docs(proposal): multi-app protocols

20260203-RIS-multi-app-protocols.md

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+# Support Multiple App Protocols (HTTP and gRPC) in a Single Dapr Application
+
+- Author(s): Tom FLENNER
+- State: Draft
+- Updated: 2026-02-03
+
+## Overview
+
+This proposal enables Dapr applications to expose both HTTP and gRPC protocols simultaneously for Service Invocation. Currently, each Dapr application is limited to a single `app-protocol`. This change affects the Dapr runtime, CLI, and SDKs.
+
+Related issue: [dapr/dapr#8897](https://github.com/dapr/dapr/issues/8897)
+
+## Background
+
+In Dapr's Service Invocation architecture, communication between sidecars always uses gRPC. The `app-protocol` setting only controls how the **local sidecar communicates with its application**.
+
+```
+[App A] <--app-protocol--> [Sidecar A] <==gRPC==> [Sidecar B] <--app-protocol--> [App B]
+```
+
+Currently, developers must choose a single protocol per application:
+
+```bash
+dapr run --app-protocol http --app-port 3000 ...
+# OR
+dapr run --app-protocol grpc --app-port 50001 ...
+```
+****
+This limitation forces developers to either duplicate APIs or deploy multiple instances when they need both protocols.
+
+### Use Cases
+
+1. **Hybrid APIs**: Expose HTTP REST for external consumers and gRPC for internal high-performance calls.
+2. **Incremental Migration**: Support both protocols during HTTP-to-gRPC transitions.
+3. **Protocol-Optimized Endpoints**: Use gRPC for streaming, HTTP for simple REST operations.
+
+## Related Items
+
+### Related proposals
+
+None
+
+### Related issues
+
+- [dapr/dapr#8897](https://github.com/dapr/dapr/issues/8897)
+
+## Expectations and alternatives
+
+### In Scope
+
+- Runtime support for multiple app protocols per application
+- CLI support for specifying multiple protocols
+- SDK support for protocol selection during invocation
+- Backward compatibility with single-protocol configurations
+
+### Out of Scope
+
+- Automatic protocol negotiation or fallback
+- Per-endpoint protocol enforcement
+- Protocol-level authorization policies
+
+### Alternatives Considered
+
+| Alternative                | Why Rejected                                  |
+| -------------------------- | --------------------------------------------- |
+| Deploy multiple instances  | Infrastructure cost and complexity            |
+| Use protocol proxy/gateway | Adds latency and defeats Dapr's unified model |
+| Keep single-protocol model | Doesn't address hybrid use cases              |
+
+## Implementation Details
+
+### Design
+
+#### CLI and Runtime Changes
+
+Accept multiple protocols via comma-separated values:
+
+```bash
+dapr run --app-protocol http,grpc --app-port 3000 --app-grpc-port 50001 ...
+```
+
+**Validation rules:**
+
+- `http` requires `--app-port`
+- `grpc` requires `--app-grpc-port`
+- Single protocol continues to use `--app-port` for backward compatibility
+
+#### SDK Changes
+
+SDKs will allow protocol selection during service invocation:
+
+```go
+// Go SDK
+resp, err := client.InvokeMethod(ctx, "my-app", "method", dapr.WithProtocol("grpc"))
+```
+
+```csharp
+// .NET SDK
+var response = await client.InvokeMethodAsync<Response>(
+    "my-app", "method",
+    new InvocationOptions { Protocol = "grpc" });
+```
+
+**Default behavior**: Use the first protocol specified in `--app-protocol`.
+
+### Feature lifecycle outline
+
+This feature adds support for multiple app protocols simultaneously. Existing single-protocol configurations remain unchanged and fully supported.
+
+### Acceptance Criteria
+
+- [ ] Runtime accepts comma-separated protocols via `--app-protocol`
+- [ ] CLI validates protocol/port combinations
+- [ ] SDKs can specify protocol during invocation
+- [ ] Single-protocol configurations continue to work
+- [ ] Clear error messages for misconfigurations
+
+## Completion Checklist
+
+**Runtime (dapr/dapr):**
+
+- [ ] Parse multiple protocols from `--app-protocol`
+- [ ] Validate required ports per protocol
+- [ ] Unit and integration tests
+
+**CLI (dapr/cli):**
+
+- [ ] Support comma-separated `--app-protocol` values
+- [ ] Validation and error messages
+- [ ] Unit tests
+
+**SDKs:**
+
+- [ ] Protocol selection API
+- [ ] Unit and integration tests
+
+**Documentation (dapr/docs):**
+
+- [ ] Update Service Invocation docs
+- [ ] Add multi-protocol configuration examples