# Parameters

Templates can contain _parameters_, which allow prompting the user for
additional information when creating workspaces in both the UI and CLI.

![Parameters in Create Workspace screen](../images/parameters.png)

```hcl
data "coder_parameter" "docker_host" {
  name        = "Region"
  description = "Which region would you like to deploy to?"
  icon        = "/emojis/1f30f.png"
  type        = "string"
  default     = "tcp://100.94.74.63:2375"

  option {
    name = "Pittsburgh, USA"
    value = "tcp://100.94.74.63:2375"
    icon = "/emojis/1f1fa-1f1f8.png"
  }

  option {
    name = "Helsinki, Finland"
    value = "tcp://100.117.102.81:2375"
    icon = "/emojis/1f1eb-1f1ee.png"
  }

  option {
    name = "Sydney, Australia"
    value = "tcp://100.127.2.1:2375"
    icon = "/emojis/1f1e6-1f1f9.png"
  }
}
```

From there, parameters can be referenced during build-time:

```hcl
provider "docker" {
  host = data.coder_parameter.docker_host.value
}
```

> For a complete list of supported parameter properties, see the
> [coder_parameter Terraform reference](https://registry.terraform.io/providers/coder/coder/latest/docs/data-sources/parameter)

## Types

The following parameter types are supported: `string`, `list(string)`, `bool`,
and `number`.

### List of strings

List of strings is a specific parameter type, that can't be easily mapped to the
default value, which is string type. Parameters with the `list(string)` type
must be converted to JSON arrays using
[jsonencode](https://developer.hashicorp.com/terraform/language/functions/jsonencode)
function.

```hcl
data "coder_parameter" "security_groups" {
  name        = "Security groups"
  icon        = "/icon/aws.png"
  type        = "list(string)"
  description = "Select appropriate security groups."
  mutable     = true
  default = jsonencode([
    "Web Server Security Group",
    "Database Security Group",
    "Backend Security Group"
  ])
}
```

## Options

A _string_ parameter can provide a set of options to limit the choice:

```hcl
data "coder_parameter" "docker_host" {
  name        = "Region"
  description = "Which region would you like to deploy to?"
  type        = "string"
  default     = "tcp://100.94.74.63:2375"

  option {
    name = "Pittsburgh, USA"
    value = "tcp://100.94.74.63:2375"
    icon = "/emojis/1f1fa-1f1f8.png"
  }

  option {
    name = "Helsinki, Finland"
    value = "tcp://100.117.102.81:2375"
    icon = "/emojis/1f1eb-1f1ee.png"
  }

  option {
    name = "Sydney, Australia"
    value = "tcp://100.127.2.1:2375"
    icon = "/emojis/1f1e6-1f1f9.png"
  }
}
```

### Incompatibility in Parameter Options for Workspace Builds

When creating Coder templates, authors have the flexibility to modify parameter
options associated with rich parameters. Such modifications can involve adding,
substituting, or removing a parameter option. It's important to note that making
these changes can lead to discrepancies in parameter values utilized by ongoing
workspace builds.

Consequently, workspace users will be prompted to select the new value from a
pop-up window or by using the command-line interface. While this additional
interactive step might seem like an interruption, it serves a crucial purpose.
It prevents workspace users from becoming trapped with outdated template
versions, ensuring they can smoothly update their workspace without any
hindrances.

Example:

- Bob creates a workspace using the `python-dev` template. This template has a
  parameter `image_tag`, and Bob selects `1.12`.
- Later, the template author Alice is notified of a critical vulnerability in a
  package installed in the `python-dev` template, which affects the image tag
  `1.12`.
- Alice remediates this vulnerability, and pushes an updated template version
  that replaces option `1.12` with `1.13` for the `image_tag` parameter. She
  then notifies all users of that template to update their workspace
  immediately.
- Bob saves their work, and selects the `Update` option in the UI. As their
  workspace uses the now-invalid option `1.12`, for the `image_tag` parameter,
  they are prompted to select a new value for `image_tag`.

## Required and optional parameters

A parameter is considered to be _required_ if it doesn't have the `default`
property. The user **must** provide a value to this parameter before creating a
workspace.

```hcl
data "coder_parameter" "account_name" {
  name        = "Account name"
  description = "Cloud account name"
  mutable     = true
}
```

If a parameter contains the `default` property, Coder will use this value if the
user does not specify any:

```hcl
data "coder_parameter" "base_image" {
  name        = "Base image"
  description = "Base machine image to download"
  default     = "ubuntu:latest"
}
```

Admins can also set the `default` property to an empty value so that the
parameter field can remain empty:

```hcl
data "coder_parameter" "dotfiles_url" {
  name        = "dotfiles URL"
  description = "Git repository with dotfiles"
  mutable     = true
  default     = ""
}
```

Terraform
[conditional expressions](https://developer.hashicorp.com/terraform/language/expressions/conditionals)
can be used to determine whether the user specified a value for an optional
parameter:

```hcl
resource "coder_agent" "main" {
  # ...
  startup_script_timeout = 180
  startup_script         = <<-EOT
    set -e

    echo "The optional parameter value is: ${data.coder_parameter.optional.value == "" ? "[empty]" : data.coder_parameter.optional.value}"

  EOT
}
```

## Mutability

Immutable parameters can be only set before workspace creation, or during update
on the first usage to set the initial value for required parameters. The idea is
to prevent users from modifying fragile or persistent workspace resources like
volumes, regions, etc.:

```hcl
data "coder_parameter" "region" {
  name        = "Region"
  description = "Region where the workspace is hosted"
  mutable     = false
  default     = "us-east-1"
}
```

It is allowed to modify the mutability state anytime. In case of emergency,
template authors can temporarily allow for changing immutable parameters to fix
an operational issue, but it is not advised to overuse this opportunity.

