> ## 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.

# Apollo Federation v2 — Overview

> Add FastQL subgraphs to an Apollo Federation supergraph.

`fastql.federation` adds Apollo Federation v2 support to any FastQL schema:
federation directives as applied-directive metadata, a federated-SDL printer with
the `@link` import header, and the `_service` / `_entities` machinery for entity
resolution.

## Installation

Federation support is built into FastQL — no extra install required. Import from
`fastql.federation`:

```python theme={null}
from fastql.federation import (
    key, shareable, reference_resolver, print_federated_schema,
)
```

## Defining a federated schema

Decorate types with federation directives and build a regular `Schema`. FastQL
injects the `_service { sdl }` root, the `_Any` / `_Entity` / `_Service` types,
and the `_entities(representations:)` root field automatically.

```python theme={null}
from fastql import Field, Query, Schema, Type
from fastql.federation import key, shareable, print_federated_schema


@Type(directives=[key("id"), shareable()])
class Product:
    id: str
    name: str


@Query
class QueryRoot:
    @Field
    def product(self, id: str) -> Product | None:
        return PRODUCTS.get(id)


schema = Schema(query=QueryRoot)
sdl = print_federated_schema(schema)   # includes the @link(url: ...) header
```

`print_federated_schema` produces the federation-annotated SDL that the Apollo
Router ingests as the subgraph SDL.

## Available directive helpers

| Helper                    | SDL directive            | Placement                 |
| ------------------------- | ------------------------ | ------------------------- |
| `key(fields)`             | `@key(fields: ...)`      | `@Type(directives=[...])` |
| `shareable()`             | `@shareable`             | `@Type` or `@Field`       |
| `external()`              | `@external`              | `@Field`                  |
| `requires(fields)`        | `@requires(fields: ...)` | `@Field`                  |
| `provides(fields)`        | `@provides(fields: ...)` | `@Field`                  |
| `inaccessible()`          | `@inaccessible`          | `@Type` or `@Field`       |
| `override(from_subgraph)` | `@override(from: ...)`   | `@Field`                  |
| `tag(name)`               | `@tag(name: ...)`        | `@Type` or `@Field`       |

All helpers are applied via `@Type(directives=[...])` or `@Field(directives=[...])`.

***

**Next:** [Entity resolution](/federation/entities) — `@key` and reference resolvers.