diff --git a/src/ZoneTree.UnitTests/InMemoryFileStreamProviderTests.cs b/src/ZoneTree.UnitTests/InMemoryFileStreamProviderTests.cs
new file mode 100644
index 0000000..cc76522
--- /dev/null
+++ b/src/ZoneTree.UnitTests/InMemoryFileStreamProviderTests.cs
@@ -0,0 +1,28 @@
+using Tenray.ZoneTree.AbstractFileStream;
+using Tenray.ZoneTree.Logger;
+using Tenray.ZoneTree.Serializers;
+using Tenray.ZoneTree.WAL;
+
+namespace Tenray.ZoneTree.UnitTests;
+
+public sealed class InMemoryFileStreamProviderTests
+{
+    [Test]
+    public void WalWithInMemoryProvider()
+    {
+        var serializer = new UnicodeStringSerializer();
+        var provider = new InMemoryFileStreamProvider();
+        var wal = new SyncFileSystemWriteAheadLog<string, string>(
+            new ConsoleLogger(),
+            provider,
+            serializer,
+            serializer,
+            "test.wal");
+        wal.Append("hello", "world", 0);
+        var result = wal.ReadLogEntries(false, false, true);
+        Assert.That(result.Success, Is.True);
+        Assert.That(result.Keys[0], Is.EqualTo("hello"));
+        Assert.That(result.Values[0], Is.EqualTo("world"));
+        wal.Drop();
+    }
+}
diff --git a/src/ZoneTree/AbstractFileStream/InMemoryFileStream.cs b/src/ZoneTree/AbstractFileStream/InMemoryFileStream.cs
new file mode 100644
index 0000000..23bee65
--- /dev/null
+++ b/src/ZoneTree/AbstractFileStream/InMemoryFileStream.cs
@@ -0,0 +1,52 @@
+using System;
+using System.IO;
+
+namespace Tenray.ZoneTree.AbstractFileStream;
+
+public sealed class InMemoryFileStream : MemoryStream, IFileStream
+{
+    readonly InMemoryFileStreamProvider Provider;
+
+    public string FilePath { get; }
+
+    public InMemoryFileStream(
+        InMemoryFileStreamProvider provider,
+        string path,
+        byte[] buffer)
+    {
+        Provider = provider;
+        FilePath = path;
+        if (buffer.Length > 0)
+            Write(buffer, 0, buffer.Length);
+        Position = 0;
+    }
+
+    public void Flush(bool flushToDisk)
+    {
+        Flush();
+    }
+
+    public int ReadFaster(byte[] buffer, int offset, int count)
+    {
+        int totalRead = 0;
+        while (totalRead < count)
+        {
+            int read = Read(buffer, offset + totalRead, count - totalRead);
+            if (read == 0)
+                throw new EndOfStreamException();
+            totalRead += read;
+        }
+        return totalRead;
+    }
+
+    public Stream ToStream() => this;
+
+    protected override void Dispose(bool disposing)
+    {
+        if (disposing)
+        {
+            Provider.UpdateFile(FilePath, ToArray());
+        }
+        base.Dispose(disposing);
+    }
+}
diff --git a/src/ZoneTree/AbstractFileStream/InMemoryFileStreamProvider.cs b/src/ZoneTree/AbstractFileStream/InMemoryFileStreamProvider.cs
new file mode 100644
index 0000000..6a604fd
--- /dev/null
+++ b/src/ZoneTree/AbstractFileStream/InMemoryFileStreamProvider.cs
@@ -0,0 +1,137 @@
+using System.Text;
+
+namespace Tenray.ZoneTree.AbstractFileStream;
+
+public sealed class InMemoryFileStreamProvider : IFileStreamProvider
+{
+    readonly Dictionary<string, byte[]> Files = new();
+    readonly HashSet<string> Directories = new();
+
+    public IFileStream CreateFileStream(
+        string path,
+        FileMode mode,
+        FileAccess access,
+        FileShare share,
+        int bufferSize = 4096,
+        FileOptions options = FileOptions.None)
+    {
+        lock (this)
+        {
+            if (!Files.ContainsKey(path))
+            {
+                if (mode == FileMode.Open)
+                    throw new FileNotFoundException(path);
+                Files[path] = Array.Empty<byte>();
+            }
+            else if (mode == FileMode.CreateNew)
