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Takes an existing OpenAPI Document and allows straightforward implementation of the API defined within it using the Cro libraries.


# Implement the OpenAPI defined in schema.yaml.
my $routes = openapi 'schema.yaml'.IO, {
	# Given an operation defined like this:
	#   summary: Updates a pet in the store with form data
	#   operationId: updatePetWithForm
	#   parameters:
	#   - name: petId
	#     in: path
	#     description: ID of pet that needs to be updated
	#     required: true
	#     schema:
	#   	type: string
	#   requestBody:
	#     content:
	#   	'application/x-www-form-urlencoded':
	#   	  schema:
	#   	   properties:
	#   		  name: 
	#   			description: Updated name of the pet
	#   			type: string
	#   		  status:
	#   			description: Updated status of the pet
	#   			type: string
	#   	   required:
	#   		 - status
	#   responses:
	#     '200':
	#   	description: Pet updated.
	#   	content: 
	#   	  'application/json': {}
	#     '400':
	#   	description: Invalid input
	#   	content: 
	#   	  'application/json': {}
	# We can implement it by receiving the route parameter as a positional
	# argument; other literal route segments need not be mentioned.
	operation 'updatePetWithForm', -> $id {
		# The request body will already have been validated, so just grab
		# it, perhaps using destructuring.
		request-body -> (:$name, :$status) {
			# Do something with it.
			$some-store.update-pet($id, $name, $status);

			# Respond (response automatically checked against schema too).
			content 'application/json', {};

The $routes object is a subclass of Cro::HTTP::Router::RouteSet, and so can be included into a route block:

my $api-routes = openapi 'schema.yaml'.IO, {
my $app = route {
	include 'api' => $api-routes;

Since it does the Cro::Transform role, it may be hosted directly as the application using Cro::HTTP::Server:

my $service = Cro::HTTP::Server:
	:host<>, :port(10000),

The openapi sub§

The openapi sub works somewhat like route from Cro::HTTP::Router. As in a route block, it is possible to:

  • Use before and after to add middleware. The before middleware will be after the validation of a request takes place, and the after middleware will be run before the validation of a response takes place. This means that middleware can rely on processing a request that has passed validation, while after middleware can add, for example, standard headers (such as rate limiting) to responses.

  • Use body-parser and body-serializer to specify body parsers and serializers. The body parsers will be put in place before validation of the body, to ensure deserialization works as desired.

By contrast, get, put, post and so forth are not valid in the context of an openapi block, and using them will produce an error. Instead, the operation sub should be used to specify the implementations of operations defined by the OpenAPI document. The URI patterns to match will be taken from the OpenAPI document, and need not be repeated. Similarly, include and delegate are not available either (a form of include may be supported in the future in order to allow for breaking up the definition of a large API over multiple files).

The openapi sub may be passed a string containing an OpenAPI document in either YAML or JSON:

openapi $json-doc, {

Or an IO object pointing to a file to read the document from:

openapi "api.yaml".IO, {

In either case, JSON will be detected by looking at the data that is read and seeing if it starts with { (with leading whitespace allowed); failing that, it will be parsed as YAML.

The openapi sub may be passed the following options:

  • :ignore-unimplemented - by default, an operation in the OpenAPI document that does not have an implementation in the openapi block will result in an error being raised by the openapi sub. This is to help you understand when an API has not been completely implemented. Setting this option will cause unimplemented operations to be ignored instead.

  • :!validate-responses - this option defaults to True, but may be set to False. If set to False, then responses will not be validated. This may be useful for increasing production performance, once confident the API has been correctly implemented.

  • :%formats and :%add-formats - passed to OpenAPI::Schema::Validate to control format validation (%add-formats adds additional formats or overrides existing ones which %formats allows for a full replacement of the available formats).

  • :%document - used to configure how the OpenAPI document itself is served. It defaults to { '/openapi.json' => 'json', '/openapi.yaml' => 'yaml' }, which means that the OpenAPI specification will be served as both JSON and YAML on requests to /openapi.json and /openapi.yaml respectively. To serve a format at the root of the API, pass :document{ '/' => json } (this also means it will not be served at /openapi.json and /openapi.yaml any longer). It is fine to register multiple paths to serve document for a given format.

All operations in the OpenAPI document should have an operationId, otherwise it is not possible to implement a Cro route handler for them.

The operation sub§

The operation sub is used to specify the implementation of an operation in the OpenAPI file. It takes a string operation ID and a block that will be run per request to that operation.

If the string operation ID does not match an operationId in the OpenAPI definition, an error will be raised.

The signature of the block may be used in order to unpack various properties of the request. This works similarly to signatures on get and similar in Cro::HTTP::Router, but with some differences.

  • The first parameter may be a session or auth object, populated according to the usual Cro::HTTP::Router rules.

