This document describes the features which hook into Micro. Micro acts as a “service loader”. Each individual feature provides new functionality to Micro. Many features of Micro are loaded by default. You can install additional features in the initializeServer function of your Service (Main.kt):



  • Default: Yes

  • Exported services:

    • Micro.backgroundScope: BackgroundScope

    • Micro.ioDispatcher: CoroutineDispatcher

Provides a BackgroundScope. This is a coroutine scope which provides a bigger thread pool, more suitable for the occasional blocking task. This includes start and shutdown hooks for UCloud.


  • Default: Yes

  • Exported services:

    • Micro.client: RpcClient

Provides an RpcClient to the micro-services. This client will be used for all RPCs to other services.

Example: Configuring the client

      host: dev.cloud.sdu.dk
      scheme: https
      port: 443
    http: true # Is HTTP enabled, default = true
    websockets: true # Is WebSockets enabled, default = true


  • Default: Yes

  • Exported services:

    • Micro.configuration: ServerConfiguration

  • Command line flags:

    • --config-dir <DIRECTORY>

    • --config <FILE>

Provides a configuration mechiansm for micro-services to use. The configuration feature will read directories or files which are formatted as either YAML or JSON.

In addition to the directories and files provided via the command line flags, this feature will read the following directories:

  • ~/sducloud (only if development mode is active)

The files which are read are merged into one big document. Micro-services can read parts of this document using the following methods:

  • fun ServerConfiguration.requestChunk<T>(node: String): T

  • fun ServerConfiguration.requestChunkAt<T>(vararg path: String): T

  • fun ServerConfiguration.requestChunkOrNull<T>(node: String): T?

  • fun ServerConfiguration.requestChunkAtOrNull<T>(vararg path: String): T?

Example: Reading a configuration file and passing it to Server.kt

override fun initializeServer(micro: Micro): CommonServer {
    val folder = micro.configuration.requestChunkAtOrNull("ceph") ?: CephConfiguration()
    val config = micro.configuration.requestChunkAtOrNull("storage") ?: StorageConfiguration()

    return Server(config, folder, micro)


  • Default: Yes

  • Exported services:

    • Micro.jdbcUrl: String

    • Micro.databaseConfig: DatabaseConfig

Provides and reads database configuration. The database configuration is read using the ConfigurationFeature at "database". The configuration uses the following format:

data class Config(
    val profile: Profile = Profile.PERSISTENT_POSTGRES,

    // Will automatically attempt to use the first valid hostname from ["postgres", "localhost"]
    val hostname: String? = null,

    val credentials: Credentials? = null,

    // Defaults to "postgres"
    val database: String? = null,

    // Defaults to 5432
    val port: Int? = null,

enum class Profile {

data class Credentials(val username: String, val password: String)

Example: Creating a database connection

val db = AsyncDBSessionFactory(micro)


  • Default: Yes

  • Exported services: None

Provides a feature to run shutdown handlers.

Example: Run a shutdown handler

micro.feature(DeinitFeature).addHandler {
    // Run my code


  • Default: Yes

  • Exported services:

    • Micro.developmentModeEnabled: Boolean

  • Command line arguments:

    • --dev

Provides a flag for development mode. Development mode can be enabled by passing the --dev command line flag. Provides a way to hardcode the location of certain services. These overrides will change both the port that the server runs on and it will be used by the client to use the correct service.

Note: This system has for the most part been superseded by the launcher module.

Example: Providing service overrides for some services

    avatar: localhost:4201
    project: localhost:4202


  • Default: No

  • Exported services:

    • Micro.elasticHighLevelClient: RestHighLevelClient

    • Micro.elasticLowLevelClient: RestClient

Provides a standardized way to retrieve a elasticsearch client.

Example: Configuration for ElasticSearch

    hostname: host
    port: 9200
      username: usernamegoeshere
      password: passwordgoeshere


  • Default: Yes

  • Exported services: None

  • Command line arguments

    • --run-script migrate-db

Provides a script handler to run database migrations, powered by Flyway. Migrations are stored in the classpath at db/migration and are simple SQL scripts. The migration scripts must follow the following convention: V${index}__${scriptName}.sql. index is 1-indexed and must be sequential.

Example: A simple migration script

-- Must be stored in example-service/src/main/resources/db/migration/V1__Initial.sql
create table foobar(
    a int primary key,
    b int


  • Default: Yes

  • Exported services: None

Provides a health-check endpoint at GET /status. This endpoint will return 200 OK if all the internal services provided by Micro are working as intended.

The following services are currently checked:

  • Redis

  • ElasticSearch

  • Ktor (webserver)

KtorServerProviderFeature and ServerFeature

  • Default: Yes

  • Exported services:

    • Micro.serverProvider: HttpServerProvider

    • Micro.server: RpcServer

Provides a webserver used by both the HTTP and WebSocket backends. As the name implies, the web server is provided by Ktor.

