Implement checkpoint modes with docs/tests and reorganize project file layout

README.md

@@ -104,6 +104,27 @@ Operational procedures, diagnostics, and escalation are documented in:
 - [`docs/runbook.md`](docs/runbook.md)
 - [`docs/troubleshooting.md`](docs/troubleshooting.md)
 
+## Checkpoint Modes
+
+`DocumentDbContext` and `StorageEngine` support explicit checkpoint modes via `CheckpointMode`:
+
+- `Passive`: non-blocking; skips if checkpoint lock is contended.
+- `Full`: applies committed WAL pages and appends a checkpoint marker, without truncating WAL.
+- `Truncate`: applies committed WAL pages and truncates WAL.
+- `Restart`: truncate + WAL writer restart.
+
+Example:
+
+```csharp
+using ZB.MOM.WW.CBDD.Core.Transactions;
+
+var result = db.Checkpoint(CheckpointMode.Full);
+if (!result.Executed)
+{
+    // passive checkpoint can be skipped under contention
+}
+```
+
 ## Security And Compliance Posture
 
 - CBDD relies on host and process-level access controls.

docs/features/storage-transactions.md

@@ -29,6 +29,24 @@ Non-goals:
 - `WriteAheadLog`
 - Storage engine modules under `src/CBDD.Core/Storage`
 
+Checkpoint APIs:
+- `DocumentDbContext.Checkpoint(CheckpointMode mode = CheckpointMode.Truncate)`
+- `DocumentDbContext.CheckpointAsync(CheckpointMode mode = CheckpointMode.Truncate, CancellationToken ct = default)`
+- `StorageEngine.Checkpoint(CheckpointMode mode)`
+- `StorageEngine.CheckpointAsync(CheckpointMode mode, CancellationToken ct = default)`
+
+Checkpoint modes:
+- `Passive`: non-blocking best-effort checkpoint. Returns `Executed = false` when lock is contended.
+- `Full`: applies committed WAL pages and appends a WAL checkpoint marker without truncating WAL.
+- `Truncate`: applies committed WAL pages and truncates WAL (default behavior).
+- `Restart`: same as truncate, then reinitializes WAL writer session.
+
+Usage guidance:
+- Use `Passive` for background/low-priority maintenance where latency matters more than immediate WAL cleanup.
+- Use `Full` when you want durable page-file sync but prefer to preserve WAL history until a later truncate.
+- Use `Truncate` for routine manual checkpoints and disk-space recovery.
+- Use `Restart` for aggressive maintenance boundaries (for example after incident remediation flows).
+
 ## Permissions And Data Handling
 
 - Database files require host-managed filesystem access controls.

docs/troubleshooting.md

@@ -28,6 +28,8 @@ Database startup fails or recovery path throws WAL/storage errors.
 ### Resolution
 - Pin consumers to last known-good package.
 - Apply fix and add regression coverage in recovery/transaction tests.
+- If WAL growth is the issue, run `CheckpointMode.Truncate` (or `Restart`) instead of `Full`.
+- If foreground latency is a concern, schedule `CheckpointMode.Passive` retries and use `Truncate` during maintenance windows.
 
 ## Query And Index Issues
 

src/CBDD.Core/Context/DocumentDbContext.cs

@@ -329,6 +329,33 @@ public abstract partial class DocumentDbContext : IDisposable, ITransactionHolde
         }
     }
 
+    /// <summary>
+    /// Executes a checkpoint using the requested mode.
+    /// </summary>
+    /// <param name="mode">Checkpoint mode to execute.</param>
+    /// <returns>The checkpoint execution result.</returns>
+    public CheckpointResult Checkpoint(CheckpointMode mode = CheckpointMode.Truncate)
+    {
+        if (_disposed)
+            throw new ObjectDisposedException(nameof(DocumentDbContext));
+
+        return Engine.Checkpoint(mode);
+    }
+
+    /// <summary>
+    /// Executes a checkpoint asynchronously using the requested mode.
+    /// </summary>
+    /// <param name="mode">Checkpoint mode to execute.</param>
+    /// <param name="ct">The cancellation token.</param>
+    /// <returns>The checkpoint execution result.</returns>
+    public Task<CheckpointResult> CheckpointAsync(CheckpointMode mode = CheckpointMode.Truncate, CancellationToken ct = default)
+    {
+        if (_disposed)
+            throw new ObjectDisposedException(nameof(DocumentDbContext));
+
+        return Engine.CheckpointAsync(mode, ct);
+    }
+
     /// <summary>
     /// Returns a point-in-time snapshot of compression telemetry counters.
     /// </summary>

src/CBDD.Core/Query/BsonExpressionEvaluator.cs

@@ -75,7 +75,7 @@ internal static class BsonExpressionEvaluator
 
                     if (name == propertyName)
                     {
-                        // Found it! Read value and compare
+                        // Found -> read value and compare
                         return Compare(ref reader, type, targetValue, op);
                     }
 

src/CBDD.Core/Storage/StorageEngine.Recovery.cs

@@ -30,148 +30,241 @@ public sealed partial class StorageEngine
     }
 
     /// <summary>
-    /// Performs a checkpoint: merges WAL into PageFile.
+    /// Performs a truncate checkpoint by default.
-    /// Uses in-memory WAL index for efficiency and consistency.
-    /// </summary>
-    /// <summary>
-    /// Performs a checkpoint: merges WAL into PageFile.
-    /// Uses in-memory WAL index for efficiency and consistency.
     /// </summary>
     public void Checkpoint()
+    {
+        _ = Checkpoint(CheckpointMode.Truncate);
+    }
+
+    /// <summary>
+    /// Performs a checkpoint using the requested mode.
+    /// </summary>
+    /// <param name="mode">Checkpoint mode to execute.</param>
+    /// <returns>The checkpoint execution result.</returns>
+    public CheckpointResult Checkpoint(CheckpointMode mode)
+    {
+        bool lockAcquired;
+        if (mode == CheckpointMode.Passive)
+        {
+            lockAcquired = _commitLock.Wait(0);
+            if (!lockAcquired)
+            {
+                var walSize = _wal.GetCurrentSize();
+                return new CheckpointResult(mode, false, 0, walSize, walSize, false, false);
+            }
+        }
+        else
         {
             _commitLock.Wait();
+            lockAcquired = true;
+        }
+
         try
         {
-            CheckpointInternal();
+            return CheckpointInternal(mode);
         }
         finally
+        {
+            if (lockAcquired)
             {
                 _commitLock.Release();
             }
         }
+    }
 
     private void CheckpointInternal()
-    {
+        => _ = CheckpointInternal(CheckpointMode.Truncate);
-        if (_walIndex.IsEmpty)
-        {
-            // WAL may still contain begin/commit records for read-only transactions.
-            if (_wal.GetCurrentSize() > 0)
-            {
-                _wal.Truncate();
-            }
-            return;
-        }
 
-        // 1. Write all committed pages from index to PageFile
+    private CheckpointResult CheckpointInternal(CheckpointMode mode)
+    {
+        var walBytesBefore = _wal.GetCurrentSize();
+        var appliedPages = 0;
+        var truncated = false;
+        var restarted = false;
+
+        // 1. Write all committed pages from index to PageFile.
         foreach (var kvp in _walIndex)
         {
             _pageFile.WritePage(kvp.Key, kvp.Value);
+            appliedPages++;
         }
 
-        // 2. Flush PageFile to ensure durability
+        // 2. Flush PageFile to ensure durability.
+        if (appliedPages > 0)
+        {
             _pageFile.Flush();
+        }
 
-        // 3. Clear in-memory WAL index (now persisted)
+        // 3. Clear in-memory WAL index (now persisted).
         _walIndex.Clear();
 
