Event-driven architecture in FastAPI - Step-by-Step Execution

Concept Flow - Event-driven architecture

Event Occurs

↓

Event Captured by Listener

↓

Listener Triggers Handler Function

↓

Handler Processes Event

↓

Optional: Emit New Event

↓

Cycle Continues or Ends

This flow shows how an event triggers a listener, which runs a handler to process the event, possibly emitting new events.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.on_event("startup")
async def startup_event():
    print("App is starting")

This FastAPI code listens for the 'startup' event and runs a function that prints a message when the app starts.

Execution Table

Step	Event	Listener Action	Handler Output	Next Action
1	App starts	startup_event listener triggered	Prints 'App is starting'	No further events
2	User sends request	Route handler triggered	Processes request and returns response	Wait for next request
3	App shuts down	shutdown_event listener triggered	Cleanup tasks run	App stops
4	No event	No listener triggered	No action	Idle state

💡 Execution stops when no new events occur or app shuts down.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
event	None	"startup"	"request"	"shutdown"	None
handler_output	None	"App is starting"	"Response sent"	"Cleanup done"	None

Key Moments - 3 Insights

Why doesn't the handler run until the event occurs?

Because in event-driven architecture, handlers wait passively for their specific event to happen before running, as shown in execution_table step 1 where the handler runs only after the 'startup' event.

Can one event trigger multiple handlers?

Yes, multiple listeners can listen to the same event and run their handlers independently, but this example shows a single listener per event for simplicity.

What happens if no event occurs?

No handlers run and the system stays idle, as shown in execution_table step 4 where no event means no action.

Visual Quiz - 3 Questions

Look at the execution table, what is the handler output at step 1?

A "Cleanup done"

B "Response sent"

C "App is starting"

D No output

💡 Hint

Check the 'Handler Output' column for step 1 in the execution_table.

At which step does the app process a user request?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look for the event 'User sends request' in the execution_table.

If the app never shuts down, which step will never occur?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Step 3 corresponds to the 'App shuts down' event in the execution_table.

Concept Snapshot

Event-driven architecture in FastAPI:
- Events trigger listeners automatically.
- Listeners run handler functions when events occur.
- Handlers process events and can emit new events.
- Common events: startup, shutdown, request handling.
- Handlers wait passively until their event happens.

Full Transcript

Event-driven architecture means your program waits for events like app startup or user requests. When an event happens, a listener notices it and runs a handler function to respond. For example, FastAPI can run a function when the app starts to print a message. Handlers only run when their event occurs, so the app stays idle otherwise. This lets your app react to things as they happen, making it flexible and efficient.

Practice

1. What is the main benefit of using event-driven architecture in FastAPI?

easy

A. It allows the app to react to actions immediately without waiting.

B. It makes the app run only on a single thread.

C. It requires no code to handle user requests.

D. It disables background tasks for faster response.

Solution

1. Step 1: Understand event-driven architecture

   Event-driven architecture lets the app respond to events as they happen, improving responsiveness.

2. Step 2: Connect to FastAPI's use of BackgroundTasks

   FastAPI uses BackgroundTasks to handle events without blocking the main user request, enabling immediate reactions.

3. Final Answer:

   It allows the app to react to actions immediately without waiting. -> Option A

4. Quick Check:

   Event-driven = Immediate reaction [OK]

Hint: Event-driven means reacting instantly to actions [OK]

Common Mistakes:

• Thinking event-driven means single-threaded only
• Believing no code is needed for requests
• Assuming background tasks are disabled

2. Which of the following is the correct way to import BackgroundTasks in FastAPI?

easy

A. import BackgroundTasks from fastapi

B. from fastapi.background import Tasks

C. from fastapi import BackgroundTasks

D. import fastapi.BackgroundTasks

Solution

1. Step 1: Recall FastAPI import syntax

   FastAPI uses standard Python import syntax: from module import class/function.

