YouTube/Code Collider/24/03/How FastAPI Handles Requests Behind the Scenes

• How FastAPI Handles Requests Behind the Scenes - YouTube

• My Notes

• Video

  • 00:00 Introduction: Basic FastAPI Endpoint Example

    • Setting up a simple endpoint with:
      • Two print statements
      • 5-second delay between prints
      • Testing through FastAPI docs interface

    • Initial Test Results

      • Single request behavior:
        • Prints "hello"
        • Waits 5 seconds
        • Prints "bye"

  • 00:31 Experiment/1: Sequential Request Handling

    • Testing multiple requests within 5-second window

    • Observations

      • Requests processed sequentially
      • Output pattern:
        • First request: hello → bye
        • Second request: hello → bye
      • Key Insight: FastAPI completes first request before processing second

  • 00:49 Experiment/2: Using Async Operations

    • Implementation Changes

      • Replaced time.sleep with asyncio.sleep

    • Results

      • Different execution pattern:
        • Both "hello" prints appear first
        • Both "bye" prints appear later
      • Key Insight: FastAPI handled requests concurrently

  • 01:07 Experiment/3: Non-Async Function

    • Implementation

      • Changed async def to regular def

    • Results

      • Parallel execution observed:
        • Both "hello" prints appear together
        • Both "bye" prints appear together
      • Key Insight: Requests handled in parallel

  • 01:41 Understanding Blocking vs Non-blocking Operations

    • Non-blocking I/O: asyncio.sleep

      • Can be awaited
      • Function execution can pause
      • Event loop handles other tasks during pause

    • Blocking I/O: time.sleep

      • Cannot be awaited
      • Blocks event loop
      • No task switching possible

  • 02:27 Summary of Request Handling Patterns

    • Pattern/1: Coroutine without Non-blocking I/O

      • Defined with async def
      • No non-blocking operations
      • Result: Sequential processing

    • Pattern/2: Coroutine with Non-blocking I/O

      • Defined with async def
      • Contains non-blocking operations
      • Result: Concurrent processing

    • Pattern/3: Normal Functions

      • Defined with regular def
      • Result: Parallel execution

  • 02:42 FastAPI Architecture Deep Dive

    • Main Thread Behavior

      • Uvicorn starts main thread
      • Coroutines run in event loop
      • Event loop runs in main thread

    • Threading Model

      • Coroutines (async def): Run in main thread
      • Normal functions (def): Run in separate threads

  • 04:26 Best Practices

    • When to Use async def

      • Non-blocking I/O operations
      • DB queries
      • External API requests
      • asyncio.sleep

    • When to Use Regular Functions

      • Blocking I/O operations
      • DB clients without await feature
      • time.sleep
      • Operations requiring separate threads