  • Route parameters, from the request target, will be passed as positional arguments. Thus, the signature of the operation must be able to accept them, and cope with optional route parameters. An error will be given if the signature of any operation block is not suitable. Note that literal route parameters must not be mentioned, and the parameter variable names are not significant (the route parameters are passed in the order they appear in the URI).

  • Query string parameters may be unpacked into named arguments (either those with no applicable source trait or those marked with is query will be considered). There is no requirement to unpack all of the query string parameters. However, it is an error to name one that does not exist in the OpenAPI document.

  • Headers and cookies may be unpacked into named arguments (using the is header and is cookie parameter traits). There is no requirement to unpack these here, and it is allowed to unpack others not mentioned in the OpenAPI specification (to provide access to "standard" headers, for example).

Otherwise, it is just like being inside a normal get, post, etc. block as with Cro::HTTP::Router. The request and response terms provide access to the request and response objects, the request-body sub is available, and the various response helpers (such as content) are also available.

Automatic validation§

A request will be validated against the OpenAPI definition. The following aspects of the request will be validated:

  • Method (GET, POST, etc.) (failure to match will result in an automatic 405 response).

  • Route (path) arguments from the target URI (failure to match these will result in an automatic 404 response).

  • Query string arguments, headers, and cookies (failure to match these will result in an automatic 400 response).

  • The content type of the request body (failure to match this will result in an automatic 400 response).

  • The request body. Cro has built-in support for JSON, multipart/form-data and application/x-www-form-urlencoded request bodies, and validation will work out of the box. For other body formats, a body-parser will be required, and it should produce output that can be traversed like a JSON data structure in order for schema validation to work. Failure to match the schema for the request body will result in an autoamtic 400 response.

A response will (unless response validation is disabled) be validated for:

  • The status code of the response. Note that 400, 404, and 405 errors that are automatically produced as a result of request validation will always be allowed through.

  • That the required headers are present and match the schema.

  • The content type of the response body.

  • The response body. Cro has built-in support for JSON response bodies, and validation will work out of the box. For other formats, a body-serializer will be needed, and the data structure to serialize should be a JSON-like tree of hashes/arrays so it can be validated against the schema.

Failure to validate the response indicates an implementation error. A 500 error will be returned to the client, and the error will be logged.

Manually handling request validation errors§

It may in some cases be desirable to handle request validation errors as part of the operation implementation. Note that this does not apply to an incorrect method or non-matching route parameters. Further, it presumes that any named unpacks in the operation signature are liberal enough to cope with the invalid data.

To manually handle request validation errors, pass :allow-invalid to the operation sub. The request-validation-error sub can then be used in order to check if there is a validation error. If there is, then it will be populated with an instance of X::Cro::OpenAPI::RoutesFromDefinition::CheckFailed, which is a subclass of Exception. It has the properties:

  • http-message - the request that failed to parse. Same as request in the scope of a handler.

  • reason - a string explaining the reason that validation failed

If there is no request validation error, then Nil is returned, meaning it can be tested using with or without.

operation 'foo', :allow-invalid, -> $path-param {
	with request-validation-error() -> $error {
		content 'application/json', { :result('error'), :reason($error.reason) };
	else {
		content 'application/json', { :result("ok") };

Security requirements§

Enforcing security requirements involves:

  • Implementing the Cro::OpenAPI::RoutesFromDefinition::SecurityChecker role

  • Passing that using the security named parameter to the openapi function

Implementing the role requires implementing a single method, which receives the security scheme to enforce, the HTTP request object, an array of requirements (optional, and only applicable to OpenID) and the operation ID (also optional). It should return True if the requester satisfies the security requirements, and False if not.

role Cro::OpenAPI::RoutesFromDefinition::SecurityChecker {
    method is-allowed(OpenAPI::Model::SecurityScheme $scheme, Cro::HTTP::Request $request,
            :@requirements, :$operation-id --> Bool) { ... }

For the case of API keys, the role provides a get-api-key($scheme, $request) method that will use the scheme to look up the API key from the request. It will return a Failure if there is no such header, cookie, or query string parameter, or if the scheme type is not apiKey.

An example implementation of the role looks like this:

class KeyChecker does Cro::OpenAPI::RoutesFromDefinition::SecurityChecker {
    method is-allowed(OpenAPI::Model::SecurityScheme $scheme, Cro::HTTP::Request $request --> Bool) {
        with self.get-api-key($scheme, $request) -> $key {
            if $key.starts-with('totally-legit') {
                $request.auth =$key);
                return True;
        return False;

Which could be used like this:

my $application = openapi $api-doc, security => KeyChecker, {
    operation 'public', -> {
        content 'text/plain', 'public ok';
    operation 'private', -> {
        content 'text/plain', 'private ok, key=' ~ request.auth.key;