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

# @link

> @link is a federation V2-only directive. Examples and usages are outlined herein.

## Definition

```graphql theme={"system"}
directive @link(
  as: String,
  for: link__Purpose,
  import: [link__Import],
  url: String!
) repeatable on SCHEMA
```

### Arguments

| Argument | Type             | Description                                                                                                           |
| -------- | ---------------- | --------------------------------------------------------------------------------------------------------------------- |
| `import` | `[link__Import]` | The name of the directive to import with leading `@` or a valid object (see [import validation](#import-validation)). |
| `url`    | `String!`        | The feature URL for the import (see [URL validation](#url-validation).                                                |

## Overview

The `@link` directive allows the importing of other directives.
This is necessary to propagate a (custom) directive into the router schema through the
[@composeDirective directive](./composeddirective).

### Usage

<Warning>
  WunderGraph Cosmo does not currently support renaming federation directives.
</Warning>

When using WunderGraph Cosmo, explicitly importing federation directives is not necessary.
These directives are inferred and injected into the schema automatically.

If importing a custom directive, the directive definition must still be provided in the schema if the directive is
used within the subgraph schema.
For example:

```graphql theme={"system"}
# subgraph 1
schema
@link(import: ["@shareable"], url: "https://specs.apollo.dev/federation/v2.3")
@link(import: ["@myDirective"], url: "https://company.org/my-feature/myDirective/v1.0") {
  query: Query
}

directive @myDirective on FIELD_DEFINITION

type Query @shareable {
    echo(string: String!): String! @myDirective
}
```

<Note>
  For brevity, "composing a directive" will henceforth serve as a substitute for providing the name of an imported an
  defined directive to the `name` argument of a `@composeDirective`.
</Note>

### Validation

#### Import validation

Directives are imported through a list.
List values can provided in one of two ways:

* A string of the directive name (with leading `@`)
* An object of the following form:

```graphql theme={"system"}
input link__Import {
  name: String!
  as: String
}
```

Any other type of value will result in a composition error.
If renaming an import, the import must also begin with a leading `@`.
An example valid import might be:

```graphql theme={"system"}
# subgraph 1
extend schema @link(
  import: ["@myDirective", { name: "@anotherDirective", as: "@hello" }],
  url: "https://company.org/directives/v1.0"
)
```

#### URL validation

A URL must contain a path component, a feature component, and end with a version component.
The version component must strictly match the format `vMajor.Minor` where `Major` and `Minor` represent integers, e.g., `v1.2`.
An example valid URL might be:

```graphql theme={"system"}
# subgraph 1
extend schema @link(
  import: ["@myDirective"],
  url: "https://path/feature-name/v1.2"
)
```

### Errors

`@link` will produce composition error(s) if:

* A feature URL is invalid.
* The same base feature URL is used across subgraphs but the major versions do not match.
* An import value is not a valid string (`@{string}`) or object (`{ name: String! as: String }`) form.
* A directive name (or its renamed value) do not both start with leading `@`.