-        // 4. Truncate WAL (all changes now in PageFile)
+        // 4. Apply mode-specific WAL handling.
+        switch (mode)
+        {
+            case CheckpointMode.Passive:
+            case CheckpointMode.Full:
+                if (walBytesBefore > 0 || appliedPages > 0)
+                {
+                    _wal.WriteCheckpointRecord();
+                    _wal.Flush();
+                }
+                break;
+            case CheckpointMode.Truncate:
+                if (walBytesBefore > 0)
+                {
                     _wal.Truncate();
+                    truncated = true;
+                }
+                break;
+            case CheckpointMode.Restart:
+                _wal.Restart();
+                truncated = true;
+                restarted = true;
+                break;
+            default:
+                throw new ArgumentOutOfRangeException(nameof(mode), mode, "Unsupported checkpoint mode.");
+        }
+
+        var walBytesAfter = _wal.GetCurrentSize();
+        return new CheckpointResult(mode, true, appliedPages, walBytesBefore, walBytesAfter, truncated, restarted);
     }
 
     /// <summary>
-    /// Performs a checkpoint asynchronously by merging WAL pages into the page file.
+    /// Performs a truncate checkpoint asynchronously by default.
     /// </summary>
     /// <param name="ct">The cancellation token.</param>
-    /// <returns>A task that represents the asynchronous checkpoint operation.</returns>
     public async Task CheckpointAsync(CancellationToken ct = default)
+    {
+        _ = await CheckpointAsync(CheckpointMode.Truncate, ct);
+    }
+
+    /// <summary>
+    /// Performs a checkpoint asynchronously using the requested mode.
+    /// </summary>
+    /// <param name="mode">Checkpoint mode to execute.</param>
+    /// <param name="ct">The cancellation token.</param>
+    /// <returns>A task that represents the asynchronous checkpoint operation.</returns>
+    public async Task<CheckpointResult> CheckpointAsync(CheckpointMode mode, CancellationToken ct = default)
+    {
+        bool lockAcquired;
+        if (mode == CheckpointMode.Passive)
+        {
+            lockAcquired = await _commitLock.WaitAsync(0, ct);
+            if (!lockAcquired)
+            {
+                var walSize = _wal.GetCurrentSize();
+                return new CheckpointResult(mode, false, 0, walSize, walSize, false, false);
+            }
+        }
+        else
         {
             await _commitLock.WaitAsync(ct);
+            lockAcquired = true;
+        }
+
         try
         {
-            if (_walIndex.IsEmpty)
+            // Checkpoint work is synchronous over MMF/page writes for now.
-            {
+            return CheckpointInternal(mode);
-                if (_wal.GetCurrentSize() > 0)
-                {
-                    _wal.Truncate();
-                }
-                return;
-            }
-
-            // 1. Write all committed pages from index to PageFile
-            // PageFile writes are sync (MMF), but that's fine as per plan (ValueTask strategy for MMF)
-            foreach (var kvp in _walIndex)
-            {
-                _pageFile.WritePage(kvp.Key, kvp.Value);
-            }
-
-            // 2. Flush PageFile to ensure durability
-            _pageFile.Flush();
-
-            // 3. Clear in-memory WAL index (now persisted)
-            _walIndex.Clear();
-
-            // 4. Truncate WAL (all changes now in PageFile)
-            // WAL truncation involves file resize and flush
-            // TODO: Add TruncateAsync to WAL? For now Truncate is sync.
-            _wal.Truncate();
         }
         finally
+        {
+            if (lockAcquired)
             {
                 _commitLock.Release();
             }
         }
+    }
 
