> ## Documentation Index
> Fetch the complete documentation index at: https://docs.tai42.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Backends (tai42_contract.backend)

> The Backend protocol a compute backend implements.

Execution-backend contract: the `Backend` ABC + the `CallbackSchema`.

## Backend

`tai42_contract.backend.base.Backend`

```python theme={null}
class Backend(ABC)
```

Abstract execution backend for a Tai app.

A backend runs the task runtime (`launch`) and executes the work its
workers pull from the broker. The app core depends only on this interface
and stays backend-agnostic; concrete backends (celery/rq/arq) implement it
and register via `@tai42_app.backends.register_backend`.

Fleet propagation of config changes is not a backend concern: it is the
app's own worker bus, internal to the app. A backend-runtime process
receives fleet ops through the app's bus subscription exactly like a serving
HTTP worker — the backend carries no control-plane surface of its own.

### Members

#### launch

`tai42_contract.backend.base.Backend.launch`

```python theme={null}
async launch(self, args: Sequence[str]) -> None
```

Start the worker runtime for the backend registered via
`@tai42_app.backends.register_backend`.

**Parameters**

| Parameter | Type            | Default | Description |
| --------- | --------------- | ------- | ----------- |
| `args`    | `Sequence[str]` | —       | —           |

## CallbackSchema

`tai42_contract.backend.callback.CallbackSchema`

```python theme={null}
class CallbackSchema(ConditionMixin, ExprMixin)
```

**Attributes**

| Attribute | Type  |
| --------- | ----- |
| `tool`    | `str` |