fastapi-events 0.2.1

Event dispatching library for FastAPI

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_events provides

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-events for 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-events for 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.

1. Register SQSForwardHandler as handlers:

   app = FastAPI()
   app.add_middleware(EventHandlerASGIMiddleware, 
                      handlers=[SQSForwardHandler(queue_url="test-queue",
                                                  region_name="eu-central-1")])   # registering handler(s)
   

2. 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)

• SQSForwardHandler:

  • import from fastapi_events.handlers.aws
  • to forward events to an AWS SQS queue

• EchoHandler:

  • import from fastapi_events.handlers.echo
  • to forward events to stdout with pprint. Great for debugging purpose

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:

1. handle_many(events): The coroutine function should expect the backlog of the events collected.

2. handle(event): In cases where handle_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:

1. I'm getting LookupError when dispatch() 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 why LookupError can occur. A common reason is dispatch() is called outside the request-response lifecycle of FastAPI/Starlette, such as calling dispatch() after a response has been returned.

   If you're getting this during testing, you may consider disabling dispatch() during testing. See Suppressing Events / Disabling dispatch() Globally for details.

Feedback, Questions?

Any form of feedback and questions are welcome! Please create an issue here.