diff --git a/IntegrationTests/IntegrationTests.csproj b/IntegrationTests/IntegrationTests.csproj
index 5578e21..9776643 100644
--- a/IntegrationTests/IntegrationTests.csproj
+++ b/IntegrationTests/IntegrationTests.csproj
@@ -73,8 +73,8 @@
     <Compile Include="Azure.cs">
       <SubType>Code</SubType>
     </Compile>
-    <Compile Include="File.cs" />
     <Compile Include="ITestScope.cs" />
+    <Compile Include="OptimisticFile.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Scenarios.cs" />
   </ItemGroup>
diff --git a/IntegrationTests/File.cs b/IntegrationTests/OptimisticFile.cs
similarity index 80%
rename from IntegrationTests/File.cs
rename to IntegrationTests/OptimisticFile.cs
index a43d836..c9b7c4e 100644
--- a/IntegrationTests/File.cs
+++ b/IntegrationTests/OptimisticFile.cs
@@ -1,48 +1,49 @@
-using System;
-using System.IO;
-using NUnit.Framework;
-using SnowMaker;
-
-namespace IntegrationTests.cs
-{
-    [TestFixture]
-    public class File : Scenarios<File.TestScope>
-    {
-        protected override TestScope BuildTestScope()
-        {
-            return new TestScope();
-        }
-
-        protected override IOptimisticDataStore BuildStore(TestScope scope)
-        {
-            return new DebugOnlyFileDataStore(scope.DirectoryPath);
-        }
-
-        public class TestScope : ITestScope
-        {
-            public TestScope()
-            {
-                var ticks = DateTime.UtcNow.Ticks;
-                IdScopeName = string.Format("snowmakertest{0}", ticks);
-
-                DirectoryPath = Path.Combine(Path.GetTempPath(), IdScopeName);
-                Directory.CreateDirectory(DirectoryPath);
-            }
-
-            public string IdScopeName { get; private set; }
-            public string DirectoryPath { get; private set; }
-
-            public string ReadCurrentPersistedValue()
-            {
-                var filePath = Path.Combine(DirectoryPath, string.Format("{0}.txt", IdScopeName));
-                return System.IO.File.ReadAllText(filePath);
-            }
-
-            public void Dispose()
-            {
-                if (Directory.Exists(DirectoryPath))
-                    Directory.Delete(DirectoryPath, true);
-            }
-        }
-    }
-}
+using System;
+using System.IO;
+using NUnit.Framework;
+using SnowMaker;
+
+namespace IntegrationTests.cs
+{
+    [TestFixture]
+    public class OptimisticFile : Scenarios<OptimisticFile.TestScope>
+    {
+        protected override TestScope BuildTestScope()
+        {
+            return new TestScope();
+        }
+
+        protected override IOptimisticDataStore BuildStore(TestScope scope)
+        {
+            return new FileOptimisticDataStore(scope.DirectoryPath);
+        }
+
+        public class TestScope : ITestScope
+        {
+            public TestScope()
+            {
+                var ticks = DateTime.UtcNow.Ticks;
+                IdScopeName = string.Format("snowmakertest{0}", ticks);
+
+                DirectoryPath = Path.Combine(Path.GetTempPath(), IdScopeName);
+                Directory.CreateDirectory(DirectoryPath);
+            }
+
+            public string IdScopeName { get; private set; }
+            public string DirectoryPath { get; private set; }
+
+            public string ReadCurrentPersistedValue()
+            {
+                var filePath = Path.Combine(DirectoryPath, string.Format("{0}.txt", IdScopeName));
+                using (TextReader reader = new StreamReader(filePath, System.Text.Encoding.Default))
+                    return reader.ReadToEnd();
+            }
+
+            public void Dispose()
+            {
+                if (Directory.Exists(DirectoryPath))
+                    Directory.Delete(DirectoryPath, true);
+            }
+        }
+    }
+}
diff --git a/SnowMaker/BlobOptimisticDataStore.cs b/SnowMaker/BlobOptimisticDataStore.cs
index d2d6048..fe6a4a0 100644
--- a/SnowMaker/BlobOptimisticDataStore.cs
+++ b/SnowMaker/BlobOptimisticDataStore.cs
@@ -4,6 +4,7 @@
 using Microsoft.WindowsAzure.Storage;
 using Microsoft.WindowsAzure.Storage.Blob;
 using System.IO;
+using System;
 
 namespace SnowMaker
 {
@@ -24,35 +25,37 @@ public BlobOptimisticDataStore(CloudStorageAccount account, string containerName
 
