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Server-side validation with Ktor

This tutorial provides a sampling of how Akkurate helps you write server-side validation with Ktor. We're going to create an HTTP API to manage the books contained within a library; its role is to ensure each book has a valid and unique ISBN, as well as a valid title.

Setting up the project

You can download a generated Ktor project by following this link. Then click Add plugins | Generate project and, once the project is downloaded, open it in IntelliJ.

Defining and persisting the data model

A book is composed of an ISBN and a title. ISBN stands for International Standard Book Number, a unique identifier assigned to each book.

To represent the book in the application, we will use the following data class:

@Serializable data class Book( val isbn: String = "", val title: String = "", )

The @Serializable annotation is necessary because this class will be used to transmit JSON data over HTTP.

To store the Book class, we will use Exposed and follow what's recommended in Database persistence with Exposed; allowing us to easily query our database and keep this tutorial as simple as possible.

First, we need to define our database schema to store all the properties of our books:

object Books : Table() { val isbn = char("isbn", length = 13) val title = varchar("title", length = 50) override val primaryKey = PrimaryKey(isbn) }

We're making the isbn the primary key, since it's already a unique identifier. It's also a char(13) type instead of a varchar because an ISBN is always composed of 13 characters.

Next, we have to create our DAO, which will provide two essential methods:

  • create() to store a new book in the database,

  • list() to retrieve all the books stored in our database.

class BookDao(database: Database) { init { transaction(database) { SchemaUtils.create(Books) } } private suspend fun <T> dbQuery(block: suspend () -> T): T = newSuspendedTransaction(Dispatchers.IO) { block() } private fun ResultRow.toBook() = Book( isbn = this[Books.isbn], title = this[Books.title], ) suspend fun create(book: Book): Unit = dbQuery { Books.insert { it[isbn] = book.isbn it[title] = book.title } } suspend fun list(): List<Book> = dbQuery { Books.selectAll().map { it.toBook() } } }

Finally, we need to instantiate our DAO with a database connection when the application starts up. Open the Databases.kt file, create a top level variable lateinit var bookDao: BookDao, and define it inside the configureDatabases function:

lateinit var bookDao: BookDao fun Application.configureDatabases() { val database = Database.connect(/* ... */) bookDao = BookDao(database) }

Handling the requests

We will need two routes for our HTTP API; POST /books to register a new book to the database, and GET /books to list all the books in the database.

Open the Routing.kt file and copy the following code in the configureRouting function:

routing { post("/books") { val book = call.receive<Book>() bookDao.create(book) call.respond(HttpStatusCode.Created) } get("/books") { val books = bookDao.list() call.respond(HttpStatusCode.OK, books) } }

The POST /books route deserializes the payload, stores it in the database, and returns a 201 HTTP status code. The GET /books route fetches all the books and serializes them into the response.

What can we improve?

Our API is done, you can run it either with IntelliJ or with the ./gradlew run command.

Create a new book with isbn=123 and title being empty:

curl -v \ --data '{"isbn": "123", "title": ""}' \ --header 'Content-Type: application/json'
HTTP/1.1 201 Created

Now list the books:

curl -v
HTTP/1.1 200 OK [ { "isbn": "123 ", "title": "" } ]

We can see two issues in this response; the ISBN is now filled up to the 13 required characters, and we shouldn't allow empty titles.

Try to run the first query again:

curl -v \ --data '{"isbn": "123", "title": ""}' \ --header 'Content-Type: application/json'
HTTP/1.1 500 Internal Server Error

Now we have an internal error because we're trying to insert a second book with the same ISBN as the first one. This is impossible due to the ISBN being the primary key of the table.

Finally, try to create a book with a title over 50 characters:

curl -v \ --data '{"isbn": "1234", "title": "this a really long title and it will not fit our database column"}' \ --header 'Content-Type: application/json'
HTTP/1.1 500 Internal Server Error

Once again, we see another internal error, because our title is composed of 64 characters meanwhile our database column can contain a maximum of 50 characters.

Validating the requests

All these issues can be fixed by validating the requests. We will use Akkurate coupled to Ktor request validation.

Enhancing the DAO

Before writing any validation code, we need to add the following method to our BookDao class to allow searching a book by its ISBN:

suspend fun existsWithIsbn(isbn: String): Boolean = dbQuery { Books.select { Books.isbn eq isbn }.singleOrNull() != null }

That way, we can check if a book exists by running bookDao.existsWithIsbn(isbn).

