Add FastAPI example (#581)

* add fastapi dir and change dep to rocksdict

* commit inital example

* update example

* use lifespan

* reorganize to have an endpoint trigger a producer

* reorganize timer to be above fast api decorator

examples/django/requirements/default.txt

@@ -1,4 +1,4 @@
 django
-faust[rocksdb]
+faust[rocksdict]
 eventlet
 faust-aioeventlet

examples/fastapi_example.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python
+import asyncio
+from contextlib import asynccontextmanager
+from typing import Union
+
+from fastapi import FastAPI
+
+import faust
+
+
+# This is just hello_world.py integrated with a FastAPI application
+
+
+def fake_answer_to_everything_ml_model(x: float):
+    return x * 42
+
+
+ml_models = {}
+
+
+# You MUST have "app" defined in order for Faust to discover the app
+# if you're using "faust" on CLI, but this doesn't work yet
+faust_app = faust.App(
+    'hello-world-fastapi',
+    broker='kafka://localhost:9092',
+    web_enabled=False,
+)
+# app = faust_app
+
+greetings_topic = faust_app.topic('greetings', value_type=str)
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    # Load the ML model
+    ml_models["answer_to_everything"] = fake_answer_to_everything_ml_model
+    await faust_app.start()
+    yield
+    # Clean up the ML models and release the resources
+    ml_models.clear()
+    await faust_app.stop()
+
+
+app = fastapi_app = FastAPI(
+    lifespan=lifespan,
+)
+# For now, run via "uvicorn fastapi_example:app"
+# then visit http://127.0.0.1:8000/docs
+
+
+@fastapi_app.get("/")
+def read_root():
+    return {"Hello": "World"}
+
+
+@fastapi_app.get("/items/{item_id}")
+def read_item(item_id: int, q: Union[str, None] = None):
+    return {"item_id": item_id, "q": q}
+
+
+@faust_app.agent(greetings_topic)
+async def print_greetings(greetings):
+    async for greeting in greetings:
+        print(greeting)
+
+
+@faust_app.timer(5)  # make sure you *always* add the timer above if you're using one
+@fastapi_app.get("/produce")
+async def produce():
+    for i in range(100):
+        await greetings_topic.send(value=f'hello {i}')
+    return {"success": True}