             blobReferences = new Dictionary<string, ICloudBlob>();
         }
-
-        public string GetData(string blockName)
+        
+        public long GetNextBatch(string blockName, int batchSize)
         {
+            if (batchSize <= 0)
+                throw new ArgumentOutOfRangeException("batchSize");
+
+            long id;
             var blobReference = GetBlobReference(blockName);
             using (var stream = new MemoryStream())
             {
                 blobReference.DownloadToStream(stream);
-                return Encoding.UTF8.GetString(stream.ToArray());
+                if (!Int64.TryParse(Encoding.UTF8.GetString(stream.ToArray()), out id))
+                    throw new Exception(String.Format("The id seed returned from the blob for blockName '{0}' was corrupt, and could not be parsed as a long. The data returned was: {1}", blockName, Encoding.UTF8.GetString(stream.ToArray())));
+                if (id <= 0)
+                    throw new Exception(String.Format("The id seed returned from the blob for blockName '{0}' was {1}", blockName, id));
             }
-        }
 
-        public bool TryOptimisticWrite(string scopeName, string data)
-        {
-            var blobReference = GetBlobReference(scopeName);
             try
             {
                 UploadText(
                     blobReference,
-                    data,
+                    (id + batchSize).ToString(),
                     AccessCondition.GenerateIfMatchCondition(blobReference.Properties.ETag));
             }
             catch (StorageException exc)
             {
                 if (exc.RequestInformation.HttpStatusCode == (int)HttpStatusCode.PreconditionFailed)
-                    return false;
-
-                throw;
+                    return -1;
             }
-            return true;
+
+            return id;
         }
 
