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)
• supports 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
• (>=0.3.0) supports event payload validation via Pydantic (See here)
• (>=0.4.0) supports event chaining: dispatching events within handlers (thank @ndopj for contributing to the idea)
• (>=0.7.0) supports OpenTelemetry: see this section for details

Installation

pip install fastapi-events

To use it with AWS handlers, install:

pip install fastapi-events[aws]

To use it with GCP handlers. install:

pip install fastapi-events[google]

To enable OpenTelemetry (OTEL) support, install:

pip install fastapi-events[otel]

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!

Event Payload Validation With Pydantic

Event payload validation is possible since version 0.3.0. To enable, simply register a Pydantic models with the corresponding event name.

import uuid
from enum import Enum
from datetime import datetime

from pydantic import BaseModel
from fastapi_events.registry.payload_schema import registry as payload_schema


class UserEvents(Enum):
    SIGNED_UP = "USER_SIGNED_UP"
    ACTIVATED = "USER_ACTIVATED"


# Registering your event payload schema
@payload_schema.register(event_name=UserEvents.SIGNED_UP)
class SignUpPayload(BaseModel):
    user_id: uuid.UUID
    created_at: datetime

Wildcard in event name is currently not supported

Payload will be validated automatically without any changes made while invoking the dispatcher.

# Events with payload schema registered
dispatch(UserEvents.SIGNED_UP)  # raises ValidationError, missing payload
dispatch(UserEvents.SIGNED_UP,
         {"user_id": "9e79cdbb-b216-40f7-9a05-20d223dee89a"})  # raises ValidationError, missing `created_at`
dispatch(UserEvents.SIGNED_UP,
         {"user_id": "9e79cdbb-b216-40f7-9a05-20d223dee89a", created_at: datetime.utcnow()})  # OK!

# Events without payload schema -> No validation will be performed
dispatch(UserEvents.ACTIVATED,
         {"user_id": "9e79cdbb-b216-40f7-9a05-20d223dee89a"})  # OK! no validation will be performed

Reminder: payload validation is optional. Payload of events without its schema registered will not be validated.

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

• GoogleCloudSimplePubSubHandler:

  • import from fastapi_events.handlers.gcp
  • to publish events to a single pubsub topic

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

OpenTelemetry (OTEL) support

Since version 0.7.0, OpenTelemetry support has been added as an optional feature.

To enable it, make sure you install the optional modules:

pip install fastapi-events[otel]

Note that no instrumentation library is needed as fastapi_events supports OTEL natively

Spans will be created when:

• fastapi_events.dispatcher.dispatch is invoked,
• fastapi_events.handlers.local.LocalHandler is handling an event

Support for other handlers will be added in the future.

Cookbook

1) 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.

2) Validating Event Payload During Dispatch

Requires Pydantic, which comes with FastAPI. If you're using Starlette, you might need to install Pydantic

See Event Payload Validation With Pydantic

3) Dispatching events within handlers (Event Chaining)

It is now possible to dispatch events within another event handlers. You'll need version 0.4 or above.

Comparison between events dispatched within the request-response cycle and event handlers are:

4) Dispatching events outside of a request

One goal of fastapi-events is to dispatch events without having to manage which instance of EventHandlerASGIMiddleware is being targeted. By default, this is handled using ContextVars. There are occasions when a user may want to dispatch events outside of the standard request sequence though. This can be accomplished by generating a custom identifier for the middleware.

By default, the middleware identifier is generated from the object id of the EventHandlerASGIMiddleware instance and is managed internally without need for user intervention. If the user needs to dispatch events outside of a request-response lifecycle, a custom middleware_id value can be generated and passed to EventHandlerASGIMiddleware during its creation. This value can then be used with dispatch() to ensure the correct EventHandlerASGIMiddleware instance is selected.

Dispatching events during a request does not require the middleware_id. These will continue to automatically discover the event handler.

In the following example, the id is being generated using the object id of the FastAPI instance. The middleware identifier must be unique int but there are no other restrictions.

import asyncio

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()
event_handler_id: int = id(app)
app.add_middleware(EventHandlerASGIMiddleware,
                   handlers=[local_handler],  # registering handler(s)
                   middleware_id=event_handler_id)  # register custom middleware id


async def dispatch_task() -> None:
    """ background task to dispatch autonomous events """

    for i in range(100):
        # without the middleware_id, this call would raise a LookupError
        dispatch("date", payload={"idx": i}, middleware_id=event_handler_id)
        await asyncio.sleep(1)


@app.on_event("startup")
async def startup_event() -> None:
    asyncio.create_task(dispatch_task())


@app.get("/")
def index(request: Request) -> JSONResponse:
    dispatch("hello", payload={"id": 1})  # Emit events anywhere in your code
    return JSONResponse({"detail": {"msg": "hello world"}})

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.

   This can be worked around by using a user-defined middleware_id.

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

2. My event handlers are not registered / Local handlers are not being executed:

   Answer:

   Make sure the module where your local event handlers are defined is loaded during runtime. A simple fix is to import the module in your __init__.py. This will ensure the modules are properly loaded during runtime.

Feedback, Questions?

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