# Use parameters to specify module values
<a name="module-using-params"></a>

In CloudFormation, you can use template parameters to customize your stacks by providing input values during stack creation or update. These parameters allow you to change certain aspects of the stack based on your needs. For more information about defining template parameters, see [CloudFormation template Parameters syntax](parameters-section-structure.md).

Similarly, modules can also have parameters. These module parameters allow you to input custom values to the module from the template (or another module) that's using it. The module can then use these custom values to set property values for the resources it contains.

You can also define template parameters that set module properties, so that you can input values that get passed to the module at the time of the stack operation. 

If a module contains a nested module that has its own module parameters, you can either:
+ Specify the values for the nested module's parameters directly in the parent module.
+ Define corresponding module parameters in the parent module that enable the nested module's parameters to be set by the template (or module) in which the parent module is contained.

## Using template parameters to specify module parameter values
<a name="module-using-params-example-1"></a>

The following example shows how to define template parameters that pass values to a module.

This template containing `My::S3::SampleBucket::MODULE` defines a template parameter, `BucketName`, that enables the user to specify an S3 bucket name during the stack operation.

```
# Template containing My::S3::SampleBucket::MODULE
Parameters:
  BucketName:
    Description: Name for your sample bucket
    Type: String
Resources:
  MyBucket:
    Type: 'My::S3::SampleBucket::MODULE'
    Properties:
      BucketName: !Ref BucketName
```

## Specifying properties on resources in a child module from the parent module
<a name="module-using-params-example-2"></a>

The following example illustrates how to specify parameter values in a module that's nested within another module.

This first module, `My::S3::SampleBucketPrivate::MODULE`, will be the child module. It defines two parameters: `BucketName` and `AccessControl`. The values specified for these parameters are used to specify the `BucketName` and `AccessControl` properties of the `AWS::S3::Bucket` resource the module contains. Below is the template fragment for `My::S3::SampleBucketPrivate::MODULE`.

```
# My::S3::SampleBucketPrivate::MODULE
AWSTemplateFormatVersion: 2010-09-09
Description: A sample S3 Bucket with Versioning and DeletionPolicy.
Parameters:
  BucketName:
    Description: Name for the bucket
    Type: String
  AccessControl:
    Description: AccessControl for the bucket
    Type: String
Resources:
  S3Bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Ref BucketName
      AccessControl: !Ref AccessControl
      DeletionPolicy: Retain
      VersioningConfiguration:
        Status: Enabled
```

Next, the previous module is nested within a parent module, `My::S3::SampleBucket::MODULE`. The parent module, `My::S3::SampleBucket::MODULE`, sets the child module parameters in the following ways:
+ It sets the `AccessControl` parameter of `My::S3::SampleBucketPrivate::MODULE` to `Private`.
+ For `BucketName`, it defines a module parameter, which will enable the bucket name to be specified in the template (or module) that contains `My::S3::SampleBucket::MODULE`.

```
# My::S3::SampleBucket::MODULE
AWSTemplateFormatVersion: 2010-09-09
Description: A sample S3 Bucket. With Private AccessControl.
Parameters:
  BucketName:
    Description: Name for your sample bucket
    Type: String
Resources:
  MyBucket:
    Type: 'My::S3::SampleBucketPrivate::MODULE'
    Properties:
      BucketName: !Ref BucketName
      AccessControl: Private
```

## Specifying constraints for module parameters
<a name="modules-using-parameters-constraints"></a>

Module parameters don't support constraint enforcement. To perform constraint checking on a module parameter, create a template parameter with the desired constraints. Then, reference that template parameter in your module parameter. For more information about defining template parameters, see [CloudFormation template Parameters syntax](parameters-section-structure.md).