         ICloudBlob GetBlobReference(string blockName)
diff --git a/SnowMaker/DebugOnlyFileDataStore.cs b/SnowMaker/DebugOnlyFileDataStore.cs
deleted file mode 100644
index fab24d4..0000000
--- a/SnowMaker/DebugOnlyFileDataStore.cs
+++ /dev/null
@@ -1,41 +0,0 @@
-using System.IO;
-
-namespace SnowMaker
-{
-    public class DebugOnlyFileDataStore : IOptimisticDataStore
-    {
-        const string SeedValue = "1";
-
-        readonly string directoryPath;
-
-        public DebugOnlyFileDataStore(string directoryPath)
-        {
-            this.directoryPath = directoryPath;
-        }
-
-        public string GetData(string blockName)
-        {
-            var blockPath = Path.Combine(directoryPath, string.Format("{0}.txt", blockName));
-            try
-            {
-                return File.ReadAllText(blockPath);
-            }
-            catch (FileNotFoundException)
-            {
-                using (var file = File.Create(blockPath))
-                using (var streamWriter = new StreamWriter(file))
-                {
-                    streamWriter.Write(SeedValue);
-                }
-                return SeedValue;
-            }
-        }
-
-        public bool TryOptimisticWrite(string blockName, string data)
-        {
-            var blockPath = Path.Combine(directoryPath, string.Format("{0}.txt", blockName));
-            File.WriteAllText(blockPath, data);
-            return true;
-        }
-    }
-}
diff --git a/SnowMaker/FileOptimisticDataStore.cs b/SnowMaker/FileOptimisticDataStore.cs
new file mode 100644
index 0000000..afb14c3
--- /dev/null
+++ b/SnowMaker/FileOptimisticDataStore.cs
@@ -0,0 +1,72 @@
+using System;
+using System.IO;
+using System.Text;
+
+namespace SnowMaker
+{
+    public class FileOptimisticDataStore : IOptimisticDataStore
+    {
+        public static readonly Encoding Encoding = Encoding.Default;
+        public const long SeedValue = 1;
+
+        readonly string directoryPath;
+
+        public FileOptimisticDataStore(string directoryPath)
+        {
+            this.directoryPath = directoryPath;
+        }
+
+        public long GetNextBatch(string blockName, int batchSize)
+        {
+            if (batchSize <= 0)
+                throw new ArgumentOutOfRangeException("batchSize");
+
+            var blockPath = Path.Combine(directoryPath, string.Format("{0}.txt", blockName));
+
+            try
+            {
+                using (FileStream stream = File.Open(blockPath, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None))
+                {
+                    if (stream.Length == 0)
+                    {
+                        // a new file was created
+                        using (StreamWriter writer = new StreamWriter(stream, Encoding))
+                            writer.Write((SeedValue + batchSize).ToString());
+                        return SeedValue;
+                    }
+
+                    // read the next available id
+                    // can't use StreamReader to read here bc it would call Dispose on the provided stream object when StreamReader is disposed
+                    StringBuilder str = new StringBuilder();
+                    byte[] buffer = new byte[128];
+                    int offset = 0, length;
+                    do
+                    {
+                        length = stream.Read(buffer, offset, buffer.Length);
+                        str.Append(Encoding.GetString(buffer, 0, length));
+                        offset += length;
+                    }
+                    while (stream.Position < stream.Length);
+
+                    long id;
+                    if (!Int64.TryParse(str.ToString(), out id))
+                        throw new Exception(String.Format("The id seed returned from the file for blockName '{0}' was corrupt, and could not be parsed as a long. The data returned was: {1}", blockName, str.ToString()));
+                    if (id <= 0)
+                        throw new Exception(String.Format("The id seed returned from the file for blockName '{0}' was {1}", blockName, id));
+
+                    // mark the next batch as taken
+                    stream.Position = 0;
+                    stream.SetLength(Encoding.GetByteCount((id + batchSize).ToString()));
+                    using (StreamWriter writer = new StreamWriter(stream, Encoding))
+                        writer.Write((id + batchSize).ToString());
+
+                    return id;
+                }
+            }
+            catch (IOException)
+            {
+                return -1;
+            }
+        }
+    }
+}
diff --git a/SnowMaker/IOptimisticDataStore.cs b/SnowMaker/IOptimisticDataStore.cs
index e608f09..3df35da 100644
--- a/SnowMaker/IOptimisticDataStore.cs
+++ b/SnowMaker/IOptimisticDataStore.cs
@@ -2,7 +2,12 @@
 {
     public interface IOptimisticDataStore
     {
-        string GetData(string blockName);
-        bool TryOptimisticWrite(string blockName, string data);
+        /// <summary>
+        /// Marks the next <paramref name="batchSize"/> ids starting at the next available id
+        /// </summary>
+        /// <param name="blockName"></param>
+        /// <param name="batchSize"></param>
+        /// <returns>The first available id in the given batch size, or -1 if the call was unable to lock the store for exclusive access</returns>
+        long GetNextBatch(string blockName, int batchSize);
     }
 }
diff --git a/SnowMaker/SnowMaker.csproj b/SnowMaker/SnowMaker.csproj
index f9082bf..66ee00e 100644
--- a/SnowMaker/SnowMaker.csproj
+++ b/SnowMaker/SnowMaker.csproj
@@ -65,7 +65,7 @@
       <SubType>Code</SubType>
     </Compile>
     <Compile Include="DictionaryExtensions.cs" />
-    <Compile Include="DebugOnlyFileDataStore.cs" />
+    <Compile Include="FileOptimisticDataStore.cs" />
     <Compile Include="IOptimisticDataStore.cs">
       <SubType>Code</SubType>
     </Compile>
diff --git a/SnowMaker/UniqueIdGenerator.cs b/SnowMaker/UniqueIdGenerator.cs
index 7261695..9ef69f0 100644
--- a/SnowMaker/UniqueIdGenerator.cs
+++ b/SnowMaker/UniqueIdGenerator.cs
@@ -65,22 +65,13 @@ void UpdateFromSyncStore(string scopeName, ScopeState state)
 
             while (writesAttempted < maxWriteAttempts)
             {
-                var data = optimisticDataStore.GetData(scopeName);
-
-                long nextId;
-                if (!long.TryParse(data, out nextId))
-                    throw new UniqueIdGenerationException(string.Format(
-                       "The id seed returned from storage for scope '{0}' was corrupt, and could not be parsed as a long. The data returned was: {1}",
-                       scopeName,
-                       data));
-
-                state.LastId = nextId - 1;
-                state.HighestIdAvailableInBatch = nextId - 1 + batchSize;
-                var firstIdInNextBatch = state.HighestIdAvailableInBatch + 1;
-
-                if (optimisticDataStore.TryOptimisticWrite(scopeName, firstIdInNextBatch.ToString(CultureInfo.InvariantCulture)))
+                long id = optimisticDataStore.GetNextBatch(scopeName, batchSize);
+                if (id != -1)
+                {
+                    state.LastId = id - 1;
+                    state.HighestIdAvailableInBatch = id - 1 + batchSize;
                     return;
-
+                }
                 writesAttempted++;
             }
 
