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Vapor's Validation API helps you validate incoming request before using the Content API to decode data.


Vapor's deep integration of Swift's type-safe Codable protocol means you don't need to worry about data validation as much compared to dynamically typed languages. However, there are still a few reasons why you might want to opt-in to explicit validation using the Validation API.

Human-Readable Errors

Decoding structs using the Content API will yield errors if any of the data is not valid. However, these error messages can sometimes lack human-readability. For example, take the following string-backed enum:

enum Color: String, Codable {
    case red, blue, green

If a user tries to pass the string "purple" to a property of type Color, they will get an error similar to the following:

Cannot initialize Color from invalid String value purple for key favoriteColor

While this error is technically correct and successfully protected the endpoint from an invalid value, it could do better informing the user about the mistake and which options are available. By using the Validation API, you can generate errors like the following:

favoriteColor is not red, blue, or green

Furthermore, Codable will stop attempting to decode a type as soon as the first error is hit. This means that even if there are many invalid properties in the request, the user will only see the first error. The Validation API will report all validation failures in a single request.

Specific Validation

Codable handles type validation well, but sometimes you want more than that. For example, validating the contents of a string or validating the size of an integer. The Validation API has validators for helping to validate data like emails, character sets, integer ranges, and more.


To validate a request, you will need to generate a Validations collection. This is most commonly done by conforming an existing type to Validatable.

Let's take a look at how you could add validation to this simple POST /users endpoint. This guide assumes you are already familiar with the Content API.

enum Color: String, Codable {
    case red, blue, green

struct CreateUser: Content {
    var name: String
    var username: String
    var age: Int
    var email: String
    var favoriteColor: Color?
}"users") { req -> CreateUser in
    let user = try req.content.decode(CreateUser.self)
    // Do something with user.
    return user

Adding Validations

The first step is to conform the type you are decoding, in this case CreateUser, to Validatable. This can be done in an extension.

extension CreateUser: Validatable {
    static func validations(_ validations: inout Validations) {
        // Validations go here.

The static method validations(_:) will be called when CreateUser is validated. Any validations you want to perform should be added to the supplied Validations collection. Let's take a look at adding a simple validation to require that the user's email is valid.

validations.add("email", as: String.self, is: .email)

The first parameter is the value's expected key, in this case "email". This should match the property name on the type being validated. The second parameter, as, is the expected type, in this case String. The type usually matches the property's type, but not always. Finally, one or more validators can be added after the third parameter, is. In this case, we are adding a single validator that checks if the value is an email address.

Validating Request Content

Once you've conformed your type to Validatable, the static validate(content:) function can be used to validate request content. Add the following line before req.content.decode(CreateUser.self) in the route handler.

try CreateUser.validate(content: req)

Now, try sending the following request containing an invalid email:

POST /users HTTP/1.1
Content-Length: 67
Content-Type: application/json

    "age": 4,
    "email": "foo",
    "favoriteColor": "green",
    "name": "Foo",
    "username": "foo"

You should see the following error returned:

email is not a valid email address

Validating Request Query

Types conforming to Validatable also have validate(query:) which can be used to validate a request's query string. Add the following lines to the route handler.

try CreateUser.validate(query: req)

Now, try sending the following request containing an invalid email in the query string.

GET /users?age=4&email=foo&favoriteColor=green&name=Foo&username=foo HTTP/1.1

You should see the following error returned:

email is not a valid email address

Integer Validation

Great, now let's try adding a validation for age.

validations.add("age", as: Int.self, is: .range(13...))

The age validation requires that the age is greater than or equal to 13. If you try the same request from above, you should see a new error now:

age is less than minimum of 13, email is not a valid email address

String Validation

Next, let's add validations for name and username.

validations.add("name", as: String.self, is: !.empty)
validations.add("username", as: String.self, is: .count(3...) && .alphanumeric)

The name validation uses the ! operator to invert the .empty validation. This will require that the string is not empty.

The username validation combines two validators using &&. This will require that the string is at least 3 characters long and contains only alphanumeric characters.

Enum Validation

Finally, let's take a look at a slightly more advanced validation to check that the supplied favoriteColor is valid.

    "favoriteColor", as: String.self,
    is: .in("red", "blue", "green"),
    required: false

Since it's not possible to decode a Color from an invalid value, this validation uses String as the base type. It uses the .in validator to verify that the value is a valid option: red, blue, or green. Since this value is optional, required is set to false to signal that validation should not fail if this key is missing from the request data.

Note that while the favorite color validation will pass if the key is missing, it will not pass if null is supplied. If you want to support null, change the validation type to String? and use the .nil || (read as: "is nil or ...") convenience.

    "favoriteColor", as: String?.self,
    is: .nil || .in("red", "blue", "green"),
    required: false

Custom Errors

You might want to add custom human-readable errors to your Validations or Validator. To do so simply provide the additional customFailureDescription parameter which will override the default error.

    as: String.self,
    is: !.empty,
    customFailureDescription: "Provided name is empty!"
    as: String.self,
    is: .count(3...) && .alphanumeric,
    customFailureDescription: "Provided username is invalid!"


Below is a list of the currently supported validators and a brief explanation of what they do.

Validation Description
.ascii Contains only ASCII characters.
.alphanumeric Contains only alphanumeric characters.
.characterSet(_:) Contains only characters from supplied CharacterSet.
.count(_:) Collection's count is within supplied bounds.
.email Contains a valid email.
.empty Collection is empty.
.in(_:) Value is in supplied Collection.
.nil Value is null.
.range(_:) Value is within supplied Range.
.url Contains a valid URL.

Validators can also be combined to build complex validations using operators.

Operator Position Description
! prefix Inverts a validator, requiring the opposite.
&& infix Combines two validators, requires both.
|| infix Combines two validators, requires one.

Custom Validators

Creating a custom validator for zip codes allows you to extend the functionality of the validation framework. In this section, we'll walk you through the steps to create a custom validator for validating zip codes.

First create a new type to represent the ZipCode validation results. This struct will be responsible for reporting whether a given string is a valid zip code.

extension ValidatorResults {
    /// Represents the result of a validator that checks if a string is a valid zip code.
    public struct ZipCode {
        /// Indicates whether the input is a valid zip code.
        public let isValidZipCode: Bool

Next, conform the new type to ValidatorResult, which defines the behavior expected from a custom validator.

extension ValidatorResults.ZipCode: ValidatorResult {
    public var isFailure: Bool {

    public var successDescription: String? {
        "is a valid zip code"

    public var failureDescription: String? {
        "is not a valid zip code"

Finally, implement the validation logic for zip codes. Use a regular expression to check whether the input string matches the format of a USA zip code.

private let zipCodeRegex: String = "^\\d{5}(?:[-\\s]\\d{4})?$"

extension Validator where T == String {
    /// Validates whether a `String` is a valid zip code.
    public static var zipCode: Validator<T> {
        .init { input in
            guard let range = input.range(of: zipCodeRegex, options: [.regularExpression]),
                  range.lowerBound == input.startIndex && range.upperBound == input.endIndex
            else {
                return ValidatorResults.ZipCode(isValidZipCode: false)
            return ValidatorResults.ZipCode(isValidZipCode: true)

Now that you've defined the custom zipCode validator, you can use it to validate zip codes in your application. Simply add the following line to your validation code:

validations.add("zipCode", as: String.self, is: .zipCode)