Add RunInChildContextAsync

Adds child-context support to the .NET Durable Execution SDK. A child
context is a logical sub-workflow with its own deterministic
operation-ID space, persisted as a CONTEXT operation so subsequent
invocations replay the cached value without re-executing the function.

Public surface:
- IDurableContext.RunInChildContextAsync<T> (reflection + AOT-safe
  ICheckpointSerializer<T> overloads, plus a void overload).
- ChildContextConfig with SubType (observability label) and
  ErrorMapping (transform exceptions before they surface to the caller).
- ChildContextException for failure surfacing.

Used as a building block for upcoming WaitForCallbackAsync.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: GarrettBeatty

Libraries/src/Amazon.Lambda.DurableExecution/ChildContextConfig.cs

@@ -0,0 +1,32 @@
+namespace Amazon.Lambda.DurableExecution;
+
+/// <summary>
+/// Configuration for a child context.
+/// </summary>
+/// <remarks>
+/// A child context is a logical sub-workflow with its own deterministic
+/// operation-ID space, persisted as a <c>CONTEXT</c> operation. Use
+/// <see cref="IDurableContext.RunInChildContextAsync{T}(System.Func{IDurableContext, System.Threading.Tasks.Task{T}}, string?, ChildContextConfig?, System.Threading.CancellationToken)"/>
+/// (and overloads) to run code inside one.
+/// </remarks>
+public sealed class ChildContextConfig
+{
+    /// <summary>
+    /// Operation sub-type label for observability (e.g. <c>"WaitForCallback"</c>).
+    /// Surfaces on the wire <c>OperationUpdate.SubType</c> field.
+    /// </summary>
+    public string? SubType { get; set; }
+
+    /// <summary>
+    /// Optional function to transform exceptions thrown by the child context's
+    /// user function before they surface to the caller. Useful for wrapping
+    /// low-level errors into domain-specific exceptions.
+    /// </summary>
+    /// <remarks>
+    /// Applied when the user function throws (the mapped exception propagates
+    /// to the caller of <c>RunInChildContextAsync</c>) and on replay of a
+    /// <c>FAILED</c> child context (the constructed
+    /// <see cref="ChildContextException"/> is mapped before being thrown).
+    /// </remarks>
+    public Func<Exception, Exception>? ErrorMapping { get; set; }
+}

Libraries/src/Amazon.Lambda.DurableExecution/DurableContext.cs

@@ -103,6 +103,55 @@ public Task WaitAsync(
             _state, _terminationManager, _durableExecutionArn, _batcher);
         return op.ExecuteAsync(cancellationToken);
     }
+
+    public Task<T> RunInChildContextAsync<T>(
+        Func<IDurableContext, Task<T>> func,
+        string? name = null,
+        ChildContextConfig? config = null,
+        CancellationToken cancellationToken = default)
+        => RunChildContext(func, name, config, cancellationToken);
+
+    public async Task RunInChildContextAsync(
+        Func<IDurableContext, Task> func,
+        string? name = null,
+        ChildContextConfig? config = null,
+        CancellationToken cancellationToken = default)
+    {
+        // Void child contexts don't carry a meaningful payload; the wrapper
+        // returns null so the registered ILambdaSerializer is never asked to
+        // serialize a real value.
+        await RunChildContext<object?>(
+            async (ctx) => { await func(ctx); return null; },
+            name, config, cancellationToken);
+    }
+
+    private Task<T> RunChildContext<T>(
+        Func<IDurableContext, Task<T>> func,
+        string? name,
+        ChildContextConfig? config,
+        CancellationToken cancellationToken)
+    {
+        var serializer = LambdaContext.Serializer
+            ?? throw new InvalidOperationException(
+                "No ILambdaSerializer is registered on ILambdaContext.Serializer. " +
+                "Register a serializer via LambdaBootstrapBuilder.Create(handler, serializer) " +
+                "(or in tests, set TestLambdaContext.Serializer).");
+
+        var operationId = _idGenerator.NextId();
+
+        // Capture this DurableContext's collaborators; the child shares state,
+        // termination, batcher, ARN, and Lambda context — but uses a child
+        // OperationIdGenerator so its operation IDs are deterministically
+        // namespaced under the parent op ID.
+        IDurableContext ChildFactory(string parentOpId) => new DurableContext(
+            _state, _terminationManager, _idGenerator.CreateChild(parentOpId),
+            _durableExecutionArn, LambdaContext, _batcher);
+
+        var op = new ChildContextOperation<T>(
+            operationId, name, func, config, serializer, ChildFactory,
+            _state, _terminationManager, _durableExecutionArn, _batcher);
+        return op.ExecuteAsync(cancellationToken);
+    }
 }
 
