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Knot.x Launcher

Knot.x Launcher provides a way to configure and run bare Knot.x instance. It contains:

  • the run-knotx starting command that runs a Vert.x instance and deploys all configured modules
  • instance configuration files
  • logger configuration files
  • distribution that contains all above and minimal set of dependencies (jar files in the lib directory) required to start the instance

It is used in the Knot.x Stack to deliver fully functional bootstrap project for Knot.x-based projects.

Launcher has two main classes are:

  • io.knotx.launcher.KnotxCommand - allows to run Knot.x instance with run-knotx command
  • io.knotx.launcher.KnotxStarterVerticle - deploys all Verticles defined in configuration files

How to run

Knot.x Launcher can be used as a JAR dependency in more complex distributions (like the Knot.x Stack) or started as a standalone Vert.x instance from the ZIP distribution.

To build the Launcher distribution:

$> ./gradlew

To start Knot.x instance:

$> cd build/dist
$> unzip -d launcher
$> cd launcher
$> ./bin/start run-knotx
13:56:15.890 [vert.x-eventloop-thread-0] INFO io.knotx.launcher.KnotxStarterVerticle - Knot.x STARTED
13:56:15.894 [vert.x-eventloop-thread-1] INFO io.vertx.core.impl.launcher.commands.VertxIsolatedDeployer - Succeeded in deploying verticle

The run-knotx command starts Vert.x instance and deploys all configured modules. This command is registered in the io.vertx.core.Launcher class that is the main class of Vert.x executable jar. Additionally it uses concepts of configuration stores to load configuration files from different locations and with different formats.

It accepts options:

  • config - allows to define the bootstrap configuration file path, if not provided, it looks for the bootstrap.json file in the classpath
  • ha - specify that the Knot.x is deployed with High Availability (HA) enabled, check Vert.x Automatic failover section
  • hagroup - HA group that denotes a logical group of nodes in the cluster, check Vert.x HA Groups section
  • quora - it is the minimum number of votes that a distributed transaction has to obtain in order to be allowed to perform an operation in a distributed system
  • cluster - enables clustering

Knot.x provides the example project that contains Docker Compose configuration file to demonstrate cluster and HA capabilities.

Please note that if you want to start Launcher with cluster option you need to provide a required cluster manager dependency and configuration files in the classpath.

How to configure

The Knot.x configuration is basically split into two configuration files:

  • bootstrap.json - a starter configuration that defines the application configuration format, location of application configurations (e.g. file, directory), as well as the ability to define whether the config should be scanned periodically (configs change discovery)
  • application.conf - a main configuration file for the Knot.x based application. Knot.x promotes a HOCON format as it provides useful mechanism, such as includes, variable substitutions, etc. but other format may be used as well.

The configuration is resolved in the following steps:

  • If not specified in command line (-conf /path/to/bootstrap.json), Knot.x looks for the bootstrap.json file in the classpath
  • The definitions of config stores are read from the bootstrap.json file and configuration files locations are loaded and validated
  • E.g. if application.conf was specified, Knot.x loads it from the defined location, parses it, resolve any includes and starts all the Verticles defined in there.
  • In case of any missing files, Knot.x stops with the proper error message.

Configuration stores

The bootstrap.json file is structured around:

  • a Configuration Retriever based on Vert.x Config module that configures a set of configuration stores and ability to scan the changes
  • a Configuration store that defines a location from where the configuration data is read and a syntax

The structure of the file defines Vert.x ConfigRetrieverOptions:

  "configRetrieverOptions": {
    "scanPeriod": 5000,
    "stores": [
        "type": "file",
        "format": "conf",
        "config": {
          "path": "conf/application.conf"
        "optional": false
  • scanPeriod in milliseconds. If property is specified, Launcher scans the defined configuration stores and redeploys the Knot.x application on changes.
  • stores it's an array of Vert.x ConfigStoreOptions:
    • type a declared data store, such as File(file), JSON(json), Environment Variables(env), System Properties(sys), HTTP endpoint(http), Event Bus (event-bus), Directory(dir), Git (git), Kubernetes Config Map(configmap), Redis(redis), Zookeeper (zookeeper), Consul (consul), Spring Config (spring-config-server), Vault (vault)
    • format a format of the configuration file, such as JSON(json), HOCON(conf) and YAML(yaml)
    • config
      • path - a relative or absolute path to the Knot.x modules configuration file, if not specified then it get the knotx.home system property and append it with conf
    • optional - whether or not the store is considered as optional. When the configuration is retrieve, if an optional store returns a failure, the failure is ignored and an empty json object is used instead (for this store). The default value is false.

In addition to the out of the box config stores and formats it's easy to provide your own custom implementation thanks to the Vert.x Config SPI.

See the Vert.x Config for details how to use and configure any type of the store.

Modules configuration

The application.conf configuration file used in Knot.x distribution supports the HOCON format. In short, the HOCON is the human optimized JSON that keeps the semantics (tree structure; set of types; encoding/escaping) from JSON, but make it more convenient as a human-editable config file format. Notable differences from JSON are comments, variables and includes.

The structure of the file is composed on the following sections:

  • modules - a map of Verticles to start
  • config - actual configuration for the given Verticle.
########### Modules to start ###########
modules {
  # alias = verticle class name
  myserver = "io.knotx.server.KnotxServerVerticle"
  # Other modules to start

########### Modules configurations ###########
config.myserver {
  options.config {
    include required("includes/server.conf")

## More configs below

The config section for each module is expected to have following structure: MODULE_ALIAS.options.config, where:

  • MODULE_ALIAS is exactly the same alias that was used in the modules JSON object (myserver for the example above),
  • options object carries-on configuration called DeploymentOptions for a given verticle. It allows you to control the verticle behaviour, such as how many instances, classpath isolation, workers, etc. Read more here:,
    • options.config contain additional config for the module that is used to create its instance that is passed as module DeploymentOptions.config.
    • options.required - Some modules are crucial for Knot.x instance (e.g. HTTP Server) and the instance should not start without them. Required (required=true) modules will fail starting the whole Knot.x instance if they fail to deploy, while not-required (required=false) let the the instance start despite the fact they failed to start. By default all modules are required.

The config section can be defined in the form that works best for you, e.g. It can be just raw JSON, or HOCONized version of it as follows:

config {
   myserver {
      options {
         config {
            # verticle configuration

Or define it in the form of path in JSON as below

config.myserver.options.config {
    # verticle configuration

Consult HOCON specification to explore all possibilities.

Knot.x Stack defines its own application.conf that uses all configuration possibilities (includes, substitution etc) and provides documentation (in comments) on how to configure Knot.x instance details.

System properties

As mentioned above, you can reference any configuration object or property using ${var} syntax. Thanks to the HOCON capabilities you can also use the same notation to get the value from JVM system properties, e.g.:


And retrieve the value in the config like:

someField = ${}

Additionally, if the value from system property is optional you can use ${?var} syntax to say that inject that value only if it's available. E.g. you can configure default value directly in the application.conf and customize it through system property if necessary.

someField = 1234
someField = ${?my.field.value}

Logback settings

Knot.x Launcher module contains default logback settings in the resources, to provide default logger configuration for the instances.


Launcher provides a way to configure and run bare Knot.x instance.



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