diff --git a/UnitTests/FileOptimisticDataStoreTests.cs b/UnitTests/FileOptimisticDataStoreTests.cs
new file mode 100644
index 0000000..cc8799c
--- /dev/null
+++ b/UnitTests/FileOptimisticDataStoreTests.cs
@@ -0,0 +1,146 @@
+using System;
+//using NSubstitute;
+using NUnit.Framework;
+using System.IO;
+using System.Threading.Tasks;
+using System.Threading;
+
+namespace SnowMaker.UnitTests
+{
+    [TestFixture]
+    public class FileOptimisticDataStoreTests
+    {
+        private class TestScope: IDisposable
+        {
+            public readonly string FilePath;
+
+            public TestScope(string scope)
+            {
+                FilePath = Path.Combine(Path.GetTempPath(), string.Format("{0}.txt", scope));
+            }
+
+            // does not lock the file
+            public long ReadCurrentPersistedValue()
+            {
+                using (TextReader reader = new StreamReader(FilePath, FileOptimisticDataStore.Encoding))
+                    return Convert.ToInt64(reader.ReadToEnd());
+            }
+
+            public void Dispose()
+            {
+                int count = 0;
+                do
+                {
+                    try
+                    {
+                        File.Delete(FilePath);
+                        Thread.Sleep(10);
+                        return;
+                    }
+                    catch
+                    {
+                        // retry, it could still be blocked by another thread from the tests
+                        count++;
+                    }
+                }
+                while(count < 5);
+            }
+        }
+
+        private const string scope = "test";
+        private const int batch = 1;
+
+        [Test]
+        public void ConstructorShouldNotCreateFile()
+        {
+            using (var testScope = new TestScope(scope))
+            {
+                var store = new FileOptimisticDataStore(Path.GetTempPath());
+                Assert.IsFalse(File.Exists(testScope.FilePath));
+            }
+        }
+
+        [Test]
+        public void ShouldCreateFileOnFirstAccess()
+        {
+            using (var testScope = new TestScope(scope))
+            {
+                var store = new FileOptimisticDataStore(Path.GetTempPath());
+                store.GetNextBatch(scope, batch);
+                Assert.IsTrue(File.Exists(testScope.FilePath));
+                Assert.AreEqual(testScope.ReadCurrentPersistedValue(), FileOptimisticDataStore.SeedValue + batch);
+            }
+        }
+
+        [Test]
+        public void GetNextBatchShouldBlockFileAccess()
+        {
+            using (var testScope = new TestScope(scope))
+            {
+                CancellationTokenSource cancelTokenSource1 = new CancellationTokenSource();
+                CancellationTokenSource cancelTokenSource2 = new CancellationTokenSource();
+
+                try
+                {
+                    var store = new FileOptimisticDataStore(Path.GetTempPath());
+                    store.GetNextBatch(scope, batch); // create the file
+
+                    CancellationToken cancelToken1 = cancelTokenSource1.Token;
+                    Task task1 = Task.Factory.StartNew(() =>
+                    {
+                        do
+                            store.GetNextBatch(scope, batch);
+                        while (!cancelToken1.IsCancellationRequested);
+                    }, cancelToken1);
+
+                    CancellationToken cancelToken2 = cancelTokenSource2.Token;
+                    Task task2 = Task.Factory.StartNew(() =>
+                    {
+                        do
+                        {
+                            try
+                            {
+                                testScope.ReadCurrentPersistedValue();
+                            }
+                            catch (IOException e)
+                            {
+                                if (e.Message.Equals("The process cannot access the file '" + testScope.FilePath + "' because it is being used by another process."))
+                                    return;
+                                throw;
+                            }
+                        }
+                        while (!cancelToken2.IsCancellationRequested);
+                    }, cancelToken2);
+
+                    if (task2.Wait(3000) && !task2.IsFaulted)
+                        Assert.Pass();
+                    else
+                    {
+                        if (task2.IsFaulted)
+                            Assert.Inconclusive("The second thread failed with error '" + task2.Exception.ToString() + "'.");
+                        else
+                            Assert.Inconclusive("The second thread was not blocked in an interval of 3000 ms.");
+                    }
+                }
+                catch
+                {
+                    cancelTokenSource1.Cancel();
+                    cancelTokenSource2.Cancel();
+                    throw;
+                }
+            }
+        }
+
+        [Test]
+        public void GetNextBatchShouldReturnMinusOneWhenBlocked()
+        {
+            using (var testScope = new TestScope(scope))
+            {
+                var store = new FileOptimisticDataStore(Path.GetTempPath());
+                store.GetNextBatch(scope, batch);
+                using (FileStream stream = File.Open(testScope.FilePath, FileMode.Open, FileAccess.Read, FileShare.None))
+                    Assert.AreEqual(-1, store.GetNextBatch(scope, batch));
+            }
+        }
+    }
+}
diff --git a/UnitTests/UniqueIdGeneratorTest.cs b/UnitTests/UniqueIdGeneratorTest.cs
index 207b1a6..3c23e92 100644
--- a/UnitTests/UniqueIdGeneratorTest.cs
+++ b/UnitTests/UniqueIdGeneratorTest.cs
@@ -13,7 +13,7 @@ public void ConstructorShouldNotRetrieveDataFromStore()
             var store = Substitute.For<IOptimisticDataStore>();
             // ReSharper disable once ObjectCreationAsStatement
             new UniqueIdGenerator(store);
-            store.DidNotReceiveWithAnyArgs().GetData(null);
+            store.DidNotReceiveWithAnyArgs().GetNextBatch(null, 1);
         }
 
