---
title: "argument | Grafana Alloy documentation"
description: "Learn about the argument configuration block"
---

# `argument`

`argument` is an optional configuration block used to specify parameterized input to a [custom component](../../../get-started/custom_components/). `argument` blocks must be given a label which determines the name of the argument.

The `argument` block may only be specified inside the definition of [a `declare` block](../../config-blocks/declare/).

## Usage

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```alloy
argument "<ARGUMENT_NAME>" {}
```

## Arguments

> Note
> 
> For clarity, “argument” in this section refers to arguments which can be given to the argument block. “Module argument” refers to the argument being defined for a module, determined by the label of the argument block.

You can use the following arguments with `argument`:

Expand table

| Name       | Type     | Description                          | Default | Required |
|------------|----------|--------------------------------------|---------|----------|
| `comment`  | `string` | Description for the argument.        | `""`    | no       |
| `default`  | `any`    | Default value for the argument.      | `null`  | no       |
| `optional` | `bool`   | Whether the argument may be omitted. | `false` | no       |

By default, all module arguments are required. The `optional` argument can be used to mark the module argument as optional. When `optional` is `true`, the initial value for the module argument is specified by `default`.

## Exported fields

The following fields are exported and can be referenced by other components:

Expand table

| Name    | Type  | Description                        |
|---------|-------|------------------------------------|
| `value` | `any` | The current value of the argument. |

If you use a custom component, you are responsible for determining the values for arguments. Other expressions within a custom component may use `argument.ARGUMENT_NAME.value` to retrieve the value you provide.

## Example

This example creates a custom component that self-collects process metrics and forwards them to an argument specified by the user of the custom component:

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```alloy
declare "self_collect" {
  argument "metrics_output" {
    optional = false
    comment  = "Where to send collected metrics."
  }

  prometheus.scrape "selfmonitor" {
    targets = [{
      __address__ = "127.0.0.1:12345",
    }]

    forward_to = [argument.metrics_output.value]
  }
}
```