update fastapi life span doc
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RadinPirouz
@@ -1,13 +1,16 @@
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# FastAPI – Application Lifespan (Startup & Shutdown Events)
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# FastAPI – Application Lifespan, Startup, and Shutdown Events
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FastAPI allows you to run code when your application **starts up** or **shuts down**.
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This is useful for initializing resources, database connections, caches, or background tasks.
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## Overview
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FastAPI allows you to execute code when the application starts and when it shuts down. This is useful for initializing and cleaning up shared resources such as database connections, cache clients, machine learning models, message queues, or background services.
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The modern recommended approach is to use the `lifespan` parameter with an async context manager. The older `@app.on_event()` method is still available, but FastAPI marks it as deprecated in favor of lifespan handlers. ([fastapi.tiangolo.com][1])
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---
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## Deprecated Method: `@app.on_event`
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# 1. Deprecated Method: `@app.on_event`
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Older FastAPI versions use the `@app.on_event` decorator for lifecycle events:
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Older FastAPI applications often use `@app.on_event("startup")` and `@app.on_event("shutdown")`.
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```python
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from fastapi import FastAPI
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@@ -25,97 +28,355 @@ def on_shutdown():
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print("App is shutting down")
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```
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### Characteristics
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## Explanation
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* `startup` runs once when the app starts
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* `shutdown` runs once when the app is stopped
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* Works for synchronous and asynchronous functions
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* Still supported but **deprecated** in favor of the `lifespan` parameter
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```python
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@app.on_event("startup")
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def on_startup():
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print("App is loading")
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```
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This function runs once when the application starts.
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```python
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@app.on_event("shutdown")
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def on_shutdown():
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print("App is shutting down")
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```
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This function runs once when the application is shutting down.
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## Important Note
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FastAPI documentation recommends using the `lifespan` parameter instead of `@app.on_event()`. Also, when a `lifespan` handler is provided, FastAPI does not call the old `startup` and `shutdown` event handlers. You should use one approach consistently, not both. ([fastapi.tiangolo.com][1])
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---
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## Recommended Modern Approach: `lifespan` with `asynccontextmanager`
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# 2. Recommended Method: `lifespan`
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FastAPI now recommends using the `lifespan` parameter in the `FastAPI` constructor.
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This uses Python's `asynccontextmanager` to define a **single lifecycle context**.
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The modern approach is to define one lifecycle function using `asynccontextmanager`.
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```python
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from fastapi import FastAPI
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from contextlib import asynccontextmanager
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from fastapi import FastAPI
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@asynccontextmanager
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async def lifespan(app: FastAPI):
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# Code to run before the app starts
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print("App is loading")
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yield # Application runs after this point
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# Code to run after the app stops
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yield
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print("App is shutting down")
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app = FastAPI(lifespan=lifespan)
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```
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### How It Works
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---
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1. Code **before `yield`** executes on startup
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2. Code **after `yield`** executes on shutdown
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3. Supports async operations, e.g., connecting to a database
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# 3. How Lifespan Works
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The `lifespan` function has two main sections:
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```python
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@asynccontextmanager
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async def lifespan(app: FastAPI):
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# Startup logic
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print("App is loading")
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yield
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# Shutdown logic
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print("App is shutting down")
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```
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## Code Before `yield`
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This runs when the application starts.
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Use this section for:
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```text
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Database connection setup
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Cache connection setup
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Loading configuration
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Initializing shared services
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Starting background clients
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```
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## Code After `yield`
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This runs when the application shuts down.
