Content Validation

Once incoming data has been deserialized, how and when do you ensure it's valid? And if you determine it's invalid, how do you report that information?

Taking a layered security approach, the sooner you can deliver validation errors, the better. Denial of Service attacks will often send invalid data in order to mire the system in long-running and processor intensive requests, thus denying service to valid requests.

Types of Validation

You know you need to validate your requests; the question is: how? With a typical HTML web application, you have forms, and server-side logic for validating the forms. What tools do you have for APIs, where the data is usually not of the application/x-www-form-urlencoded media type?

One option is JSON Schema, which is both a way to describe the data format, as well as validate it. This approach requires having tools server-side for transforming the schema into validation rules that you can run against your code.

Another option is to treat the incoming data as form data; deserialize it into an array, and pass it to the same logic you would use to validate a form. This requires that your form validation logic does not operate directly on $_POST or $_GET, but instead allows passing the data set to validate.

Zend Framework 2 offers an approach similar to the latter, via the Laminas\InputFilter component. This component allows you to describe and validate data sets of arbitrary complexity. Additionally, it allows for the ability to both set custom error messages as well as retrieve validation error messages in a structured format. API Tools' api-tools-content-validation module provides functionality for mapping Zend Framework 2 input filters to services, and utilizes API Problem in order to return validation error messages to the end-user of the API.

If the data provided does not overlap with the set described by the input filter, API Tools will return a 400 Bad Request status code. If any portion of the data set does overlap, but is invalid, instead a 422 Unprocessable Entity status will be returned with an application/problem+json payload that contains a validation_messages key.

As an example, consider a "Status" service that accepts two fields, "message" and "user"; the first cannot be empty, and must be less than or equal to 140 characters; the second will be validated against a regular expression of valid users. Let's consider a request that provides an empty message and an invalid user:

POST /status HTTP/1.1
Accept: application/vnd.status.v2+json
Content-Type: application/json

{
    "message": " ",
    "user": "matthew"
}

API Tools will deserialize the data and pass it to the configured input filter, which will then determine that the data is invalid. The following response will be provided:

HTTP/1.1 422 Unprocessable Entity
Content-Type: application/problem+json

{
    "detail": "Failed Validation",
    "status": 422,
    "title": "Unprocessable Entity",
    "type": "http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html",
    "validation_messages": {
        "message": {
            "isEmpty": "Value is required and can't be empty"
        },
        "user": {
            "regexNotMatch": "Invalid user supplied."
        }
    }
}

Validation errors from API Tools will always follow this format, thereby providing predictability to consumers of your APIs.

HTTP Method-Specific Validation

Sometimes the validation rules for a given URI may change based on which HTTP method is being used. As an example, during creation of a user, via POST, you may want to specify just a name and email. However, during a later operation to update a password via PATCH, you may be able to receive only the password. An operation that replaces all details of the user via PUT may need to validate each and every field representing the user.

The api-tools-content-validation module provides granularity beyond just mapping input filters to services; it also allows you to map input filters to specific HTTP methods for a given service. In the case of REST services, it also differentiates between collection and entity URIs, allowing an input filter for each HTTP method for each.

Summary

Zend Framework 2 provides the ability to short-circuit the request lifecycle at any point by returning a "response" object. API Tools leverages this fact by registering an event listener after content negotiation completes, but before the service itself executes, ensuring we intercept validation errors early.

Read the content validation chapter for more details.

Images in the documentation, and the API Tools Admin UI itself, still refer to Apigility. This is due to the fact that we only recently transitioned the project to its new home in the Laminas API Tools. Rest assured that the functionality remains the same.