 internal sealed class DurableExecutionContext : IExecutionContext

Libraries/src/Amazon.Lambda.DurableExecution/DurableExecutionException.cs

@@ -67,3 +67,31 @@ public StepInterruptedException(string message) : base(message) { }
     /// <summary>Creates a <see cref="StepInterruptedException"/> wrapping an inner exception.</summary>
     public StepInterruptedException(string message, Exception innerException) : base(message, innerException) { }
 }
+
+/// <summary>
+/// Thrown when a child context's user function fails. Surfaces from
+/// <c>RunInChildContextAsync</c>; the underlying error is preserved on the
+/// <see cref="ErrorType"/>/<see cref="ErrorData"/>/<see cref="OriginalStackTrace"/>
+/// fields. Use <see cref="ChildContextConfig.ErrorMapping"/> to remap into a
+/// domain-specific exception.
+/// </summary>
+public class ChildContextException : DurableExecutionException
+{
+    /// <summary>
+    /// The child context's <see cref="ChildContextConfig.SubType"/>, if any.
+    /// </summary>
+    public string? SubType { get; init; }
+    /// <summary>The fully-qualified type name of the original exception.</summary>
+    public string? ErrorType { get; init; }
+    /// <summary>Optional structured error data attached by the user.</summary>
+    public string? ErrorData { get; init; }
+    /// <summary>Stack trace of the original exception, captured before serialization.</summary>
+    public IReadOnlyList<string>? OriginalStackTrace { get; init; }
+
+    /// <summary>Creates an empty <see cref="ChildContextException"/>.</summary>
+    public ChildContextException() { }
+    /// <summary>Creates a <see cref="ChildContextException"/> with the given message.</summary>
+    public ChildContextException(string message) : base(message) { }
+    /// <summary>Creates a <see cref="ChildContextException"/> wrapping an inner exception.</summary>
+    public ChildContextException(string message, Exception innerException) : base(message, innerException) { }
+}

Libraries/src/Amazon.Lambda.DurableExecution/IDurableContext.cs

@@ -60,6 +60,37 @@ Task WaitAsync(
         TimeSpan duration,
         string? name = null,
         CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Run a user function inside a logical sub-workflow (a "child context").
+    /// The child has its own deterministic operation-ID space; its result is
+    /// checkpointed as a <c>CONTEXT</c> operation so subsequent invocations
+    /// replay the cached value without re-executing the func.
+    /// </summary>
+    /// <remarks>
+    /// Use child contexts to group related durable operations (e.g. a step plus
+    /// a wait plus a step) into a single observability/error-handling boundary.
+    /// On failure, surfaces as <see cref="ChildContextException"/>; supply
+    /// <see cref="ChildContextConfig.ErrorMapping"/> to remap into a
+    /// domain-specific exception.
+    /// The child context's return value is serialized to a checkpoint using the
+    /// <see cref="ILambdaSerializer"/> registered on
+    /// <see cref="ILambdaContext.Serializer"/>.
+    /// </remarks>
+    Task<T> RunInChildContextAsync<T>(
+        Func<IDurableContext, Task<T>> func,
+        string? name = null,
+        ChildContextConfig? config = null,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Run a user function inside a child context that returns no value.
+    /// </summary>
+    Task RunInChildContextAsync(
+        Func<IDurableContext, Task> func,
+        string? name = null,
+        ChildContextConfig? config = null,
+        CancellationToken cancellationToken = default);
 }
 