     /// <summary>
     /// Recovers from crash by replaying WAL.
-    /// Applies all committed transactions to PageFile, then truncates WAL.
+    /// Applies committed transactions to PageFile in deterministic WAL order, then truncates WAL.
     /// </summary>
     public void Recover()
     {
         _commitLock.Wait();
         try
         {
-            // 1. Read WAL and identify committed transactions
+            // 1. Read WAL and locate the latest checkpoint boundary.
             var records = _wal.ReadAll();
-            var committedTxns = new HashSet<ulong>();
+            var startIndex = 0;
-            var txnWrites = new Dictionary<ulong, List<(uint pageId, byte[] data)>>();
+            for (var i = records.Count - 1; i >= 0; i--)
-
-            foreach (var record in records)
             {
-                if (record.Type == WalRecordType.Commit)
+                if (records[i].Type == WalRecordType.Checkpoint)
-                    committedTxns.Add(record.TransactionId);
-                else if (record.Type == WalRecordType.Write)
                 {
-                    if (!txnWrites.ContainsKey(record.TransactionId))
+                    startIndex = i + 1;
-                        txnWrites[record.TransactionId] = new List<(uint, byte[])>();
+                    break;
-
-                    if (record.AfterImage != null)
-                    {
-                        txnWrites[record.TransactionId].Add((record.PageId, record.AfterImage));
                 }
             }
+
+            // 2. Replay WAL in source order with deterministic commit application.
+            var pendingWrites = new Dictionary<ulong, List<(uint pageId, byte[] data)>>();
+            var appliedAny = false;
+
+            for (var i = startIndex; i < records.Count; i++)
+            {
+                var record = records[i];
+                switch (record.Type)
+                {
+                    case WalRecordType.Begin:
+                        if (!pendingWrites.ContainsKey(record.TransactionId))
+                        {
+                            pendingWrites[record.TransactionId] = new List<(uint, byte[])>();
+                        }
+                        break;
+                    case WalRecordType.Write:
+                        if (record.AfterImage == null)
+                        {
+                            break;
                         }
 
-            // 2. Apply committed transactions to PageFile
+                        if (!pendingWrites.TryGetValue(record.TransactionId, out var writes))
-            foreach (var txnId in committedTxns)
                         {
-                if (!txnWrites.ContainsKey(txnId))
+                            writes = new List<(uint, byte[])>();
-                    continue;
+                            pendingWrites[record.TransactionId] = writes;
+                        }
 
-                foreach (var (pageId, data) in txnWrites[txnId])
+                        writes.Add((record.PageId, record.AfterImage));
+                        break;
+                    case WalRecordType.Commit:
+                        if (!pendingWrites.TryGetValue(record.TransactionId, out var committedWrites))
+                        {
+                            break;
+                        }
+
+                        foreach (var (pageId, data) in committedWrites)
                         {
                             _pageFile.WritePage(pageId, data);
+                            appliedAny = true;
+                        }
+
+                        pendingWrites.Remove(record.TransactionId);
+                        break;
+                    case WalRecordType.Abort:
+                        pendingWrites.Remove(record.TransactionId);
+                        break;
+                    case WalRecordType.Checkpoint:
+                        pendingWrites.Clear();
+                        break;
                 }
             }
 
-            // 3. Flush PageFile to ensure durability
+            // 3. Flush PageFile to ensure durability.
+            if (appliedAny)
+            {
                 _pageFile.Flush();
+            }
 
-            // 4. Clear in-memory WAL index (redundant since we just recovered)
+            // 4. Clear in-memory WAL index (redundant since we just recovered).
             _walIndex.Clear();
 
-            // 5. Truncate WAL (all changes now in PageFile)
+            // 5. Truncate WAL (all changes now in PageFile).
+            if (_wal.GetCurrentSize() > 0)
+            {
                 _wal.Truncate();
             }
+        }
         finally
         {
             _commitLock.Release();

src/CBDD.Core/Transactions/CheckpointMode.cs

@@ -7,26 +7,46 @@ namespace ZB.MOM.WW.CBDD.Core.Transactions;
 public enum CheckpointMode
 {
     /// <summary>
-    /// Passive checkpoint: Non-blocking, best-effort transfer from WAL to database.
+    /// Passive checkpoint: non-blocking, best-effort transfer from WAL to database.
-    /// Does not wait for readers or writers. May not checkpoint all frames.
+    /// If the checkpoint lock is busy, the operation is skipped.
+    /// WAL content is preserved and a checkpoint marker is appended when work is applied.
     /// </summary>
     Passive = 0,
 
