feat: Implement idempotency for B/P requests

Examples/BookingSystem.AspNetCore/BookingSystem.AspNetCore.csproj

@@ -7,6 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="2.1.2" />
+    <PackageReference Include="System.Runtime.Caching" Version="7.0.0" />
   </ItemGroup>
 
   <ItemGroup>

Examples/BookingSystem.AspNetCore/Settings/EngineConfig.cs

@@ -166,6 +166,9 @@ public static StoreBookingEngine CreateStoreBookingEngine(AppSettings appSetting
                         ),
                     HasSingleSeller = appSettings.FeatureFlags.SingleSeller,
 
+                    // IdempotencyStore used for storing the response to Order Creation B/P requests
+                    IdempotencyStore = new AcmeIdempotencyStore(),
+
                     OpenDataFeeds = new Dictionary<OpportunityType, IOpportunityDataRpdeFeedGenerator> {
                         {
                             OpportunityType.ScheduledSession, new AcmeScheduledSessionRpdeGenerator(fakeBookingSystem)

Examples/BookingSystem.AspNetCore/Stores/IdempotencyStore.cs

@@ -0,0 +1,25 @@
+using OpenActive.Server.NET.OpenBookingHelper;
+using System;
+using System.Threading.Tasks;
+using System.Runtime.Caching;
+
+namespace BookingSystem
+{
+    public class AcmeIdempotencyStore : IdempotencyStore
+    {
+        private readonly ObjectCache _cache = MemoryCache.Default;
+
+        protected override ValueTask<string> GetSuccessfulOrderCreationResponse(string idempotencyKey)
+        {
+            return new ValueTask<string>((string)_cache.Get(idempotencyKey));
+        }
+
+        protected override ValueTask SetSuccessfulOrderCreationResponse(string idempotencyKey, string responseJson)
+        {
+            var policy = new CacheItemPolicy();
+            policy.AbsoluteExpiration = DateTimeOffset.Now.AddMinutes(5);
+            _cache.Set(idempotencyKey, responseJson, policy);
+            return new ValueTask();
+        }
+    }
+}

OpenActive.Server.NET/CustomBookingEngine/CustomBookingEngine.cs

@@ -414,9 +414,6 @@ private async Task<ResponseContent> ProcessCheckpoint(string clientId, Uri selle
         }
         public async Task<ResponseContent> ProcessOrderCreationB(string clientId, Uri sellerId, string uuidString, string orderJson, Uri customerAccountId = null)
         {
-
-            // Note B will never contain OrderItem level errors, and any issues that occur will be thrown as exceptions.
-            // If C1 and C2 are used correctly, B should not fail except in very exceptional cases.
             Order order = OpenActiveSerializer.Deserialize<Order>(orderJson);
             if (order == null || order.GetType() != typeof(Order))
             {
@@ -425,21 +422,23 @@ public async Task<ResponseContent> ProcessOrderCreationB(string clientId, Uri se
             var (orderId, sellerIdComponents, seller, customerAccountIdComponents) = await ConstructIdsFromRequest(clientId, sellerId, customerAccountId, uuidString, OrderType.Order);
             using (await asyncDuplicateLock.LockAsync(GetParallelLockKey(orderId)))
             {
+                // Attempt to use idempotency cache if it exists
+                var cachedResponse = await GetResponseFromIdempotencyStoreIfExists(settings, orderId, orderJson);
+                if (cachedResponse != null) 
+                {
+                    return cachedResponse;
+                }
+
                 var response = order.OrderProposalVersion != null ?
                      await ProcessOrderCreationFromOrderProposal(orderId, settings.OrderIdTemplate, seller, sellerIdComponents, customerAccountIdComponents, order) :
                      await ProcessFlowRequest(ValidateFlowRequest<Order>(orderId, sellerIdComponents, seller, customerAccountIdComponents, FlowStage.B, order), order);
 
-                // Return a 409 status code if any OrderItem level errors exist
-                return ResponseContent.OpenBookingResponse(OpenActiveSerializer.Serialize(response),
-                    response.OrderedItem.Exists(x => x.Error?.Count > 0) ? HttpStatusCode.Conflict : HttpStatusCode.Created);
+                return await CreateResponseViaIdempotencyStoreIfExists(settings, orderId, orderJson, response);
             }
         }
 
         public async Task<ResponseContent> ProcessOrderProposalCreationP(string clientId, Uri sellerId, string uuidString, string orderJson, Uri customerAccountId = null)
         {
-
-            // Note B will never contain OrderItem level errors, and any issues that occur will be thrown as exceptions.
-            // If C1 and C2 are used correctly, P should not fail except in very exceptional cases.
             OrderProposal order = OpenActiveSerializer.Deserialize<OrderProposal>(orderJson);
             if (order == null || order.GetType() != typeof(OrderProposal))
             {
@@ -448,14 +447,47 @@ public async Task<ResponseContent> ProcessOrderProposalCreationP(string clientId
             var (orderId, sellerIdComponents, seller, customerAccountIdComponents) = await ConstructIdsFromRequest(clientId, sellerId, customerAccountId, uuidString, OrderType.OrderProposal);
             using (await asyncDuplicateLock.LockAsync(GetParallelLockKey(orderId)))
             {
+                // Attempt to use idempotency cache if it exists
+                var cachedResponse = await GetResponseFromIdempotencyStoreIfExists(settings, orderId, orderJson);
+                if (cachedResponse != null) 
+                {
+                    return cachedResponse;
+                }
+
                 var response = await ProcessFlowRequest(ValidateFlowRequest<OrderProposal>(orderId, sellerIdComponents, seller, customerAccountIdComponents, FlowStage.P, order), order);
 