+            {
+                throw new IOException($"File {path} already exists.");
+            }
+            else if (mode == FileMode.Create)
+            {
+                Files[path] = Array.Empty<byte>();
+            }
+            else if (mode == FileMode.Truncate)
+            {
+                Files[path] = Array.Empty<byte>();
+            }
+            var bytes = Files[path];
+            var stream = new InMemoryFileStream(this, path, bytes);
+            if (mode == FileMode.Append)
+                stream.Seek(0, SeekOrigin.End);
+            return stream;
+        }
+    }
+
+    public bool FileExists(string path)
+    {
+        lock (this) return Files.ContainsKey(path);
+    }
+
+    public bool DirectoryExists(string path)
+    {
+        lock (this) return Directories.Contains(path);
+    }
+
+    public void CreateDirectory(string path)
+    {
+        lock (this) Directories.Add(path);
+    }
+
+    public void DeleteFile(string path)
+    {
+        lock (this) Files.Remove(path);
+    }
+
+    public void DeleteDirectory(string path, bool recursive)
+    {
+        lock (this)
+        {
+            Directories.Remove(path);
+            if (recursive)
+            {
+                var toRemove = Files.Keys.Where(x => x.StartsWith(path)).ToList();
+                foreach (var f in toRemove)
+                    Files.Remove(f);
+            }
+        }
+    }
+
+    public string ReadAllText(string path)
+    {
+        lock (this) return Encoding.UTF8.GetString(Files[path]);
+    }
+
+    public byte[] ReadAllBytes(string path)
+    {
+        lock (this)
+        {
+            var b = Files[path];
+            var copy = new byte[b.Length];
+            Buffer.BlockCopy(b, 0, copy, 0, b.Length);
+            return copy;
+        }
+    }
+
+    public void Replace(
+        string sourceFileName,
+        string destinationFileName,
+        string destinationBackupFileName)
+    {
+        lock (this)
+        {
+            if (destinationBackupFileName != null && Files.ContainsKey(destinationFileName))
+            {
+                Files[destinationBackupFileName] = Files[destinationFileName];
+            }
+            Files[destinationFileName] = Files.ContainsKey(sourceFileName) ? Files[sourceFileName] : Array.Empty<byte>();
+            Files.Remove(sourceFileName);
+        }
+    }
+
+    public DurableFileWriter GetDurableFileWriter()
+    {
+        return new DurableFileWriter(this);
+    }
+
+    public IReadOnlyList<string> GetDirectories(string path)
+    {
+        lock (this)
+        {
+            return Directories.Where(x => x.StartsWith(path)).ToArray();
+        }
+    }
+
+    public string CombinePaths(string path1, string path2)
+    {
+        return Path.Combine(path1, path2);
+    }
+
+    internal void UpdateFile(string path, byte[] data)
+    {
+        lock (this)
+        {
+            Files[path] = data;
+        }
+    }
+}
diff --git a/src/ZoneTree/docs/ZoneTree/guide/quick-start.md b/src/ZoneTree/docs/ZoneTree/guide/quick-start.md
index e424c9c..c3569e0 100644
--- a/src/ZoneTree/docs/ZoneTree/guide/quick-start.md
+++ b/src/ZoneTree/docs/ZoneTree/guide/quick-start.md
@@ -162,4 +162,14 @@ while(iterator.Next()) {
    var key = iterator.CurrentKey;
    var value = iterator.CurrentValue;
 } 
-```
\ No newline at end of file
+```
+## Using the InMemoryFileStreamProvider
+
+The `InMemoryFileStreamProvider` keeps all files entirely in memory. It is useful for unit testing or scenarios that require fast, temporary storage without touching the disk.
+
+```csharp
+var provider = new InMemoryFileStreamProvider();
+using var zoneTree = new ZoneTreeFactory<int, string>(provider)
+    .OpenOrCreate();
+zoneTree.Upsert(1, "value");
+```