## Ephemeral parameters

Ephemeral parameters are introduced to users in the form of "build options."
This functionality can be used to model specific behaviors within a Coder
workspace, such as reverting to a previous image, restoring from a volume
snapshot, or building a project without utilizing cache.

As these parameters are ephemeral in nature, subsequent builds will proceed in
the standard manner.

```hcl
data "coder_parameter" "force_rebuild" {
  name         = "force_rebuild"
  type         = "bool"
  description  = "Rebuild the Docker image rather than use the cached one."
  mutable      = true
  default      = false
  ephemeral    = true
}
```

## Validation

Rich parameters support multiple validation modes - min, max, monotonic numbers,
and regular expressions.

### Number

A _number_ parameter can be limited to boundaries - min, max. Additionally, the
monotonicity (`increasing` or `decreasing`) between the current parameter value
and the new one can be verified too. Monotonicity can be enabled for resources
that can't be shrunk without implications, for instance - disk volume size.

```hcl
data "coder_parameter" "instances" {
  name        = "Instances"
  type        = "number"
  description = "Number of compute instances"
  validation {
    min       = 1
    max       = 8
    monotonic = "increasing"
  }
}
```

### String

A _string_ parameter can have a regular expression defined to make sure that the
parameter value matches the pattern. The `regex` property requires a
corresponding `error` property.

```hcl
data "coder_parameter" "project_id" {
  name        = "Project ID"
  description = "Alpha-numeric project ID"
  validation {
    regex = "^[a-z0-9]+$"
    error = "Unfortunately, it isn't a valid project ID"
  }
}
```

## Legacy

### Legacy parameters are unsupported now

In Coder, workspaces using legacy parameters can't be deployed anymore. To
address this, it is necessary to either remove or adjust incompatible templates.
In some cases, deleting a workspace with a hard dependency on a legacy parameter
may be challenging. To cleanup unsupported workspaces, administrators are
advised to take the following actions for affected templates:

1. Enable the `feature_use_managed_variables` provider flag.
2. Ensure that every legacy variable block has defined missing default values,
   or convert it to `coder_parameter`.
3. Push the new template version using UI or CLI.
4. Update unsupported workspaces to the newest template version.
5. Delete the affected workspaces that have been updated to the newest template
   version.

### Migration

> ⚠️ Migration is available until v0.24.0 (Jun 2023) release.

Terraform `variable` shouldn't be used for workspace scoped parameters anymore,
and it's required to convert `variable` to `coder_parameter` resources. To make
the migration smoother, there is a special property introduced -
`legacy_variable` and `legacy_variable_name` , which can link `coder_parameter`
with a legacy variable.

```hcl
variable "legacy_cpu" {
  sensitive   = false
  description = "CPU cores"
  default     = 2
}

data "coder_parameter" "cpu" {
  name        = "CPU cores"
  type        = "number"
  description = "Number of CPU cores"
  mutable     = true

  legacy_variable_name = "legacy_cpu"
  legacy_variable = var.legacy_cpu
}
```

#### Steps

1. Prepare and update a new template version:

   - Add `coder_parameter` resource matching the legacy variable to migrate.
   - Use `legacy_variable_name` and `legacy_variable` to link the
     `coder_parameter` to the legacy variable.
   - Mark the new parameter as `mutable`, so that Coder will not block updating
     existing workspaces.

2. Update all workspaces to the updated template version. Coder will populate
   the added `coder_parameter`s with values from legacy variables.
3. Prepare another template version:

   - Remove the migrated variables.
   - Remove properties `legacy_variable` and `legacy_variable_name` from
     `coder_parameter`s.

4. Update all workspaces to the updated template version (2nd).
5. Prepare a third template version:

   - Enable the `feature_use_managed_variables` provider flag to use managed
     Terraform variables for template customization. Once the flag is enabled,
     legacy variables won't be used.

6. Update all workspaces to the updated template version (3rd).
7. Delete legacy parameters.

As a template improvement, the template author can consider making some of the
new `coder_parameter` resources `mutable`.

## Terraform template-wide variables

> ⚠️ Flag `feature_use_managed_variables` is available until v0.25.0 (Jul 2023)
> release. After this release, template-wide Terraform variables will be enabled
> by default.

As parameters are intended to be used only for workspace customization purposes,
Terraform variables can be freely managed by the template author to build
templates. Workspace users are not able to modify template variables.

The template author can enable Terraform template-wide variables mode by
specifying the following flag:

```hcl
provider "coder" {
  feature_use_managed_variables = "true"
}
```

Once it's defined, coder will allow for modifying variables by using CLI and UI
forms, but it will not be possible to use legacy parameters.