Detect and recover projectors that are ahead of the event store.

Samples/ExampleHost/App.config

@@ -15,7 +15,7 @@
       </dependentAssembly>
       <dependentAssembly>
         <assemblyIdentity name="Microsoft.Owin" publicKeyToken="31bf3856ad364e35" culture="neutral" />
-        <bindingRedirect oldVersion="0.0.0.0-3.0.1.0" newVersion="3.0.1.0" />
+        <bindingRedirect oldVersion="0.0.0.0-3.1.0.0" newVersion="3.1.0.0" />
       </dependentAssembly>
       <dependentAssembly>
         <assemblyIdentity name="System.Web.Http.Owin" publicKeyToken="31bf3856ad364e35" culture="neutral" />
@@ -25,6 +25,10 @@
         <assemblyIdentity name="Microsoft.Owin.Hosting" publicKeyToken="31bf3856ad364e35" culture="neutral" />
         <bindingRedirect oldVersion="0.0.0.0-3.0.1.0" newVersion="3.0.1.0" />
       </dependentAssembly>
+      <dependentAssembly>
+        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
+        <bindingRedirect oldVersion="0.0.0.0-10.0.0.0" newVersion="10.0.0.0" />
+      </dependentAssembly>
     </assemblyBinding>
   </runtime>
 </configuration>

Samples/ExampleHost/CountsProjector.cs

@@ -23,7 +23,7 @@ public CountsProjector(Dispatcher dispatcher, InMemoryDatabase store)
 
         public void Start()
         {
-            dispatcher.Subscribe(null, async transactions =>
+            dispatcher.Subscribe(null, async (transactions, info) =>
             {
                 await documentCountProjector.Handle(transactions);
             });

Samples/ExampleHost/ExampleHost.csproj

@@ -39,25 +39,20 @@
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
-    <Reference Include="Microsoft.Owin, Version=3.0.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
-      <HintPath>..\..\packages\Microsoft.Owin.3.0.1\lib\net45\Microsoft.Owin.dll</HintPath>
-      <Private>True</Private>
+    <Reference Include="Microsoft.Owin, Version=3.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Microsoft.Owin.3.1.0\lib\net45\Microsoft.Owin.dll</HintPath>
     </Reference>
-    <Reference Include="Microsoft.Owin.Diagnostics, Version=3.0.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
-      <HintPath>..\..\packages\Microsoft.Owin.Diagnostics.3.0.1\lib\net45\Microsoft.Owin.Diagnostics.dll</HintPath>
-      <Private>True</Private>
+    <Reference Include="Microsoft.Owin.Diagnostics, Version=3.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Microsoft.Owin.Diagnostics.3.1.0\lib\net45\Microsoft.Owin.Diagnostics.dll</HintPath>
     </Reference>
-    <Reference Include="Microsoft.Owin.Host.HttpListener, Version=3.0.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
-      <HintPath>..\..\packages\Microsoft.Owin.Host.HttpListener.3.0.1\lib\net45\Microsoft.Owin.Host.HttpListener.dll</HintPath>
-      <Private>True</Private>
+    <Reference Include="Microsoft.Owin.Host.HttpListener, Version=3.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Microsoft.Owin.Host.HttpListener.3.1.0\lib\net45\Microsoft.Owin.Host.HttpListener.dll</HintPath>
     </Reference>
-    <Reference Include="Microsoft.Owin.Hosting, Version=3.0.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
-      <HintPath>..\..\packages\Microsoft.Owin.Hosting.3.0.1\lib\net45\Microsoft.Owin.Hosting.dll</HintPath>
-      <Private>True</Private>
+    <Reference Include="Microsoft.Owin.Hosting, Version=3.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Microsoft.Owin.Hosting.3.1.0\lib\net45\Microsoft.Owin.Hosting.dll</HintPath>
     </Reference>
-    <Reference Include="Newtonsoft.Json, Version=6.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed, processorArchitecture=MSIL">
-      <HintPath>..\..\packages\Newtonsoft.Json.6.0.4\lib\net45\Newtonsoft.Json.dll</HintPath>
-      <Private>True</Private>
+    <Reference Include="Newtonsoft.Json, Version=10.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed, processorArchitecture=MSIL">
+      <HintPath>..\..\packages\Newtonsoft.Json.10.0.2\lib\net45\Newtonsoft.Json.dll</HintPath>
     </Reference>
     <Reference Include="Owin, Version=1.0.0.0, Culture=neutral, PublicKeyToken=f0ebd12fd5e55cc5, processorArchitecture=MSIL">
       <HintPath>..\..\packages\Owin.1.0\lib\net40\Owin.dll</HintPath>

