> ## Documentation Index
> Fetch the complete documentation index at: https://fastql.vachagan.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Subscriptions

> Stream a result per source event, over WebSocket or SSE.

A subscription resolver returns an async iterator; the executor runs the selection
set once per yielded value and emits one `ExecutionResult` per event.

```python theme={null}
from typing import AsyncGenerator
from fastql import Field, Subscription, subscribe


@Subscription
class Subscriptions:
    @Field
    async def counter(self, to: int = 3) -> AsyncGenerator[int, None]:
        for value in range(to):
            yield value


async for result in await subscribe(schema, "subscription { counter(to: 2) }"):
    print(result.data)          # {'counter': 0}, then {'counter': 1}
```

`subscribe()` returns an async iterator of results, or a single
`ExecutionResult` (with `executed=False`) when the document is invalid or is not a
subscription. A subscription operation must select exactly one root field — this is
enforced during validation. Per-event resolver errors are isolated to that event,
and the source generator's `aclose()` is guaranteed on disconnect or cancellation.

## Transports

The shared HTTP handler drives subscriptions over a streaming transport, selected by
the request's `Accept` header:

* **`graphql-transport-ws`** over WebSocket (`connection_init` → `next` → `complete`).
* **Server-Sent Events** (`Accept: text/event-stream`).
* **`multipart/mixed`** (`Accept: multipart/mixed`).

The ASGI, Starlette, FastAPI, AIOHTTP, Quart, Litestar, and Channels adapters expose
the WebSocket transport; every HTTP adapter supports SSE and multipart streaming.