Migrate from FastAPI to Starlette (#2171)

doc/source/changelog.md

@@ -26,6 +26,10 @@
 
   The (experimental) `start_client` argument `rest` was deprecated in favor of a new argument `transport`. `start_client(transport="rest")` will yield the same behaviour as `start_client(rest=True)` did before. All code should migrate to the new argument `transport`. The deprecated argument `rest` will be removed in a future release.
 
+- **Migrate experimental REST API to Starlette** ([2171](https://github.com/adap/flower/pull/2171))
+
+  The (experimental) REST API used to be implemented in [FastAPI](https://fastapi.tiangolo.com/), but it has now been migrated to use [Starlette](https://www.starlette.io/) directly.
+
 - **General improvements** ([#1872](https://github.com/adap/flower/pull/1872), [#1866](https://github.com/adap/flower/pull/1866), [#1884](https://github.com/adap/flower/pull/1884))
 
 ### Incompatible changes

examples/mt-pytorch/README.md

@@ -1,6 +1,6 @@
 # Multi-Tenant Federated Learning with Flower and PyTorch
 
-This example contains highly experimental code. Please consult the regular PyTorch code examples ([quickstart](https://github.com/adap/flower/tree/main/examples/quickstart-pytorch), [advanced](https://github.com/adap/flower/tree/main/examples/advanced-pytorch)) to learn how to use Flower with PyTorch.
+This example contains experimental code. Please consult the regular PyTorch code examples ([quickstart](https://github.com/adap/flower/tree/main/examples/quickstart-pytorch), [advanced](https://github.com/adap/flower/tree/main/examples/advanced-pytorch)) to learn how to use Flower with PyTorch.
 
 ## Setup
 
@@ -16,7 +16,7 @@ Terminal 1: start Flower server
 flower-server
 ```
 
-Terminal 2+3: start two clients
+Terminal 2+3: start two Flower client nodes
 
 ```bash
 python client.py

pyproject.toml

@@ -71,14 +71,13 @@ iterators = "^0.0.2"
 ray = { version = "==2.5.1", extras = ["default"], optional = true }
 pydantic = { version = "<2.0.0", optional = true }
 # Optional dependencies (REST transport layer)
-requests = { version = "^2.28.2", optional = true }
-fastapi = { version = "^0.95.0", optional = true }
-starlette = { version = "^0.27.0", optional = true }
-uvicorn = { version = "^0.21.1", extras = ["standard"], optional = true }
+requests = { version = "^2.31.0", optional = true }
+starlette = { version = "^0.29.0", optional = true }
+uvicorn = { version = "^0.22.0", extras = ["standard"], optional = true }
 
 [tool.poetry.extras]
 simulation = ["ray", "pydantic"]
-rest = ["fastapi", "requests", "starlette", "uvicorn"]
+rest = ["requests", "starlette", "uvicorn"]
 
 [tool.poetry.group.dev.dependencies]
 types-dataclasses = "==0.6.6"

src/py/flwr/server/app.py

@@ -282,8 +282,7 @@ def run_fleet_api() -> None:
     # Start Fleet API
     if args.fleet_api_type == TRANSPORT_TYPE_REST:
         if (
-            importlib.util.find_spec("fastapi")
-            and importlib.util.find_spec("requests")
+            importlib.util.find_spec("requests")
             and importlib.util.find_spec("starlette")
             and importlib.util.find_spec("uvicorn")
         ) is None:
@@ -376,8 +375,7 @@ def run_server() -> None:
     # Start Fleet API
     if args.fleet_api_type == TRANSPORT_TYPE_REST:
         if (
-            importlib.util.find_spec("fastapi")
-            and importlib.util.find_spec("requests")
+            importlib.util.find_spec("requests")
             and importlib.util.find_spec("starlette")
             and importlib.util.find_spec("uvicorn")
         ) is None:

src/py/flwr/server/fleet/rest_rere/rest_api.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 # ==============================================================================
-"""REST API server."""
+"""Experimental REST API server."""
 
 
 import sys
@@ -23,16 +23,16 @@
 from flwr.server.state import State
 
 try:
-    from fastapi import FastAPI, HTTPException, Request, Response
+    from starlette.applications import Starlette
     from starlette.datastructures import Headers
+    from starlette.exceptions import HTTPException
+    from starlette.requests import Request
+    from starlette.responses import Response
+    from starlette.routing import Route
 except ModuleNotFoundError:
     sys.exit(MISSING_EXTRA_REST)
 
 
-app: FastAPI = FastAPI()
-
-
-@app.post("/api/v0/fleet/pull-task-ins", response_class=Response)
 async def pull_task_ins(request: Request) -> Response:
     """Pull TaskIns."""
     _check_headers(request.headers)
@@ -62,7 +62,6 @@ async def pull_task_ins(request: Request) -> Response:
     )
 
 
-@app.post("/api/v0/fleet/push-task-res", response_class=Response)
 async def push_task_res(request: Request) -> Response:  # Check if token is needed here
     """Push TaskRes."""
     _check_headers(request.headers)
@@ -92,6 +91,17 @@ async def push_task_res(request: Request) -> Response:  # Check if token is need
     )
 
 
+routes = [
+    Route("/api/v0/fleet/pull-task-ins", pull_task_ins, methods=["POST"]),
+    Route("/api/v0/fleet/push-task-res", push_task_res, methods=["POST"]),
+]
+
+app: Starlette = Starlette(
+    debug=False,
+    routes=routes,
+)
+
+
 def _check_headers(headers: Headers) -> None:
     """Check if expected headers are set."""
     if "content-type" not in headers: