Unclear possible values for contentType in AppConfig CDK

[jeqcho]

aws-cdk/packages/aws-cdk-lib/aws-appconfig/lib/configuration.ts

Lines 906 to 909 in adb9c64

	/**
	* The configuration content type.
	*/
	public abstract readonly contentType: string;

All constructor methods for ConfigurationContent accept the contentType parameter, but no guidance is found here nor the docs as to which strings are valid (e.g. I found text/plain in the guide, but I can only guess if text/json, text/yaml are also accepted). It will be helpful if this is clarified in the docstring.

[pahud]

Root Cause Analysis

From examining the source code in packages/aws-cdk-lib/aws-appconfig/lib/configuration.ts, I found:

/**
 * Defines the hosted configuration content from a file.
 *
 * @param inputPath The path to the file that defines configuration content
 * @param contentType The content type of the configuration
 */
public static fromFile(inputPath: string, contentType?: string): ConfigurationContent {
  return {
    content: fs.readFileSync(path.resolve(inputPath)).toString(),
    contentType: contentType || mimeTypes.lookup(inputPath) || 'application/json',
  };
}

The code shows that:

1. When contentType is not provided, it uses mimeTypes.lookup() or defaults to 'application/json'
2. The CDK uses standard MIME types internally
3. From test files, I can see these content types are used:
   • 'text/plain' (for fromInlineText)
   • 'application/json' (for fromFile with .json files and fromInlineJson)
   • 'application/octet-stream' (for fromInline default)
   • 'application/x-yaml' (for fromInlineYaml)

The AWS AppConfig API documentation states:

"A standard MIME type describing the format of the configuration content. For more information, see Content-Type." and references the standard HTTP Content-Type header specification.

Document Evidence with links

• ConfigurationContent source code
• AWS AppConfig API CreateHostedConfigurationVersion
• CloudFormation HostedConfigurationVersion
• Test examples showing various content types

Possible Fixes/Solutions

1. Add JSDoc documentation to the contentType parameter explaining:
   • It accepts standard MIME types
   • Common examples: text/plain, application/json, application/x-yaml, text/yaml
   • Reference to IANA Media Types registry
   • Note that AWS AppConfig accepts any valid MIME type

2. Update the ConfigurationContent class documentation with examples of valid content types

3. Add a link to MIME type standards in the parameter documentation

---

Hi @jeqcho,

Thank you for reporting this documentation gap! You're absolutely right that the contentType parameter documentation could be much clearer about which values are accepted.

The contentType parameter accepts standard MIME types as defined by the IANA Media Types registry. AWS AppConfig accepts any valid MIME type, including but not limited to:

• text/plain - Plain text content
• application/json - JSON format
• application/x-yaml or text/yaml - YAML format
• text/xml - XML format
• application/octet-stream - Binary data
• Any other standard MIME type

Looking at the ConfigurationContent source code, you can see that when no contentType is specified, the CDK automatically detects it using the mime-types library or provides sensible defaults.

This is definitely a documentation enhancement that would help other developers. Would you be interested in contributing a PR to improve the JSDoc comments for these methods? The fix would involve adding better documentation to the contentType parameters in the ConfigurationContent class methods, including examples of common MIME types and a reference to the MIME type standards.

The relevant file is packages/aws-cdk-lib/aws-appconfig/lib/configuration.ts around lines 906-909 and similar methods.

Thanks for bringing this to our attention!

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