2. Step 2: Identify correct module and class

   The BackgroundTasks class is imported from fastapi directly, so the correct syntax is 'from fastapi import BackgroundTasks'.

3. Final Answer:

   from fastapi import BackgroundTasks -> Option C

4. Quick Check:

   Correct import syntax = from fastapi import BackgroundTasks [OK]

Hint: Use 'from fastapi import BackgroundTasks' to import correctly [OK]

Common Mistakes:

• Using wrong import order or syntax
• Trying to import from a non-existent submodule
• Using dot notation incorrectly in import

3. What will be the output when this FastAPI endpoint is called?

from fastapi import FastAPI, BackgroundTasks
app = FastAPI()

results = []

def write_log(message: str):
    results.append(message)

@app.post('/send')
async def send_message(background_tasks: BackgroundTasks):
    background_tasks.add_task(write_log, 'Message sent')
    return {'status': 'ok'}

# After calling POST /send once, what is the value of results?

medium

A. ['Message sent']

B. [] (empty list)

C. ['Message sent', 'Message sent']

D. Error: background_tasks not defined

Solution

1. Step 1: Understand BackgroundTasks behavior

   BackgroundTasks run after the response is sent, but in testing or immediate inspection, the task may not have run yet.

2. Step 2: Check when results list updates

   The results list is updated inside the background task, which runs asynchronously after returning the response, so immediately after the call, results is still empty.

3. Final Answer:

   [] (empty list) -> Option B

4. Quick Check:

   Background task runs after response = empty results [OK]

Hint: BackgroundTasks run after response, so immediate results stay empty [OK]

Common Mistakes:

• Assuming background task runs before return
• Expecting results updated synchronously
• Confusing background_tasks parameter usage

4. Identify the error in this FastAPI code using BackgroundTasks:

from fastapi import FastAPI, BackgroundTasks
app = FastAPI()

@app.post('/notify')
async def notify():
    background_tasks.add_task(print, 'Notify event')
    return {'message': 'Notification sent'}

medium

A. BackgroundTasks must be imported from fastapi.background module

B. print function cannot be used in background tasks

C. Missing await keyword before add_task call

D. background_tasks parameter missing in function definition

Solution

1. Step 1: Check function parameters for BackgroundTasks

   The function uses background_tasks but does not declare it as a parameter, so it is undefined inside the function.

2. Step 2: Confirm correct usage of BackgroundTasks

   BackgroundTasks must be accepted as a parameter in the endpoint function to be used, e.g., 'async def notify(background_tasks: BackgroundTasks)'.

3. Final Answer:

   background_tasks parameter missing in function definition -> Option D

4. Quick Check:

   Missing parameter = background_tasks parameter missing in function definition [OK]

Hint: Always add BackgroundTasks as a function parameter to use it [OK]

Common Mistakes:

• Forgetting to add background_tasks parameter
• Thinking print can't be used in background tasks
• Adding await to add_task (it's not a coroutine)

5. You want to send an email notification after a user registers without delaying the response. Which FastAPI pattern best fits this event-driven need?

hard

A. Use BackgroundTasks to send the email after returning the response

B. Send the email synchronously inside the endpoint before returning

C. Use a global variable to store emails and send later manually

D. Call the email function inside a separate thread without FastAPI support

Solution

1. Step 1: Understand the goal of non-blocking email sending

   We want the user to get a quick response while the email sends in the background.

2. Step 2: Match FastAPI's event-driven feature

   BackgroundTasks lets you schedule tasks to run after the response, perfect for sending emails without delay.

3. Final Answer:

   Use BackgroundTasks to send the email after returning the response -> Option A

4. Quick Check:

   BackgroundTasks = non-blocking post-response tasks [OK]

Hint: Use BackgroundTasks for post-response actions like emails [OK]

Common Mistakes:

• Sending email synchronously causing delay
• Using global variables for task management
• Manually managing threads without FastAPI support