child-contexts.md

Child Contexts

RunInChildContextAsync runs a sub-workflow inside its own deterministic operation-ID space. The child's return value is checkpointed as a single CONTEXT operation, so subsequent invocations replay the cached value without re-executing the contained operations. Use to group related steps under a shared error/observability boundary.

Signatures

Task<T> RunInChildContextAsync<T>(
    Func<IDurableContext, CancellationToken, Task<T>> func,
    string? name = null,
    ChildContextConfig? config = null,
    CancellationToken cancellationToken = default);

Task RunInChildContextAsync(
    Func<IDurableContext, CancellationToken, Task> func,
    string? name = null,
    ChildContextConfig? config = null,
    CancellationToken cancellationToken = default);

The CancellationToken parameter is a linked token combining the caller-supplied token with the SDK's workflow-shutdown signal — forward it to StepAsync and other operations inside the child so cancellation propagates uniformly.

Example

var phaseResult = await ctx.RunInChildContextAsync<string>(
    async (childCtx, ct) =>
    {
        var validated = await childCtx.StepAsync(async (_, c) => Validate(input, c), name: "validate", cancellationToken: ct);
        await childCtx.WaitAsync(TimeSpan.FromSeconds(2), name: "short_wait", cancellationToken: ct);
        var processed = await childCtx.StepAsync(async (_, c) => Process(validated, c), name: "process", cancellationToken: ct);
        return processed;
    },
    name: "phase",
    config: new ChildContextConfig { SubType = "OrderProcessing" });

Configuration

public sealed class ChildContextConfig
{
    public string? SubType { get; set; }                         // observability label
    public Func<Exception, Exception>? ErrorMapping { get; set; } // remap thrown exceptions
}

SubType is an optional label recorded on the child context's checkpoint. When null or empty, no subtype is sent. It must be 1 to 32 characters from [a-zA-Z0-9-_]; invalid values throw ArgumentException when the child context runs.

ErrorMapping lets you translate exceptions thrown inside the child context into a domain-specific exception type before they propagate to the parent.

Custom serializer

Set ChildContextConfig.Serializer to override the ILambdaSerializer used to serialize and (on replay) deserialize the child context's result. When null (default), the globally-registered serializer on ILambdaContext.Serializer is used. See Steps → Custom serializer for details and replay/AOT notes.