 /// <summary>

Libraries/src/Amazon.Lambda.DurableExecution/Internal/ChildContextOperation.cs

@@ -0,0 +1,196 @@
+using System.IO;
+using System.Text;
+using Amazon.Lambda.Core;
+using SdkErrorObject = Amazon.Lambda.Model.ErrorObject;
+using SdkOperationUpdate = Amazon.Lambda.Model.OperationUpdate;
+
+namespace Amazon.Lambda.DurableExecution.Internal;
+
+/// <summary>
+/// Durable child context operation. Runs a user-supplied function inside a
+/// nested <see cref="DurableContext"/> with its own deterministic operation-ID
+/// space, persisting the function's result so subsequent invocations replay
+/// the cached value without re-executing.
+/// </summary>
+/// <remarks>
+/// Replay branches — example: <c>await ctx.RunInChildContextAsync(child =&gt; ..., name: "phase")</c>
+/// <list type="bullet">
+///   <item><b>Fresh</b>: no prior state → sync-flush CONTEXT START → run user
+///       func → on success emit CONTEXT SUCCEED → on failure emit CONTEXT FAIL
+///       and throw <see cref="ChildContextException"/>.</item>
+///   <item><b>SUCCEEDED</b>: return cached deserialized result; user func is
+///       NOT re-executed.</item>
+///   <item><b>FAILED</b>: throw <see cref="ChildContextException"/> with the
+///       recorded error; if <see cref="ChildContextConfig.ErrorMapping"/> is
+///       set, the mapped exception is thrown instead.</item>
+///   <item><b>STARTED</b> / <b>PENDING</b>: re-run the user func without
+///       re-checkpointing START. The child's own operations recover from their
+///       own checkpoints, so this is replay propagation; if a wait/callback
+///       inside the child is still pending, the user func re-suspends.</item>
+/// </list>
+/// Unlike <see cref="StepOperation{T}"/>, child contexts have no retry strategy:
+/// failure is terminal and surfaces immediately via
+/// <see cref="ChildContextException"/>.
+/// </remarks>
+internal sealed class ChildContextOperation<T> : DurableOperation<T>
+{
+    private readonly Func<IDurableContext, Task<T>> _func;
+    private readonly ChildContextConfig? _config;
+    private readonly ILambdaSerializer _serializer;
+    private readonly Func<string, IDurableContext> _childContextFactory;
+
+    public ChildContextOperation(
+        string operationId,
+        string? name,
+        Func<IDurableContext, Task<T>> func,
+        ChildContextConfig? config,
+        ILambdaSerializer serializer,
+        Func<string, IDurableContext> childContextFactory,
+        ExecutionState state,
+        TerminationManager termination,
+        string durableExecutionArn,
+        CheckpointBatcher? batcher = null)
+        : base(operationId, name, state, termination, durableExecutionArn, batcher)
+    {
+        _func = func;
+        _config = config;
+        _serializer = serializer;
+        _childContextFactory = childContextFactory;
+    }
+
+    protected override string OperationType => OperationTypes.Context;
+
+    protected override async Task<T> StartAsync(CancellationToken cancellationToken)
+    {
+        // Sync-flush CONTEXT START before user code so the service has a record
+        // of the parent context if the inner func suspends (e.g. a Wait inside
+        // the child terminates the workflow before SUCCEED is reached).
+        await EnqueueAsync(new SdkOperationUpdate
+        {
+            Id = OperationId,
+            Type = OperationTypes.Context,
+            Action = "START",
+            SubType = _config?.SubType,
+            Name = Name
+        }, cancellationToken);
+
+        return await ExecuteFunc(cancellationToken);
+    }
+
+    protected override Task<T> ReplayAsync(Operation existing, CancellationToken cancellationToken)
+    {
+        switch (existing.Status)
+        {
+            case OperationStatuses.Succeeded:
+                // Side-effecting code runs at most once: replay returns the
+                // cached result without invoking the user func.
+                return Task.FromResult(DeserializeResult(existing.ContextDetails?.Result));
+
+            case OperationStatuses.Failed:
+                throw MapFailureException(BuildChildContextException(existing));
+
+            case OperationStatuses.Started:
+            case OperationStatuses.Pending:
