FastAPI Backend Development Bootcamp · Lesson

Real-Time GraphQL Subscriptions

Stream live updates to clients over WebSocket-backed GraphQL subscriptions with backpressure awareness.

Why Subscriptions Exist

GraphQL gives you three root operation types: query (read), mutation (write), and subscription (live stream). A subscription is the only one that keeps a long-lived connection open and pushes multiple results to the client over time.

Query/Mutation: one request, one response, connection closes.
Subscription: one request, a stream of responses, until either side closes it.

In Strawberry, a subscription resolver is an async generator: every yield becomes one payload delivered to the client. Under the hood, FastAPI carries this over a WebSocket, because plain HTTP cannot push server-initiated frames cleanly.

An Async Generator Is the Core Idea

Before wiring GraphQL, understand the engine: Python async generators. Each yield hands one value to the consumer and then suspends until the consumer asks for the next one. This natural pull-based flow is what gives subscriptions backpressure for free: the producer only advances when the consumer is ready.

This snippet runs anywhere — it is the exact shape a Strawberry subscription resolver takes, minus the decorator.

import asyncio

async def counter(limit: int):
    for i in range(limit):
        await asyncio.sleep(0.1)  # simulate work / waiting for an event
        yield i

async def main():
    async for value in counter(5):
        print(f"received: {value}")

asyncio.run(main())

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