Client Credentials Flow with Micronaut and Auth0

Learn how to use Client Credentials Flow between Micronaut microservices with an Authorization Server provided by Auth0.

Authors: Sergio del Amo

Micronaut Version: 4.6.3

1. Getting Started

In this guide, we will create a Micronaut application written in Kotlin.

2. What you will need

To complete this guide, you will need the following:

Some time on your hands
A decent text editor or IDE (e.g. IntelliJ IDEA)
JDK 21 or greater installed with JAVA_HOME configured appropriately
An Auth0 account.

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

Download and unzip the source

4. Application Diagram

Download the complete solution of the Consul and Micronaut Framework - Microservices Service Discovery guide. You will use the sample app as a starting point. The application contains three microservices:

bookcatalogue - This returns a list of books. It uses a domain consisting of a book name and an ISBN.
bookinventory - This exposes an endpoint to check whether a book has sufficient stock to fulfil an order. It uses a domain consisting of a stock level and an ISBN.
bookrecommendation - This consumes previous services and exposes an endpoint that recommends book names that are in stock.

The bookcatalogue service consumes endpoints exposed by the other services. The following image illustrates the original application flow:

A request to bookrecommendation (http://localhost:8080/books) triggers several requests through our microservices mesh.

In this guide, you are going to secure the communication between the microservices. You will use a client credentials flow and obtain an access token from an Auth0 authorization server.

5. OAuth 2.0

To provide authentication, sign in to your Auth0 account.

5.1. Create an application

5.2. Obtain client id and client secret

You can obtain the application’s domain, client id, and secret in the Auth0 console.

5.3. Obtain API audience

Go to Applications → APIs and copy the API Audience:

5.4. Authorize application

In the API Settings, authorize the application in the Machine to Machine Applications tab:

6. Writing the application

Setup IntelliJ IDEA to develop Micronaut Applications.

6.1. Dependencies

Modify every application (bookinventory, bookinventory, and bookrecommendation). Add Micronaut JWT and Micronaut OAuth 2.0 dependencies:

build.gradle

implementation("io.micronaut.security:micronaut-security-oauth2")
implementation("io.micronaut.security:micronaut-security-jwt")

6.2. Changes to Book Inventory service

Annotate the Controller’s method with @Secured:

bookinventory/src/main/kotlin/example/micronaut/BooksController.kt

package example.micronaut

import io.micronaut.http.MediaType.TEXT_PLAIN
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Produces
import java.util.Optional
import jakarta.validation.constraints.NotBlank
import io.micronaut.security.rules.SecurityRule.IS_AUTHENTICATED
import io.micronaut.security.annotation.Secured

@Controller("/books")
open class BooksController {

    @Produces(TEXT_PLAIN)
    @Get("/stock/{isbn}")
    @Secured(IS_AUTHENTICATED) (1)
    open fun stock(@NotBlank isbn: String): Boolean? =
        bookInventoryByIsbn(isbn).map { (_, stock) -> stock > 0 }.orElse(null)

    private fun bookInventoryByIsbn(isbn: String): Optional<BookInventory> {
        if (isbn == "1491950358") {
            return Optional.of(BookInventory(isbn, 4))
        }
        if (isbn == "1680502395") {
            return Optional.of(BookInventory(isbn, 0))
        }
        return Optional.empty()
    }
}

1	Annotate with `io.micronaut.security.Secured` to configure secured access. The `SecurityRule.IS_AUTHENTICATED` expression allows only access to authenticated users.

To validate the tokens issued by Auth0, configure Validation with Remote JKWS:

Since Micronaut Framework 4.0, to use YAML configuration, you have to add the YAML dependency.

bookinventory/src/main/resources/application.yml

micronaut:
  security:
    token:
      jwt:
        signatures:
          jwks:
            auth0:
              url: '${OAUTH_JWKS:`https://micronautguides.eu.auth0.com/.well-known/jwks.json`}'

You can obtain the JWKS URL in the .well-known/openid-configuration endpoint.