+                // Re-run the user func: the child's own operations replay from
+                // their own checkpoints. Do NOT re-checkpoint START — the
+                // original is still authoritative. If something inside the
+                // child is still pending (Wait, callback, retry) the user func
+                // will re-suspend on its own.
+                return ExecuteFunc(cancellationToken);
+
+            default:
+                throw new NonDeterministicExecutionException(
+                    $"Child context operation '{Name ?? OperationId}' has unexpected status '{existing.Status}' on replay.");
+        }
+    }
+
+    private async Task<T> ExecuteFunc(CancellationToken cancellationToken)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var childContext = _childContextFactory(OperationId);
+
+        T result;
+        try
+        {
+            result = await _func(childContext);
+        }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            throw;
+        }
+        catch (Exception ex)
+        {
+            await EnqueueAsync(new SdkOperationUpdate
+            {
+                Id = OperationId,
+                Type = OperationTypes.Context,
+                Action = "FAIL",
+                SubType = _config?.SubType,
+                Name = Name,
+                Error = ToSdkError(ex)
+            }, cancellationToken);
+
+            throw MapFailureException(new ChildContextException(ex.Message, ex)
+            {
+                SubType = _config?.SubType,
+                ErrorType = ex.GetType().FullName
+            });
+        }
+
+        await EnqueueAsync(new SdkOperationUpdate
+        {
+            Id = OperationId,
+            Type = OperationTypes.Context,
+            Action = "SUCCEED",
+            SubType = _config?.SubType,
+            Name = Name,
+            Payload = SerializeResult(result)
+        }, cancellationToken);
+
+        return result;
+    }
+
+    private Exception MapFailureException(ChildContextException ex)
+    {
+        var mapper = _config?.ErrorMapping;
+        if (mapper == null) return ex;
+
+        var mapped = mapper(ex);
+        return mapped ?? ex;
+    }
+
+    private ChildContextException BuildChildContextException(Operation failedOp)
+    {
+        var err = failedOp.ContextDetails?.Error;
+        return new ChildContextException(err?.ErrorMessage ?? "Child context failed")
+        {
+            SubType = failedOp.SubType ?? _config?.SubType,
+            ErrorType = err?.ErrorType,
+            ErrorData = err?.ErrorData,
+            OriginalStackTrace = err?.StackTrace
+        };
+    }
+
+    private T DeserializeResult(string? serialized)
+    {
+        if (serialized == null) return default!;
+        var bytes = Encoding.UTF8.GetBytes(serialized);
+        using var ms = new MemoryStream(bytes);
+        return _serializer.Deserialize<T>(ms);
+    }
+
+    private string SerializeResult(T value)
+    {
+        using var ms = new MemoryStream();
+        _serializer.Serialize(value, ms);
+        return Encoding.UTF8.GetString(ms.ToArray());
+    }
+
+    private static SdkErrorObject ToSdkError(Exception ex) => new()
+    {
+        ErrorType = ex.GetType().FullName,
+        ErrorMessage = ex.Message,
+        StackTrace = ex.StackTrace?.Split(new[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries).ToList()
+    };
+}

Libraries/src/Amazon.Lambda.DurableExecution/Services/LambdaDurableServiceClient.cs

@@ -132,6 +132,15 @@ private static Operation MapFromSdkOperation(SdkOperation sdkOp)
             ExecutionDetails = sdkOp.ExecutionDetails != null ? new ExecutionDetails
             {
                 InputPayload = sdkOp.ExecutionDetails.InputPayload
+            } : null,
+            ContextDetails = sdkOp.ContextDetails != null ? new Internal.ContextDetails
+            {
+                Result = sdkOp.ContextDetails.Result,
+                Error = sdkOp.ContextDetails.Error != null ? new ErrorObject
+                {
+                    ErrorType = sdkOp.ContextDetails.Error.ErrorType,
+                    ErrorMessage = sdkOp.ContextDetails.Error.ErrorMessage
+                } : null
             } : null
         };
     }