fastapi-events
An event dispatching/handling library for FastAPI, and Starlette.
Features:
- straightforward API to emit events anywhere in your code
- events are handled after responses are returned (doesn't affect response time)
- support event piping to remote queues
- powerful built-in handlers to handle events locally and remotely
- coroutine functions (
async def) are the first-class citizen - write your handlers, never be limited to just what
fastapi_eventsprovides
Installation
pip install fastapi-events
To use it with AWS handlers, install:
pip install fastapi-events[aws]
Usage
fastapi-events supports both FastAPI and Starlette. To use it, simply configure it as middleware.
-
Configuring
fastapi-eventsfor FastAPI:from fastapi import FastAPI from fastapi.requests import Request from fastapi.responses import JSONResponse from fastapi_events.dispatcher import dispatch from fastapi_events.middleware import EventHandlerASGIMiddleware from fastapi_events.handlers.local import local_handler app = FastAPI() app.add_middleware(EventHandlerASGIMiddleware, handlers=[local_handler]) # registering handler(s) @app.get("/") def index(request: Request) -> JSONResponse: dispatch("my-fancy-event", payload={"id": 1}) # Emit events anywhere in your code return JSONResponse()
-
Configuring
fastapi-eventsfor Starlette:from starlette.applications import Starlette from starlette.middleware import Middleware from starlette.requests import Request from starlette.responses import JSONResponse from fastapi_events.dispatcher import dispatch from fastapi_events.handlers.local import local_handler from fastapi_events.middleware import EventHandlerASGIMiddleware app = Starlette(middleware=[ Middleware(EventHandlerASGIMiddleware, handlers=[local_handler]) # registering handlers ]) @app.route("/") async def root(request: Request) -> JSONResponse: dispatch("new event", payload={"id": 1}) # Emit events anywhere in your code return JSONResponse()
Dispatching events
Events can be dispatched anywhere in the code, as long as they are dispatched before a response is made.
# anywhere in code
from fastapi_events.dispatcher import dispatch
dispatch(
"cat-requested-a-fish", # Event name, accepts any valid string
payload={"cat_id": "fd375d23-b0c9-4271-a9e0-e028c4cd7230"} # Event payload, accepts any arbitrary data
)
dispatch("a_cat_is_spotted") # This works too!
Handling Events
Handle events locally
The flexibility of fastapi-events allows us to customise how the events should be handled. For starters, you might want to handle your events locally.
# ex: in handlers.py
from fastapi_events.handlers.local import local_handler
from fastapi_events.typing import Event
@local_handler.register(event_name="cat*")
def handle_all_cat_events(event: Event):
"""
this handler will match with an events prefixed with `cat`.
ex: "cat_eats_a_fish", "cat_is_cute", etc
"""
# the `event` argument is nothing more than a tuple of event name and payload
event_name, payload = event
# TODO do anything you'd like with the event
@local_handler.register(event_name="cat*") # Tip: You can register several handlers with the same event name
def handle_all_cat_events_another_way(event: Event):
pass
@local_handler.register(event_name="*")
async def handle_all_events(event: Event):
# event handlers can be coroutine function too (`async def`)
pass
Piping Events To Remote Queues
For larger projects, you might have services dedicated to handling events separately.
For instance, fastapi-events comes with AWS SQS forwarder to forward events to a remote queue.
-
Register
SQSForwardHandleras handlers:app = FastAPI() app.add_middleware(EventHandlerASGIMiddleware, handlers=[SQSForwardHandler(queue_url="test-queue", region_name="eu-central-1")]) # registering handler(s)
-
Start dispatching events! Events will be serialised into JSON format by default:
["event name", {"payload": "here is the payload"}]
Tip: to pipe events to multiple queues, provide multiple handlers while adding
EventHandlerASGIMiddleware.
Built-in handlers
Here is a list of built-in event handlers:
-
LocalHandler/local_handler:- import from
fastapi_events.handlers.local - for handling events locally. See examples above
- event name pattern matching is done using Unix shell-style matching (
fnmatch)
- import from
-
SQSForwardHandler:- import from
fastapi_events.handlers.aws - to forward events to an AWS SQS queue
- import from
-
EchoHandler:- import from
fastapi_events.handlers.echo - to forward events to stdout with
pprint. Great for debugging purpose
- import from
Creating your own handler
Creating your own handler is nothing more than inheriting from the BaseEventHandler class in fastapi_events.handlers.base.
To handle events, fastapi_events calls one of these methods, in the following priority order:
-
handle_many(events): The coroutine function should expect the backlog of the events collected. -
handle(event): In cases wherehandle_many()weren't defined in your custom handler,handle()will be called by iterating through the events in the backlog.
from typing import Iterable
from fastapi_events.typing import Event
from fastapi_events.handlers.base import BaseEventHandler
class MyOwnEventHandler(BaseEventHandler):
async def handle(self, event: Event) -> None:
"""
Handle events one by one
"""
pass
async def handle_many(self, events: Iterable[Event]) -> None:
"""
Handle events by batch
"""
pass
Suppressing Events / Disabling dispatch() Globally
In case you want to suppress events globally especially during testing, you can do so without having to mock or patch the dispatch() function. Simple set the environment variable FASTAPI_EVENTS_DISABLE_DISPATCH to 1, True or any truthy values.
FAQs:
-
I'm getting
LookupErrorwhendispatch()is used:def dispatch(event_name: str, payload: Optional[Any] = None) -> None: > q: Deque[Event] = event_store.get() E LookupError: <ContextVar name='fastapi_context' at 0x400a1f12b0>
Answer:
dispatch()relies on ContextVars to work properly. There are many reasons whyLookupErrorcan occur. A common reason isdispatch()is called outside the request-response lifecycle of FastAPI/Starlette, such as callingdispatch()after a response has been returned.If you're getting this during testing, you may consider disabling
dispatch()during testing. See Suppressing Events / Disablingdispatch()Globally for details.
Feedback, Questions?
Any form of feedback and questions are welcome! Please create an issue here.
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