Samples/ExampleHost/JsonFileEventStore.cs

@@ -5,6 +5,7 @@
 using System.Linq;
 using System.Runtime.Serialization.Formatters;
 using System.Threading.Tasks;
+using LiquidProjections.Abstractions;
 using Newtonsoft.Json;
 
 namespace LiquidProjections.ExampleHost
@@ -25,9 +26,11 @@ public JsonFileEventStore(string filePath, int pageSize)
             entryQueue = new Queue<ZipArchiveEntry>(zip.Entries.Where(e => e.Name.EndsWith(".json")));
         }
 
-        public IDisposable Subscribe(long? lastProcessedCheckpoint, Func<IReadOnlyList<Transaction>, Task> handler, string subscriptionId)
+        public IDisposable Subscribe(long? lastProcessedCheckpoint, Subscriber subscriber, string subscriptionId)
         {
-            var subscriber = new Subscriber(lastProcessedCheckpoint ?? 0, handler);
+            var subscription = new Subscription(
+                lastProcessedCheckpoint ?? 0, 
+                transactions => subscriber.HandleTransactions(transactions, null));
 
             Task.Run(async () =>
             {
@@ -39,13 +42,13 @@ public IDisposable Subscribe(long? lastProcessedCheckpoint, Func<IReadOnlyList<T
                     // Start loading the next page on a separate thread while we have the subscriber handle the previous transactions.
                     loader = LoadNextPageAsync();
 
-                    await subscriber.Send(transactions);
+                    await subscription.Send(transactions);
 
                     transactions = await loader;
                 }
             });
 
-            return subscriber;
+            return subscription;
         }
 
         private Task<Transaction[]> LoadNextPageAsync()
@@ -72,7 +75,7 @@ private Task<Transaction[]> LoadNextPageAsync()
                             Body = JsonConvert.DeserializeObject(json, new JsonSerializerSettings
                             {
                                 TypeNameHandling = TypeNameHandling.All,
-                                TypeNameAssemblyFormat = FormatterAssemblyStyle.Full
+                                TypeNameAssemblyFormatHandling = TypeNameAssemblyFormatHandling.Full
                             })
                         });
                     }
@@ -105,13 +108,13 @@ public void Dispose()
             zip = null;
         }
 
-        internal class Subscriber : IDisposable
+        internal class Subscription : IDisposable
         {
             private readonly long lastProcessedCheckpoint;
             private readonly Func<IReadOnlyList<Transaction>, Task> handler;
             private bool disposed;
 
-            public Subscriber(long lastProcessedCheckpoint, Func<IReadOnlyList<Transaction>, Task> handler)
+            public Subscription(long lastProcessedCheckpoint, Func<IReadOnlyList<Transaction>, Task> handler)
             {
                 this.lastProcessedCheckpoint = lastProcessedCheckpoint;
                 this.handler = handler;

Samples/ExampleHost/packages.config

@@ -4,12 +4,12 @@
   <package id="Microsoft.AspNet.WebApi.Client" version="5.2.3" targetFramework="net45" />
   <package id="Microsoft.AspNet.WebApi.Core" version="5.2.3" targetFramework="net45" />
   <package id="Microsoft.AspNet.WebApi.Owin" version="5.2.3" targetFramework="net452" />
-  <package id="Microsoft.Owin" version="3.0.1" targetFramework="net452" />
-  <package id="Microsoft.Owin.Diagnostics" version="3.0.1" targetFramework="net452" />
-  <package id="Microsoft.Owin.Host.HttpListener" version="3.0.1" targetFramework="net452" />
-  <package id="Microsoft.Owin.Hosting" version="3.0.1" targetFramework="net452" />
-  <package id="Microsoft.Owin.SelfHost" version="3.0.1" targetFramework="net452" />
-  <package id="Newtonsoft.Json" version="6.0.4" targetFramework="net45" />
+  <package id="Microsoft.Owin" version="3.1.0" targetFramework="net45" />
+  <package id="Microsoft.Owin.Diagnostics" version="3.1.0" targetFramework="net45" />
+  <package id="Microsoft.Owin.Host.HttpListener" version="3.1.0" targetFramework="net45" />
+  <package id="Microsoft.Owin.Hosting" version="3.1.0" targetFramework="net45" />
+  <package id="Microsoft.Owin.SelfHost" version="3.1.0" targetFramework="net45" />
+  <package id="Newtonsoft.Json" version="10.0.2" targetFramework="net45" />
   <package id="Owin" version="1.0" targetFramework="net452" />
   <package id="TinyIoC" version="1.3" targetFramework="net452" developmentDependency="true" />
 </packages>

Src/LiquidProjections.Abstractions/CreateSubscription.cs

@@ -7,13 +7,40 @@ namespace LiquidProjections.Abstractions
     /// <summary>
     /// Creates a subscription on an event store, starting at the transaction following 
     /// <paramref name="lastProcessedCheckpoint"/>, identified by <paramref name="subscriptionId"/>, and which 
-    /// passed transactions to the provided <paramref name="handler"/>.
+    /// passes any transactions to the provided <paramref name="subscriber"/>.
     /// </summary>
-    /// <param name="lastProcessedCheckpoint"></param>
-    /// <param name="handler"></param>
-    /// <param name="subscriptionId"></param>
-    /// <returns></returns>
+    /// <param name="lastProcessedCheckpoint"> 
+    /// The checkpoint of the transaction the subscriber has last seen, or <c>null</c> to start from the beginning.
+    /// </param>
+    /// <param name="subscriber">
+    /// An object wrapping the various handlers that the subscription will use.
+    /// </param>
+    /// <param name="subscriptionId">
+    /// Identifies this subscription and helps distinct multiple subscriptions. 
+    /// </param>
+    /// <returns>
+    /// A disposable object that will cancel the subscription.
+    /// </returns>
     public delegate IDisposable CreateSubscription(long? lastProcessedCheckpoint,
-        Func<IReadOnlyList<Transaction>, Task> handler,
-        string subscriptionId);
+        Subscriber subscriber, string subscriptionId);
+
+    public class Subscriber
+    {
+        /// <summary>
+        /// Represents a handler that will receive the transactions that the event store pushes to the subscriber.
+        /// </summary>
+        public Func<IReadOnlyList<Transaction>, SubscriptionInfo, Task> HandleTransactions { get; set; }
+
+        /// <summary>
+        /// Represents a handler that the event store will use if the requested checkpoint does not exist. 
+        /// </summary>
+        public Func<SubscriptionInfo, Task> NoSuchCheckpoint { get; set; }
+    }
+
+    public class SubscriptionInfo
+    {
+        public string Id { get; set; }
+
+        public IDisposable Subscription { get; set; }
+    }
 }

Src/LiquidProjections.Abstractions/LiquidProjections.Abstractions.v3.ncrunchproject

@@ -0,0 +1,5 @@
+<ProjectConfiguration>
+  <Settings>
+    <PreviouslyBuiltSuccessfully>True</PreviouslyBuiltSuccessfully>
+  </Settings>
+</ProjectConfiguration>

Src/LiquidProjections.Testing/LiquidProjections.Testing.v3.ncrunchproject

@@ -0,0 +1,5 @@
+<ProjectConfiguration>
+  <Settings>
+    <PreviouslyBuiltSuccessfully>True</PreviouslyBuiltSuccessfully>
+  </Settings>
+</ProjectConfiguration>

Src/LiquidProjections.Testing/MemoryEventSource.cs

@@ -4,41 +4,50 @@
 using System.Globalization;
 using System.Linq;
 using System.Threading.Tasks;
-using LiquidProjections.Testing;
+using LiquidProjections.Abstractions;
 
-namespace LiquidProjections
+namespace LiquidProjections.Testing
 {
     public class MemoryEventSource
     {
         private readonly int batchSize;
         private long lastCheckpoint;
 