The ktor engine to use is provided by KtorServerProviderFeature while ServerFeature uses this server to initialize and start the server. This will also install middleware to provide additional features (e.g. auditing).


  • Default: Yes

  • Exported services: None

  • Command line arguments:

    • --dev (turns on global debug logs)

    • --debug (turns on global debug logs but without development mode)

Provides a default configuration for the logging framework used in UCloud. The default configuration will check if the server is running in development mode. If the server is in development mode then the default log level will be DEBUG otherwise INFO will be used. Logs are written using Log4j 2 and are written to stdout.

The logging levels can be changed programmatically, see example below.

Example: Change log levels programmatically

    // Enables trace level debugging for the dk.sdu.cloud.avatar package
        "dk.sdu.cloud.avatar" to Level.TRACE

Example: Obtaining a logging instance

class MyService {
    // Service code goes here
    fun myServiceFunction() {
        log.info("Performing some work")
    companion object : Loggable {
        override val log = logger()

Redis Feature

  • Default: Yes

  • Exported services:

    • Micro.redisConnectionManager: RedisConnectionManager

    • Micro.eventStreamService: RedisStreamService

Provides configuration needed for event streams and distributed locks.

Example: Configuration for Redis

  hostname: localhost # defaults to first valid hostname in ["redis", "localhost"]
  port: 6379 # defaults to 6379

Example: Broadcasting stream

val broadcastingStream = RedisBroadcastingStream(micro.redisConnectionManager)
broadcastingStream.broadcast(MyMessage(42), MyStreams.stream)

Example: Producing a message

val eventProducer = micro.eventStreamService.createProducer(ProjectEvents.events)

Example: Creating a distributed lock

val distributedLocks = DistributedLockBestEffortFactory(micro)
val lock = distributedLockFactory.create("metadata-recovery-lock", duration = 60_000)
while (true) {
    val didAcquire = lock.acquire()
    if (didAcquire) {
        processing@while (true) {
            // Do work here
            if (!lock.renew(60_000)) {
                log.info("We lost the lock!")
    // Introduce randomness to make it more likely that clients don't try simultaneously
    delay(15000 + Random.nextLong(5000))


  • Default: Yes

  • Exported services:

    • Micro.scriptsToRun: List<String>

    • fun Micro.optionallyAddScriptHandler(scriptName: String, handler: ScriptHandler)

    • fun Micro.runScriptHandler()

  • Command line arguments:

    • --run-script <SCRIPTNAME>

Provides a way to run ad-hoc scripts at the Micro feature level. This is, for example, utilized by the FlywayFeature. This feature is mostly intended to be used by other Micro features. The scripts are run before before the server is started but after initializeServer.

Example: Add a script handler and terminate UCloud

micro.optionallyAddScriptHandler("my-script") {
    // Code goes here

Example: Add a script handler and continue launching UCloud

micro.optionallyAddScriptHandler("my-script") {
    // Code goes here


  • Default: Yes

  • Exported services: None

Provides a way to provide service overrides programmatically. This is similar to the code used in DevelopmentOverrides.


  • Default: Yes

  • Exported services:

    • Micro.serviceInstance: ServiceInstance

Provides a ServiceInstance to the micro-service. This contains information valuable for auditing.

data class ServiceInstance(
    val definition: ServiceDefinition,
    val hostname: String,
    val port: Int,
    val ipAddress: String? = null

data class ServiceDefinition(
    val name: String, 
    val version: String


  • Default: Yes

  • Exported services:

    • Micro.tokenValidation: TokenValidation<Any>

Provides a way for services to validate JSON Web Tokens (JWTs). The feature supports both public certificate signing (RSA256) and shared secret signing (HMAC512). The details are described here.

Example: Shared secret configuration

    sharedSecret: notverysecret

AuthenticatorFeature (formerly RefreshingJWTCloudFeature)

  • Default: No

  • Exported services:

    • Micro.authenticator: RefreshingJWTAuthenticator

Provides a way to use automatically renew accessTokens from a refreshToken. The RefreshingJWTAuthenticator can authenticate individual calls as well as a complete client (returning an AuthenticatedClient). It will read a refreshToken from configuration for the service itself. This will be used to make calls on behalf of the server.

It is also possible to create a RefreshingJWTAuthenticator with a different refreshToken (see below). This is useful if the service has extended a user’s token for some purpose. User tokens are extended when a service needs to act on the user’s behalf on a later point in time.

Example: Configuration file

refreshToken: my-refresh-token-goes-here

Example: Creating an authenticated client

val serviceClientHttp = micro.authenticator.authenticateClient(OutgoingHttpCall)
val serviceClientWebSockets = micro.authenticator.authenticateClient(OutgoingWSCall)

Example: Creating an AuthenticatedClient from an extended refreshToken

val clientFromToken = RefreshingJWTAuthenticator(
    micro.tokenValidation as TokenValidationJWT