-                // Return a 409 status code if any OrderItem level errors exist
-                return ResponseContent.OpenBookingResponse(OpenActiveSerializer.Serialize(response),
-                    response.OrderedItem.Exists(x => x.Error?.Count > 0) ? HttpStatusCode.Conflict : HttpStatusCode.Created);
+                return await CreateResponseViaIdempotencyStoreIfExists(settings, orderId, orderJson, response);
             }
         }
 
+        private async Task<ResponseContent> GetResponseFromIdempotencyStoreIfExists(BookingEngineSettings settings, OrderIdComponents orderId, string orderJson)
+        {
+             // Attempt to use idempotency cache if it exists
+            if (settings.IdempotencyStore != null)
+            {
+                var cachedResponse = await settings.IdempotencyStore.GetSuccessfulOrderCreationResponse(orderId, orderJson);
+                if (cachedResponse != null)
+                {
+                    return ResponseContent.OpenBookingResponse(cachedResponse, HttpStatusCode.Created);
+                }
+            }
+            return null;
+        }
+
+        private async Task<ResponseContent> CreateResponseViaIdempotencyStoreIfExists(BookingEngineSettings settings, OrderIdComponents orderId, string orderJson, Order response) {
+            // Return a 409 status code if any OrderItem level errors exist
+            var httpStatusCode = response.OrderedItem.Exists(x => x.Error?.Count > 0) ? HttpStatusCode.Conflict : HttpStatusCode.Created;
+            var responseJson = OpenActiveSerializer.Serialize(response);
+
+            // Store response in idempotency cache if it exists, and if the response is successful
+            if (settings.IdempotencyStore != null && httpStatusCode == HttpStatusCode.Created)
+            {
+                await settings.IdempotencyStore.SetSuccessfulOrderCreationResponse(orderId, orderJson, responseJson);
+            }
+
+            return ResponseContent.OpenBookingResponse(responseJson, httpStatusCode);
+        }
+
         private SimpleIdComponents GetSimpleIdComponentsFromApiKey(Uri sellerId)
         {
             // Return empty SimpleIdComponents in Single Seller mode, as it is not required in the API Key

OpenActive.Server.NET/OpenBookingHelper/Settings/BookingEngineSettings.cs

@@ -28,6 +28,7 @@ public class BookingEngineSettings
         public OrdersRPDEFeedGenerator OrdersFeedGenerator { get; set; }
         public OrdersRPDEFeedGenerator OrderProposalsFeedGenerator { get; set; }
         public SellerStore SellerStore { get; set; }
+        public IdempotencyStore IdempotencyStore { get; set; }
         public bool HasSingleSeller { get; set; } = false;
         /// <summary>
         /// TTL in the Cache-Control header for all RPDE pages that contain greater than zero items
@@ -44,5 +45,5 @@ public class BookingEngineSettings
         /// See https://developer.openactive.io/publishing-data/data-feeds/scaling-feeds for CDN configuration instructions
         /// </summary>
         public TimeSpan DatasetSiteCacheDuration { get; set; } = TimeSpan.FromMinutes(15);
-    }
+  }
 }

OpenActive.Server.NET/OpenBookingHelper/Stores/IdempotencyStore.cs

@@ -0,0 +1,34 @@
+using OpenActive.NET;
+using System;
+using System.Threading.Tasks;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace OpenActive.Server.NET.OpenBookingHelper
+{
+    public abstract class IdempotencyStore
+    {
+        protected abstract ValueTask<string> GetSuccessfulOrderCreationResponse(string idempotencyKey);
+        protected abstract ValueTask SetSuccessfulOrderCreationResponse(string idempotencyKey, string responseJson);
+
+        internal ValueTask<string> GetSuccessfulOrderCreationResponse(OrderIdComponents orderId, string orderJson) {
+            return GetSuccessfulOrderCreationResponse(CalculateIdempotencyKey(orderId, orderJson));
+        }
+
+        internal ValueTask SetSuccessfulOrderCreationResponse(OrderIdComponents orderId, string orderJson, string responseJson) {
+            return SetSuccessfulOrderCreationResponse(CalculateIdempotencyKey(orderId, orderJson), responseJson);
+        }
+
+        protected string CalculateIdempotencyKey(OrderIdComponents orderId, string orderJson) {
+            return $"{orderId.ClientId}|{orderId.uuid}|{orderId.OrderType.ToString()}|{ComputeSHA256Hash(orderJson)}";
+        }
+
+        protected static string ComputeSHA256Hash(string text)
+        {
+            using (var sha256 = SHA256.Create())
+            {
+                return BitConverter.ToString(sha256.ComputeHash(Encoding.UTF8.GetBytes(text))).Replace("-", "");
+            }                
+        }
+    }
+}