-        private readonly List<Subscriber> subscribers = new List<Subscriber>();
+        private readonly List<Subscription> subscribers = new List<Subscription>();
         private readonly List<Transaction> history = new List<Transaction>();
 
         public MemoryEventSource(int batchSize = 10)
         {
             this.batchSize = batchSize;
         }
 
-        public IDisposable Subscribe(long? lastProcessedCheckpoint, Func<IReadOnlyList<Transaction>, Task> handler, string subscriptionId)
+        public IDisposable Subscribe(long? lastProcessedCheckpoint, Subscriber subscriber, string subscriptionId)
         {
             lastCheckpoint = lastProcessedCheckpoint ?? 0;
-            var subscriber = new Subscriber(lastCheckpoint, batchSize, handler);
+            var subscription = new Subscription(lastCheckpoint, batchSize, subscriber, subscriptionId);
 
-            subscribers.Add(subscriber);
+            subscribers.Add(subscription);
 
-            Func<Task> asyncAction = async () =>
+            async Task AsyncAction()
             {
+                if (history.LastOrDefault()?.Checkpoint < lastProcessedCheckpoint)
+                {
+                    await subscriber.NoSuchCheckpoint(new SubscriptionInfo
+                    {
+                        Id = subscriptionId,
+                        Subscription = subscription
+                    });
+                }
+
                 foreach (Transaction transaction in history)
                 {
-                    await subscriber.Send(new[] { transaction }).ConfigureAwait(false);
+                    await subscription.Send(new[] {transaction}).ConfigureAwait(false);
                 }
-            };
+            }
 
-            asyncAction().ConfigureAwait(false).GetAwaiter().GetResult();
+            AsyncAction().ConfigureAwait(false).GetAwaiter().GetResult();
 
-            return subscriber;
+            return subscription;
         }
 
 
@@ -102,29 +111,46 @@ public async Task<Transaction> WriteWithHeaders(object anEvent, IDictionary<stri
 
             return transaction;
         }
+
+        public bool HasSubscriptionForId(string subscriptionId)
+        {
+            Subscription subscription = subscribers.SingleOrDefault(s => s.Id == subscriptionId);
+            return (subscription != null) && !subscription.IsDisposed;
+        }
     }
 
-    internal class Subscriber : IDisposable
+    internal class Subscription : IDisposable
     {
         private readonly long lastProcessedCheckpoint;
         private readonly int batchSize;
-        private readonly Func<IReadOnlyList<Transaction>, Task> handler;
+        private readonly Subscriber subscriber;
+        private readonly string subscriptionId;
         private bool disposed = false;
-
-        public Subscriber(long lastProcessedCheckpoint, int batchSize, Func<IReadOnlyList<Transaction>, Task> handler)
+
+        public Subscription(long lastProcessedCheckpoint, int batchSize,
+            Subscriber subscriber, string subscriptionId)
         {
             this.lastProcessedCheckpoint = lastProcessedCheckpoint;
             this.batchSize = batchSize;
-            this.handler = handler;
+            this.subscriber = subscriber;
+            this.subscriptionId = subscriptionId;
         }
 
         public async Task Send(IEnumerable<Transaction> transactions)
         {
             if (!disposed)
             {
-                foreach (var batch in transactions.Where(t => t.Checkpoint > lastProcessedCheckpoint).InBatchesOf(batchSize))
+                var subscriptionInfo = new SubscriptionInfo
                 {
-                    await handler(new ReadOnlyCollection<Transaction>(batch.ToList())).ConfigureAwait(false);
+                    Id = subscriptionId,
+                    Subscription = this
+                };
+
+                Transaction[] requestedTransactions = transactions.Where(t => t.Checkpoint > lastProcessedCheckpoint).ToArray();
+                foreach (var batch in requestedTransactions.InBatchesOf(batchSize))
+                {
+                    await subscriber.HandleTransactions(new ReadOnlyCollection<Transaction>(batch.ToList()), subscriptionInfo)
+                        .ConfigureAwait(false);
                 }
             }
         }
@@ -133,5 +159,9 @@ public void Dispose()
         {
             disposed = true;
         }
+
+        public bool IsDisposed => disposed;
+
+        public string Id => subscriptionId;
     }
 }