RFC: Durable Task middleware design review

Summary

Requesting design review for the .NET Durable Task middleware proposal and reference implementation.

• Reference spec: https://github.com/microsoft/durabletask-dotnet/blob/feature/durable-task-middleware/doc/cross-sdk-middleware.md
• .NET user guide: https://github.com/microsoft/durabletask-dotnet/blob/feature/durable-task-middleware/doc/durable-task-middleware.md
• .NET feature branch: https://github.com/microsoft/durabletask-dotnet/tree/feature/durable-task-middleware
• Azure Functions usage sample: https://github.com/Azure/azure-functions-durable-extension/tree/feature/durable-middleware-functions/samples/durable-middleware
• Motivation: https://github.com/Azure/azure-functions-durable-extension/issues/3054
• Cross-SDK tracking: https://github.com/microsoft/durabletask-dotnet/issues/720

cc @torosent @jviau @stevendarby @LockTar

Review focus

Please review the proposed v1 shape and call out any blocking concerns around:

• The async middleware pipeline shape for orchestrations and activities.
• Orchestration replay determinism requirements and runtime illegal-await guard expectations.
• The v1 decision that orchestration middleware must call
  ext(context) exactly once and cannot short-circuit or replace successful orchestration results.
• Activity middleware short-circuiting through SetResult.
• The type-keyed feature collection for host-specific context such as Azure Functions FunctionContext.
• Migration guidance away from Azure Functions IFunctionBindingsFeature reflection/private protobuf workarounds.
• Cross-SDK portability for JavaScript/TypeScript, Python, Java, and Go.

Current .NET scope

• SDK-level orchestration and activity middleware in Microsoft.DurableTask.Worker.Middleware.
• Standalone Durable Task worker registration through AddDurableTaskWorker() and UseOrchestrationMiddleware / UseActivityMiddleware.
• .NET isolated Durable Functions registration through ConfigureDurableWorker().
• Azure Functions GetFunctionContext() extensions for orchestration and activity middleware contexts.
• Function-syntax and class-based Durable Functions orchestration/activity support.
• Standalone and Durable Functions samples.

Out of scope for v1

• Entity middleware.
• Wire protocol or protobuf changes.
• Orchestration middleware short-circuiting / SetResult.
• Static analyzer work for orchestration middleware determinism.

Start with doc/cross-sdk-middleware.md and doc/durable-task-middleware.md on the linked feature branch, then inspect the middleware registration entry points named in the issue. Review replay determinism, pipeline semantics, feature collection design, and cross-SDK portability against the listed focus areas. Done means documenting blocking concerns or confirming the proposed v1 shape.