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Use this section for:
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```text
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Closing database connections
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Closing cache clients
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Flushing logs
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Releasing resources
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Stopping background services
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```
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FastAPI passes the lifespan context manager into the application and executes the code before `yield` on startup and after `yield` on shutdown. ([fastapi.tiangolo.com][1])
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---
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## Example: Using Lifespan for Database Initialization
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# 4. Example Application
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Create or update `main.py` with the following content:
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```python
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from fastapi import FastAPI
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from contextlib import asynccontextmanager
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from fastapi import FastAPI
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@asynccontextmanager
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async def lifespan(app: FastAPI):
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app.state.db = await connect_to_db()
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print("Database connected")
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print("App is loading")
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yield
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await app.state.db.close()
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print("Database disconnected")
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print("App is shutting down")
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app = FastAPI(lifespan=lifespan)
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@app.get("/")
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def root():
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return {"message": "API is working"}
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```
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* `app.state` is used to store shared resources
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* Clean startup and shutdown handling
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* Ensures proper resource cleanup
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---
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## Benefits of the Lifespan Approach
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# 5. Running the Application
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* Centralized lifecycle management
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* Cleaner async support
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* Avoids multiple scattered `@app.on_event` decorators
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* Better for testing and production-ready apps
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---
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## Running the Application
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Start the service with `uvicorn`:
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Start the service using `uvicorn`:
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```bash
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uvicorn main:app --reload
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```
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* On startup, `App is loading` prints to the console
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* On shutdown (Ctrl+C), `App is shutting down` prints to the console
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When the application starts, the terminal prints:
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```text
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App is loading
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```
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When you stop the application, for example with `Ctrl + C`, the terminal prints:
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```text
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App is shutting down
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```
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---
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## Best Practices
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# 6. Example: Database Initialization Pattern
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* Always use `lifespan` for new applications
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* Use `app.state` to store shared resources
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* Close database connections, caches, or background services in shutdown
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* Keep startup logic lightweight to avoid blocking the server
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* Avoid printing in production; use logging instead
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In real applications, lifespan is commonly used to initialize and close database connections.
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```python
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from contextlib import asynccontextmanager
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from fastapi import FastAPI
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async def connect_to_db():
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# Replace this with a real database connection
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return "database-connection"
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@asynccontextmanager
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async def lifespan(app: FastAPI):
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app.state.db = await connect_to_db()
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print("Database connected")
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yield
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# Replace this with real cleanup logic
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print("Database disconnected")
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app = FastAPI(lifespan=lifespan)
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@app.get("/")
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def root():
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return {"message": "API is working"}
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```
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---
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This approach provides a **modern, production-ready pattern** for managing application startup and shutdown events in FastAPI.
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# 7. Using `app.state`
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`app.state` is useful for storing shared application-level resources.
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Example:
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```python
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app.state.db = await connect_to_db()
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```
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Later, this resource can be accessed from the application.
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Common resources stored in `app.state` include:
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```text
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Database clients
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Redis clients
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HTTP clients
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Configuration objects
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Service clients
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Loaded models
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```
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---
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# 8. Better Database Cleanup Example
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If the database client has a `close()` method, close it after `yield`.
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```python
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from contextlib import asynccontextmanager
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from fastapi import FastAPI
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@asynccontextmanager
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async def lifespan(app: FastAPI):
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app.state.db = await connect_to_db()
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print("Database connected")
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yield
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await app.state.db.close()
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print("Database disconnected")
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app = FastAPI(lifespan=lifespan)
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```
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This ensures that the application does not leave open connections after shutdown.
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---
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# 9. Why Lifespan Is Better
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The lifespan approach is better for modern FastAPI applications because it keeps startup and shutdown logic in one place.
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It helps with:
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```text
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Centralized lifecycle management
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Cleaner async resource handling
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Better application structure
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Easier testing
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More predictable production behavior
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Cleaner resource cleanup
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```
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---
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# 10. Complete Recommended Version
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```python
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from contextlib import asynccontextmanager
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from fastapi import FastAPI
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@asynccontextmanager
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async def lifespan(app: FastAPI):
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print("Application startup started")
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# Initialize shared resources here
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app.state.service_status = "ready"
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print("Application startup completed")
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yield
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print("Application shutdown started")
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# Clean up shared resources here
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app.state.service_status = "stopped"
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print("Application shutdown completed")
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app = FastAPI(lifespan=lifespan)
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@app.get("/")
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def root():
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return {"message": "API is working"}
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@app.get("/health")
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def health_check():
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return {
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"status": "healthy",
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"service": app.state.service_status
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}
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```
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---
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# 11. Testing the Application
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Run the app:
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```bash
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uvicorn main:app --reload
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```
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Open:
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```text
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http://localhost:8000/
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```
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Expected response:
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```json
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{
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"message": "API is working"
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}
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```
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Open:
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```text
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http://localhost:8000/health
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```
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Expected response:
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```json
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{
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"status": "healthy",
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"service": "ready"
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}
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```
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---
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# 12. Best Practices
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Use `lifespan` for new FastAPI applications.
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Avoid using `@app.on_event()` in new code because it is deprecated.
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Do not mix `lifespan` with `startup` and `shutdown` event decorators.
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Use `app.state` for shared application resources.
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Close database connections, cache clients, HTTP clients, and background services during shutdown.
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Keep startup logic lightweight.
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Avoid using `print()` in production.
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Use structured logging instead of printing to the console.
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Do not use `--reload` in production.
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