     /// <summary>
-    /// Full checkpoint: Waits for concurrent readers/writers, then checkpoints all
+    /// Full checkpoint: waits for the checkpoint lock, transfers committed pages to
-    /// committed transactions from WAL to database. Blocks until complete.
+    /// the page file, and preserves WAL content by appending a checkpoint marker.
     /// </summary>
     Full = 1,
 
     /// <summary>
-    /// Truncate checkpoint: Same as Full, but also truncates the WAL file after
+    /// Truncate checkpoint: same as <see cref="Full"/> but truncates WAL after
-    /// successful checkpoint. Use this to reclaim disk space.
+    /// successfully applying committed pages. Use this to reclaim disk space.
     /// </summary>
     Truncate = 2,
 
     /// <summary>
-    /// Restart checkpoint: Truncates WAL and restarts with a new WAL file.
+    /// Restart checkpoint: same as <see cref="Truncate"/> and then reinitializes
-    /// Forces a fresh start. Most aggressive mode.
+    /// the WAL stream for a fresh writer session.
     /// </summary>
     Restart = 3
 }
+
+/// <summary>
+/// Result of a checkpoint execution.
+/// </summary>
+/// <param name="Mode">Requested checkpoint mode.</param>
+/// <param name="Executed">True when checkpoint logic ran; false when skipped (for passive mode contention).</param>
+/// <param name="AppliedPages">Number of pages copied from WAL index to page file.</param>
+/// <param name="WalBytesBefore">WAL size before the operation.</param>
+/// <param name="WalBytesAfter">WAL size after the operation.</param>
+/// <param name="Truncated">True when WAL was truncated by this operation.</param>
+/// <param name="Restarted">True when WAL stream was restarted by this operation.</param>
+public readonly record struct CheckpointResult(
+    CheckpointMode Mode,
+    bool Executed,
+    int AppliedPages,
+    long WalBytesBefore,
+    long WalBytesAfter,
+    bool Truncated,
+    bool Restarted);

src/CBDD.Core/Transactions/WriteAheadLog.cs

@@ -225,6 +225,61 @@ public sealed class WriteAheadLog : IDisposable
     }
 
 
+    /// <summary>
+    /// Writes a checkpoint marker record.
+    /// </summary>
+    public void WriteCheckpointRecord()
+    {
+        _lock.Wait();
+        try
+        {
+            WriteCheckpointRecordInternal();
+        }
+        finally
+        {
+            _lock.Release();
+        }
+    }
+
+    /// <summary>
+    /// Writes a checkpoint marker record asynchronously.
+    /// </summary>
+    /// <param name="ct">The cancellation token.</param>
+    /// <returns>A task that represents the asynchronous write operation.</returns>
+    public async ValueTask WriteCheckpointRecordAsync(CancellationToken ct = default)
+    {
+        await _lock.WaitAsync(ct);
+        try
+        {
+            var buffer = System.Buffers.ArrayPool<byte>.Shared.Rent(17);
+            try
+            {
+                buffer[0] = (byte)WalRecordType.Checkpoint;
+                BitConverter.TryWriteBytes(buffer.AsSpan(1, 8), 0UL);
+                BitConverter.TryWriteBytes(buffer.AsSpan(9, 8), DateTimeOffset.UtcNow.ToUnixTimeMilliseconds());
+                await _walStream!.WriteAsync(new ReadOnlyMemory<byte>(buffer, 0, 17), ct);
+            }
+            finally
+            {
+                System.Buffers.ArrayPool<byte>.Shared.Return(buffer);
+            }
+        }
+        finally
+        {
+            _lock.Release();
+        }
+    }
+
+    private void WriteCheckpointRecordInternal()
+    {
+        Span<byte> buffer = stackalloc byte[17];  // type(1) + reserved(8) + timestamp(8)
+        buffer[0] = (byte)WalRecordType.Checkpoint;
+        BitConverter.TryWriteBytes(buffer[1..9], 0UL);
+        BitConverter.TryWriteBytes(buffer[9..17], DateTimeOffset.UtcNow.ToUnixTimeMilliseconds());
+        _walStream!.Write(buffer);
+    }
+
+
     /// <summary>
     /// Writes a data modification record
     /// </summary>
@@ -399,6 +454,28 @@ public sealed class WriteAheadLog : IDisposable
         }
     }
 
