16. Asynchronous Endpoints and Streaming Responses

Difference Between Sync and Async Routes

FastAPI supports both synchronous and asynchronous route handlers. Synchronous routes block the worker thread during execution, while asynchronous routes release the event loop during I/O operations. Choosing between them affects scalability and resource utilization under concurrent load.

from fastapi import FastAPI

app = FastAPI()

@app.get("/sync")
def sync_route():
    return {"type": "sync"}

@app.get("/async")
async def async_route():
    return {"type": "async"}

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When Async Improves Performance and When It Does Not

Asynchronous execution improves performance primarily for I/O-bound workloads such as database queries and network calls. It does not provide benefits for CPU-bound operations and may even introduce overhead if misused. Understanding this distinction is critical for designing efficient FastAPI applications.

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Streaming Large Responses and Long-Running Operations

FastAPI supports streaming responses to handle large datasets or long-running processes without exhausting memory. Streaming sends data incrementally to the client, reducing latency and improving user experience. This pattern is essential for file downloads, real-time feeds, and progressive data delivery.

from fastapi import FastAPI
from fastapi.responses import StreamingResponse

app = FastAPI()

def data_stream():
    for i in range(5):
        yield f"chunk {i}\n"

@app.get("/stream")
def stream_data():
    return StreamingResponse(data_stream(), media_type="text/plain")