# channel-parameters-defined

Source: https://apigenie.pl/docs/linter/asyncapi/rules/channel-parameters-defined

This rule checks that the parameters used in a channel address are declared in
the channel's `parameters` map, and that the map declares no parameters the
address does not use.

## Rule Details

A channel address may contain parameter expressions, such as `{userId}` in
`users.{userId}.signedUp`. The AsyncAPI specification requires every such
expression to have a matching entry in the channel's `parameters` map, whose
key is the parameter name. Parameters that are declared but never used in the
address are almost always leftovers from an earlier version of the address.

The `channel-parameters-defined` rule checks every channel defined in the
root `channels` map and in `components/channels` (references to channels are
checked where the channel is defined). For each channel it reports:

- `Channel parameter '<name>' is not defined` for every parameter used in the
  address that has no entry in the `parameters` map,
- `Channel parameter '<name>' is not used in the channel address` for every
  entry of the `parameters` map that does not appear in the address.

Channels without an `address` (or with `address: null`) are ignored, since
their parameters cannot be verified.

## Configuration

None.

## Examples

**Good**. The address and the parameters map agree.

```yaml
asyncapi: 3.0.0
info:
  title: User Events
  version: 1.0.0
channels:
  userSignedUp:
    address: users.{userId}.signedUp
    parameters:
      userId:
        description: Identifier of the user.
```

**Bad**. `userId` is used in the address but not declared, and `region` is
declared but not used.

```yaml
asyncapi: 3.0.0
info:
  title: User Events
  version: 1.0.0
channels:
  userSignedUp:
    address: users.{userId}.signedUp
    parameters:
      region:
        description: Region the user signed up in.
```

## Automatic fixes

When the linter is invoked with the `--fix` option, this rule rewrites the
`parameters` map of every reported channel so that it matches the address:
declared parameters that the address uses are kept unchanged (including
references), missing parameters are added as empty Parameter objects, and
unused ones are removed. The parameters are ordered as they first appear in
the address. If the address uses no parameters at all, the `parameters` map is
removed.

## When Not to Use It

We recommend to always enable this rule.

## Compatibility

This rule is compatible with AsyncAPI 3.x documents. AsyncAPI 2.x documents
are rejected by the reader and are not linted.