+    /// <summary>
+    /// Truncates and reopens the WAL stream to start a fresh writer session.
+    /// </summary>
+    public void Restart()
+    {
+        _lock.Wait();
+        try
+        {
+            _walStream?.Dispose();
+            _walStream = new FileStream(
+                _walPath,
+                FileMode.Create,
+                FileAccess.ReadWrite,
+                FileShare.None,
+                bufferSize: 64 * 1024);
+        }
+        finally
+        {
+            _lock.Release();
+        }
+    }
+
 
     /// <summary>
     /// Reads all WAL records (for recovery)
@@ -440,6 +517,7 @@ public sealed class WriteAheadLog : IDisposable
                     case WalRecordType.Begin:
                     case WalRecordType.Commit:
                     case WalRecordType.Abort:
+                    case WalRecordType.Checkpoint:
                         // Read common fields (txnId + timestamp = 16 bytes)
                         var bytesRead = _walStream.Read(headerBuf);
                         if (bytesRead < 16)

src/CBDD.SourceGenerators/ZB.MOM.WW.CBDD.SourceGenerators.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFramework>net10.0</TargetFramework>
     <AssemblyName>ZB.MOM.WW.CBDD.SourceGenerators</AssemblyName>
     <RootNamespace>ZB.MOM.WW.CBDD.SourceGenerators</RootNamespace>
     <LangVersion>latest</LangVersion>
@@ -29,8 +29,6 @@
 
   <ItemGroup>
     <None Include="$(OutputPath)\$(AssemblyName).dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
-
-    <None Include="_._" />
   </ItemGroup>
 
   <ItemGroup>

tests/CBDD.Tests/Context/TestDbContext.cs

@@ -5,6 +5,7 @@ using ZB.MOM.WW.CBDD.Core.Compression;
 using ZB.MOM.WW.CBDD.Core.Indexing;
 using ZB.MOM.WW.CBDD.Core.Metadata;
 using ZB.MOM.WW.CBDD.Core.Storage;
+using ZB.MOM.WW.CBDD.Core.Transactions;
 
 namespace ZB.MOM.WW.CBDD.Shared;
 
@@ -243,6 +244,15 @@ public partial class TestDbContext : DocumentDbContext
         Engine.Checkpoint();
     }
 
+    /// <summary>
+    /// Executes ForceCheckpoint with the requested checkpoint mode.
+    /// </summary>
+    /// <param name="mode">Checkpoint mode to execute.</param>
+    public CheckpointResult ForceCheckpoint(CheckpointMode mode)
+    {
+        return Engine.Checkpoint(mode);
+    }
+
     /// <summary>
     /// Gets or sets the Storage.
     /// </summary>