         [Test]
@@ -40,79 +40,42 @@ public void MaxWriteAttemptsShouldThrowArgumentOutOfRangeExceptionWhenValueIsNeg
             };
         }
 
-        [Test]
-        [ExpectedException(typeof(UniqueIdGenerationException))]
-        public void NextIdShouldThrowExceptionOnCorruptData()
-        {
-            var store = Substitute.For<IOptimisticDataStore>();
-            store.GetData("test").Returns("abc");
-
-            var generator = new UniqueIdGenerator(store);
-
-            generator.NextId("test");
-        }
-
-        [Test]
-        [ExpectedException(typeof(UniqueIdGenerationException))]
-        public void NextIdShouldThrowExceptionOnNullData()
-        {
-            var store = Substitute.For<IOptimisticDataStore>();
-            store.GetData("test").Returns((string)null);
-
-            var generator = new UniqueIdGenerator(store);
-
-            generator.NextId("test");
-        }
-
         [Test]
         public void NextIdShouldReturnNumbersSequentially()
         {
             var store = Substitute.For<IOptimisticDataStore>();
-            store.GetData("test").Returns("0", "250");
-            store.TryOptimisticWrite("test", "3").Returns(true);
+            store.GetNextBatch("test", 3).Returns(1, 251);
 
-            var subject = new UniqueIdGenerator(store)
-            {
-                BatchSize = 3
-            };
+            var subject = new UniqueIdGenerator(store) { BatchSize = 3 };
 
-            Assert.AreEqual(0, subject.NextId("test"));
             Assert.AreEqual(1, subject.NextId("test"));
             Assert.AreEqual(2, subject.NextId("test"));
+            Assert.AreEqual(3, subject.NextId("test"));
         }
 
         [Test]
         public void NextIdShouldRollOverToNewBlockWhenCurrentBlockIsExhausted()
         {
             var store = Substitute.For<IOptimisticDataStore>();
-            store.GetData("test").Returns("0", "250");
-            store.TryOptimisticWrite("test", "3").Returns(true);
-            store.TryOptimisticWrite("test", "253").Returns(true);
+            store.GetNextBatch("test", 3).Returns(1, 251);
 
-            var subject = new UniqueIdGenerator(store)
-            {
-                BatchSize = 3
-            };
+            var subject = new UniqueIdGenerator(store) { BatchSize = 3 };
 
-            Assert.AreEqual(0, subject.NextId("test"));
             Assert.AreEqual(1, subject.NextId("test"));
             Assert.AreEqual(2, subject.NextId("test"));
-            Assert.AreEqual(250, subject.NextId("test"));
+            Assert.AreEqual(3, subject.NextId("test"));
             Assert.AreEqual(251, subject.NextId("test"));
             Assert.AreEqual(252, subject.NextId("test"));
+            Assert.AreEqual(253, subject.NextId("test"));
         }
 
         [Test]
         public void NextIdShouldThrowExceptionWhenRetriesAreExhausted()
         {
             var store = Substitute.For<IOptimisticDataStore>();
-            store.GetData("test").Returns("0");
-            store.TryOptimisticWrite("test", "3").Returns(false, false, false, true);
+            store.GetNextBatch("test", 100).Returns(-1, -1, -1, 1);
 
-            var generator = new UniqueIdGenerator(store)
-            {
-                MaxWriteAttempts = 3
-            };
+            var generator = new UniqueIdGenerator(store) { MaxWriteAttempts = 3 };
 
             try
             {
diff --git a/UnitTests/UnitTests.csproj b/UnitTests/UnitTests.csproj
index 4ea996b..60198d5 100644
--- a/UnitTests/UnitTests.csproj
+++ b/UnitTests/UnitTests.csproj
@@ -74,6 +74,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="DictionaryExtentionsTests.cs" />
+    <Compile Include="FileOptimisticDataStoreTests.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="UniqueIdGeneratorTest.cs" />
   </ItemGroup>