1-9]\\d*)(?:\\.\\d+)?$",
+            "type": [
+              "number",
+              "string"
+            ],
+            "format": "double"
+          },
+          "totalPrice": {
+            "pattern": "^-?(?:0|[1-9]\\d*)(?:\\.\\d+)?$",
+            "type": [
+              "number",
+              "string"
+            ],
+            "format": "double"
+          }
+        }
+      },
       "CatalogBrand": {
         "required": [
           "brand"