tests/CBDD.Tests/Storage/CheckpointModeTests.cs

@@ -0,0 +1,228 @@
+using System.Reflection;
+using ZB.MOM.WW.CBDD.Bson;
+using ZB.MOM.WW.CBDD.Core.Storage;
+using ZB.MOM.WW.CBDD.Core.Transactions;
+using ZB.MOM.WW.CBDD.Shared;
+
+namespace ZB.MOM.WW.CBDD.Tests;
+
+public class CheckpointModeTests
+{
+    /// <summary>
+    /// Verifies default checkpoint mode truncates WAL.
+    /// </summary>
+    [Fact]
+    public void Checkpoint_Default_ShouldUseTruncate()
+    {
+        var dbPath = NewDbPath();
+        try
+        {
+            using var db = new TestDbContext(dbPath);
+            db.Users.Insert(new User { Name = "checkpoint-default", Age = 42 });
+            db.SaveChanges();
+
+            db.Storage.GetWalSize().ShouldBeGreaterThan(0);
+            var result = db.Checkpoint();
+
+            result.Mode.ShouldBe(CheckpointMode.Truncate);
+            result.Executed.ShouldBeTrue();
+            result.Truncated.ShouldBeTrue();
+            db.Storage.GetWalSize().ShouldBe(0);
+        }
+        finally
+        {
+            CleanupFiles(dbPath);
+        }
+    }
+
+    /// <summary>
+    /// Verifies passive mode skips when checkpoint lock is contended.
+    /// </summary>
+    [Fact]
+    public void Checkpoint_Passive_ShouldSkip_WhenLockIsContended()
+    {
+        var dbPath = NewDbPath();
+        try
+        {
+            using var storage = new StorageEngine(dbPath, PageFileConfig.Default);
+            var gate = GetCommitGate(storage);
+            gate.Wait(TestContext.Current.CancellationToken);
+            try
+            {
+                var result = storage.Checkpoint(CheckpointMode.Passive);
+                result.Mode.ShouldBe(CheckpointMode.Passive);
+                result.Executed.ShouldBeFalse();
+                result.Truncated.ShouldBeFalse();
+                result.Restarted.ShouldBeFalse();
+            }
+            finally
+            {
+                gate.Release();
+            }
+        }
+        finally
+        {
+            CleanupFiles(dbPath);
+        }
+    }
+
+    /// <summary>
+    /// Verifies full checkpoint applies data and appends a checkpoint marker without truncating WAL.
+    /// </summary>
+    [Fact]
+    public void Checkpoint_Full_ShouldAppendMarker_AndPreserveWal()
+    {
+        var dbPath = NewDbPath();
+        var walPath = Path.ChangeExtension(dbPath, ".wal");
+
+        try
+        {
+            using (var db = new TestDbContext(dbPath))
+            {
+                db.Users.Insert(new User { Name = "checkpoint-full", Age = 50 });
+                db.SaveChanges();
+
+                var walBefore = db.Storage.GetWalSize();
+                walBefore.ShouldBeGreaterThan(0);
+
+                var result = db.Checkpoint(CheckpointMode.Full);
+                result.Mode.ShouldBe(CheckpointMode.Full);
+                result.Executed.ShouldBeTrue();
+                result.Truncated.ShouldBeFalse();
+                result.WalBytesAfter.ShouldBeGreaterThan(0);
+                db.Storage.GetWalSize().ShouldBeGreaterThan(0);
+            }
+
+            using var wal = new WriteAheadLog(walPath);
+            wal.ReadAll().Any(r => r.Type == WalRecordType.Checkpoint).ShouldBeTrue();
+        }
+        finally
+        {
+            CleanupFiles(dbPath);
+        }
+    }
+
+    /// <summary>
+    /// Verifies restart checkpoint clears WAL and allows subsequent writes.
+    /// </summary>
+    [Fact]
+    public void Checkpoint_Restart_ShouldResetWal_AndAcceptNewWrites()
+    {
+        var dbPath = NewDbPath();
+        try
+        {
+            using var db = new TestDbContext(dbPath);
+            db.Users.Insert(new User { Name = "restart-before", Age = 30 });
+            db.SaveChanges();
+