Writing validation constraints

First, we must install Akkurate dependencies:

Install in a single-platform project

  1. Add KSP to your plugin list; make sure to use the appropriate version, depending on the Kotlin version you're using.

    plugins { kotlin("jvm") version "1.9.10" id("com.google.devtools.ksp") version "1.9.10-1.0.13" }
  2. Add the dependencies and register the compiler plugin through KSP.

    dependencies { implementation("dev.nesk.akkurate:akkurate-core:0.7.0") implementation("dev.nesk.akkurate:akkurate-ksp-plugin:0.7.0") ksp("dev.nesk.akkurate:akkurate-ksp-plugin:0.7.0") }

Then mark the Book class with the @Validate annotation:

@Validate @Serializable data class Book(/* ... */)

Just like in the Getting Started guide, we create a Validator instance and add our constraints to it:

val validateBook = Validator.suspendable<BookDao, Book> { dao -> isbn { val (isValidIsbn) = isMatching(Regex("""\d{13}""")) otherwise { "Must be a valid ISBN (13 digits)" } if (isValidIsbn) { constrain { !dao.existsWithIsbn(it) } otherwise { "This ISBN is already registered" } } } title { isNotBlank() hasLengthLowerThanOrEqualTo(50) } }

There are multiple things to explain here:

  • We use a suspendable validator with a context. Those allow our validator to call the BookDao.existsWithIsbn method, to ensure a book isn't already registered in our database.

  • The call to existsWithIsbn is done within an inline constraint and only if the ISBN is valid, to avoid a useless query to the database.

  • We ensure the title is not blank but also that it isn't longer than 50 characters, otherwise the database will reject it with an exception.

Wiring to Ktor validation

Akkurate runs the validation and returns a result, but it needs to provide the latter to Ktor in order to generate a response. This requires the Request Validation plugin:


This plugin allows configuring a validation function for a specific class; it will be executed on each deserialization. A validation result must be returned, we can generate it from Akkurate's own result:

import dev.nesk.akkurate.ValidationResult.Failure as AkkurateFailure import dev.nesk.akkurate.ValidationResult.Success as AkkurateSuccess fun Application.configureValidation() { install(RequestValidation) { validate<Book> { book -> when (val result = validateBook(bookDao, book)) { is AkkurateSuccess -> ValidationResult.Valid is AkkurateFailure -> { val reasons = result.violations.map { "${it.path.joinToString(".")}: ${it.message}" } ValidationResult.Invalid(reasons) } } } } }

Notice how we execute our validateBook function with the provided book, then we map the result to Ktor's result.

We also have to call our configureValidation function on application start, this is done in the Application.kt file:

fun Application.module() { configureSerialization() configureDatabases() configureRouting() configureValidation() // Register the validation handlers }

When the validation fails, the plugin throws a RequestValidationException. To handle this exception and return a proper response, we use the Status Pages plugin.

Open the Routing.kt file, navigate to the configureRouting function, then the install(StatusPages) {} lambda, and add the following code:

exception<RequestValidationException> { call, cause -> call.respond(HttpStatusCode.UnprocessableEntity, cause.reasons) }

When the RequestValidationException is thrown, the Status Page plugin catches it and returns a response with a 422 HTTP status code, along with a JSON array of validation messages.

Conformance checking

It's time to test our API once again.

Create a new book with invalid values:

curl -v \ --data '{"isbn": "123", "title": ""}' \ --header 'Content-Type: application/json'
HTTP/1.1 422 Unprocessable Entity [ "isbn: Must be a valid ISBN (13 digits)", "title: Must not be blank" ]

This time, our validation took care of rejecting the request.

Now try to create a book with valid values:

curl -v \ --data '{"isbn": "1234567891234", "title": "The Lord of the Rings"}' \ --header 'Content-Type: application/json'
HTTP/1.1 201 Created

The request is considered valid and we received a 201 HTTP status code.

What if we try to create the same book a second time?

HTTP/1.1 422 Unprocessable Entity [ "isbn: This ISBN is already registered" ]

As expected, we can't register the same ISBN twice.

Our API is now fully validated, which means security is improved, and the users can understand why the request failed.

Last modified: 27 February 2024