6.3. Changes to Book Catalogue service

Annotate the Controller’s method with @Secured:

bookcatalogue/src/main/kotlin/example/micronaut/BooksController.kt

package example.micronaut

import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.security.rules.SecurityRule.IS_AUTHENTICATED
import io.micronaut.security.annotation.Secured

@Controller("/books")
class BooksController {

    @Get
    @Secured(IS_AUTHENTICATED) (1)
    fun index(): List<Book> = listOf(
            Book("1491950358", "Building Microservices"),
            Book("1680502395", "Release It!"),
            Book("0321601912", "Continuous Delivery:"))
}

1	Annotate with `io.micronaut.security.Secured` to configure secured access. The `SecurityRule.IS_AUTHENTICATED` expression allows only access to authenticated users.

To validate the tokens issued by Auth0, configure Validation with Remote JKWS:

bookcatalogue/src/main/resources/application.yml

micronaut:
  security:
    token:
      jwt:
        signatures:
          jwks:
            auth0:
              url: '${OAUTH_JWKS:`https://micronautguides.eu.auth0.com/.well-known/jwks.json`}'

You can obtain the JWKS URL in the .well-known/openid-configuration endpoint.

6.4. Changes to Book Recommendations service

6.4.1. Books Controller Security

The GET /books in the booksrecommendation service is open.

Annotate the Controller’s method with @Secured:

bookrecommendation/src/main/kotlin/example/micronaut/BookController.kt

package example.micronaut

import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import org.reactivestreams.Publisher
import reactor.core.publisher.Flux
import io.micronaut.security.rules.SecurityRule.IS_ANONYMOUS
import io.micronaut.security.annotation.Secured

@Controller("/books")
class BookController(
        private val bookCatalogueOperations: BookCatalogueOperations,
        private val bookInventoryOperations: BookInventoryOperations) {

    @Get
    @Secured(IS_ANONYMOUS) (1)
    fun index(): Publisher<BookRecommendation> =

        Flux.from(bookCatalogueOperations.findAll())
                .flatMap { b ->
                    Flux.from(bookInventoryOperations.stock(b.isbn))
                            .filter { hasStock -> hasStock }
                            .map { b }
                }.map { (_, name) -> BookRecommendation(name) }
}

1	Annotate with `io.micronaut.security.Secured` to configure secured access. The `SecurityRule.IS_ANONYMOUS` expression will allow access without authentication.

6.4.2. Configuration of HTTP Services URLs

Modify application-dev.yml to point the declarative HTTP clients to the other microservices URLs.

bookrecommendation/src/main/resources/application-dev.yml

micronaut:
  http:
    services:
      bookcatalogue:
        url: 'http://localhost:8081'
      bookinventory:
        url: 'http://localhost:8082'

6.5. Configuration

Add the following OAuth2 configuration:

bookrecommendation/src/main/resources/application.yml

micronaut:
  security:
    oauth2:
      clients:
        auth0: (1)
          client-id: '${OAUTH_CLIENT_ID:xxx}' (2)
          client-secret: '${OAUTH_CLIENT_SECRET:yyy}' (3)
          grant-type: 'client_credentials' (4)
          token:
            url: '${OAUTH_TOKEN_URL:`https://micronautguides.eu.auth0.com/oauth/token`}' (5)
            auth-method: 'client_secret_post' (6)
          client-credentials:
            service-id-regex: 'bookcatalogue|bookinventory' (7)
            additional-request-params:
              audience: '${AUTH0_API_IDENTIFIER:`https://micronautguides.eu.auth0.com/api/v2/`}' (8)

1	OAuth 2.0 client name.
2	Client id. See previous screenshot.
3	Client secret. See previous screenshot.
4	Specify GrantType#CLIENT_CREDENTIALS `client-credentials` as grant type for this OAuth 2.0 client.
5	Specify the token endpoint URL. You can obtain the token endpoint URL in the `.well-known/openid-configuration`.
6	Specify AuthenticationMethod#CLIENT_SECRET_POST as the authentication method. This means the client id and client secret are specified in the body of the HTTP request sent to the token endpoint.
7	Propagate the access token obtained from Auth0 to requests sent to the services `bookinventory` and `bookcatalogue`. This uses the Micronaut Client Credentials HTTP Client Filter.
8	Auth0 requires the API Identifier with an audience key in the token endpoint request for the client credentials flow.