+            db.Storage.GetWalSize().ShouldBeGreaterThan(0);
+            var result = db.Checkpoint(CheckpointMode.Restart);
+            result.Mode.ShouldBe(CheckpointMode.Restart);
+            result.Executed.ShouldBeTrue();
+            result.Truncated.ShouldBeTrue();
+            result.Restarted.ShouldBeTrue();
+            db.Storage.GetWalSize().ShouldBe(0);
+
+            db.Users.Insert(new User { Name = "restart-after", Age = 31 });
+            db.SaveChanges();
+            db.Storage.GetWalSize().ShouldBeGreaterThan(0);
+        }
+        finally
+        {
+            CleanupFiles(dbPath);
+        }
+    }
+
+    /// <summary>
+    /// Verifies recovery remains deterministic after a full checkpoint boundary.
+    /// </summary>
+    [Fact]
+    public void Recover_AfterFullCheckpoint_ShouldApplyLatestCommitDeterministically()
+    {
+        var dbPath = NewDbPath();
+        try
+        {
+            uint pageId;
+
+            using (var storage = new StorageEngine(dbPath, PageFileConfig.Default))
+            {
+                pageId = storage.AllocatePage();
+
+                using (var tx1 = storage.BeginTransaction())
+                {
+                    var first = new byte[storage.PageSize];
+                    first[0] = 1;
+                    storage.WritePage(pageId, tx1.TransactionId, first);
+                    tx1.Commit();
+                }
+
+                storage.Checkpoint(CheckpointMode.Full);
+
+                using (var tx2 = storage.BeginTransaction())
+                {
+                    var second = new byte[storage.PageSize];
+                    second[0] = 2;
+                    storage.WritePage(pageId, tx2.TransactionId, second);
+                    tx2.Commit();
+                }
+            }
+
+            using (var recovered = new StorageEngine(dbPath, PageFileConfig.Default))
+            {
+                var buffer = new byte[recovered.PageSize];
+                recovered.ReadPage(pageId, 0, buffer);
+                buffer[0].ShouldBe((byte)2);
+                recovered.GetWalSize().ShouldBe(0);
+            }
+        }
+        finally
+        {
+            CleanupFiles(dbPath);
+        }
+    }
+
+    /// <summary>
+    /// Verifies asynchronous mode-based checkpoints return expected result metadata.
+    /// </summary>
+    [Fact]
+    public async Task CheckpointAsync_Full_ShouldReturnResult()
+    {
+        var dbPath = NewDbPath();
+        try
+        {
+            using var db = new TestDbContext(dbPath);
+            db.Users.Insert(new User { Name = "checkpoint-async", Age = 38 });
+            db.SaveChanges();
+
+            var result = await db.CheckpointAsync(CheckpointMode.Full, TestContext.Current.CancellationToken);
+            result.Mode.ShouldBe(CheckpointMode.Full);
+            result.Executed.ShouldBeTrue();
+            result.Truncated.ShouldBeFalse();
+        }
+        finally
+        {
+            CleanupFiles(dbPath);
+        }
+    }
+
+    private static SemaphoreSlim GetCommitGate(StorageEngine storage)
+    {
+        var field = typeof(StorageEngine).GetField("_commitLock", BindingFlags.Instance | BindingFlags.NonPublic);
+        field.ShouldNotBeNull();
+        return (SemaphoreSlim)field!.GetValue(storage)!;
+    }
+
+    private static string NewDbPath()
+        => Path.Combine(Path.GetTempPath(), $"checkpoint_mode_{Guid.NewGuid():N}.db");
+
+    private static void CleanupFiles(string dbPath)
+    {
+        if (File.Exists(dbPath)) File.Delete(dbPath);
+
+        var walPath = Path.ChangeExtension(dbPath, ".wal");
+        if (File.Exists(walPath)) File.Delete(walPath);
+
+        var markerPath = $"{dbPath}.compact.state";
+        if (File.Exists(markerPath)) File.Delete(markerPath);
+    }
+}