Startup and shutdown events in FastAPI - Step-by-Step Execution

Concept Flow - Startup and shutdown events

App starts

↓

Run startup event

↓

App ready to handle requests

↓

App receives shutdown signal

↓

Run shutdown event

↓

App stops

When the FastAPI app starts, it runs startup events first. After that, it handles requests. When stopping, it runs shutdown events before fully stopping.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.on_event("startup")
async def startup_event():
    print("Starting up...")

@app.on_event("shutdown")
async def shutdown_event():
    print("Shutting down...")

This code prints messages when the app starts and stops using startup and shutdown events.

Execution Table

Step	Trigger	Action	Output	State
1	App starts	Run startup_event	Prints 'Starting up...'	App ready
2	App running	Handle requests	Respond to clients	App ready
3	Shutdown signal	Run shutdown_event	Prints 'Shutting down...'	App stopping
4	App stopped	Exit process	No output	App stopped

💡 App stops after running shutdown event on shutdown signal

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
App state	Not started	Ready	Ready	Stopping	Stopped

Key Moments - 3 Insights

Why does the startup event run before the app handles requests?

The startup event prepares the app before it can serve requests, as shown in execution_table step 1 where the app prints 'Starting up...' before becoming ready.

What happens if the shutdown event is not defined?

If no shutdown event is defined, the app stops immediately without running any cleanup code, unlike step 3 where shutdown_event prints a message.

Can startup and shutdown events be asynchronous?

Yes, they can be async functions as in the example code, allowing for async operations during startup and shutdown.

Visual Quiz - 3 Questions

Look at the execution table, what is the app state after step 1?

A Ready

B Stopping

C Not started

D Stopped

💡 Hint

Check the 'State' column in row for step 1 in execution_table

At which step does the app print 'Shutting down...'?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Output' column in execution_table for the shutdown event

If the startup event took longer, which step would be delayed?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Startup event runs at step 1; app becomes ready at step 2 to handle requests

Concept Snapshot

FastAPI runs startup events before handling requests.
Shutdown events run before app stops.
Use @app.on_event("startup") and @app.on_event("shutdown") decorators.
Events can be async for async setup/cleanup.
Startup prepares app; shutdown cleans up resources.

Full Transcript

In FastAPI, startup and shutdown events let you run code when the app starts and stops. When the app starts, it runs the startup event first to prepare resources. Then it handles incoming requests. When the app receives a shutdown signal, it runs the shutdown event to clean up before stopping. You define these events using @app.on_event decorators with 'startup' or 'shutdown'. These functions can be asynchronous to handle async tasks. The execution table shows the app state changing from not started to ready after startup, then stopping after shutdown. This helps manage resources safely during app lifecycle.