The previous configuration uses several placeholders with default values. You will need to set up OAUTH_CLIENT_ID, OAUTH_CLIENT_SECRET, and OAUTH_TOKEN_URL environment variables in your Auth0 application.

export OAUTH_CLIENT_ID=XXXXXXXXXX
export OAUTH_CLIENT_SECRET=YYYYYYYYYY
export OAUTH_TOKEN_URL=https://micronautguides.eu.auth0.com/oauth/token

7. Running the Application

7.1. Run `bookcatalogue` microservice

To run the application, execute ./gradlew run.

...
14:28:34.034 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 499ms. Server Running: http://localhost:8081

7.2. Run `bookinventory` microservice

To run the application, execute ./gradlew run.

...
14:31:13.104 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 506ms. Server Running: http://localhost:8082

7.3. Run `bookrecommendation` microservice

To run the application, execute ./gradlew run.

...
14:31:57.389 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 523ms. Server Running: http://localhost:8080

You can run a cURL command to test the whole application:

curl http://localhost:8080/books

[{"name":"Building Microservices"}]

8. Generate a Micronaut Application Native Executable with GraalVM

We will use GraalVM, an advanced JDK with ahead-of-time Native Image compilation, to generate a native executable of this Micronaut application.

Compiling Micronaut applications ahead of time with GraalVM significantly improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

8.1. GraalVM Installation

The easiest way to install GraalVM on Linux or Mac is to use SDKMan.io.

Java 21

sdk install java 21.0.5-graal

For installation on Windows, or for a manual installation on Linux or Mac, see the GraalVM Getting Started documentation.

The previous command installs Oracle GraalVM, which is free to use in production and free to redistribute, at no cost, under the GraalVM Free Terms and Conditions.

Alternatively, you can use the GraalVM Community Edition:

Java 21

sdk install java 21.0.2-graalce

8.2. Native Executable Generation

To generate a native executable using Gradle, run:

./gradlew nativeCompile

The native executable is created in build/native/nativeCompile directory and can be run with build/native/nativeCompile/micronautguide.

It is possible to customize the name of the native executable or pass additional parameters to GraalVM:

build.gradle

graalvmNative {
    binaries {
        main {
            imageName.set('mn-graalvm-application') (1)
            buildArgs.add('-Ob') (2)
        }
    }
}

1	The native executable name will now be `mn-graalvm-application`
2	It is possible to pass extra build arguments to `native-image`. For example, `-Ob` enables the quick build mode.

Run the native executables and execute a cURL command to test the whole application:

curl http://localhost:8080/books

[{"name":"Building Microservices"}]

9. Next Steps

Read Micronaut OAuth 2.0 Documentation to learn more.

10. Help with the Micronaut Framework

The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.

11. License

All guides are released with an Apache license 2.0 license for the code and a Creative Commons Attribution 4.0 license for the writing and media (images…).

Table of Contents

Client Credentials Flow with Micronaut and Auth0

1. Getting Started

2. What you will need

3. Solution

4. Application Diagram

5. OAuth 2.0

5.1. Create an application

5.2. Obtain client id and client secret

5.3. Obtain API audience

5.4. Authorize application

6. Writing the application

6.1. Dependencies

6.2. Changes to Book Inventory service

6.3. Changes to Book Catalogue service

6.4. Changes to Book Recommendations service

6.4.1. Books Controller Security

6.4.2. Configuration of HTTP Services URLs

6.5. Configuration

7. Running the Application

7.1. Run bookcatalogue microservice

7.2. Run bookinventory microservice

7.3. Run bookrecommendation microservice

8. Generate a Micronaut Application Native Executable with GraalVM

8.1. GraalVM Installation

8.2. Native Executable Generation

9. Next Steps

10. Help with the Micronaut Framework

11. License

7.1. Run `bookcatalogue` microservice

7.2. Run `bookinventory` microservice

7.3. Run `bookrecommendation` microservice