Kovenant Core Configuration

part of kovenant-core


The Context object is basically the current configuration. It can be obtained from Kovenant.context and configured by Kovenant.context {...}. Refer to the configuration section for the options. To create a completely new Context just use Kovenant.createContext {...} which uses the exact same options as Kovenant.context {...}.

Functions like deferred and task have a first parameter which is actually a Context instance. By default this is Kovenant.context so normally you don't have worry about this. Just for that case you want to work with multiple configurations at once you have the possibility.

fun main(args: Array<String>) {
    val ctx = Kovenant.createContext {
        callbackContext.dispatcher = buildDispatcher { name = "cb-new" }
        workerContext.dispatcher = buildDispatcher { name = "work-new" }

    task {
        println("default task $threadName")
    } success {
        println("default success $threadName")

    task(ctx) {
        println("ctx task $threadName")
    } success {
        println("ctx success $threadName")

private val threadName : String get() = Thread.currentThread().getName()


Configuration of Kovenant is done entirely in code and any changes to the Context are completely threadsafe, so Kovenant can be reconfigured during a running application from multiple threads. But you probably want to do this when your application starts.

Configuring is done by simply calling Kovenant.context { ... }.


Kovenant operates with two Dispatchers, a worker and callback Dispatcher. They are configured as follows:

Kovenant.context {
    workerContext.dispatcher = ...
    // or
    callbackContext {
        dispatcher = ...

You can provide your own Dispatcher implementation, convert an existing Executor (Jvm) or use buildDispatcher (Jvm) to create configure the build in Dispatcher.

CONFIGURE ONE THREAD FOR THE CALLBACK DISPATCHER understand the implications when using more then one thread


Let me state upfront that this method is not threadsafe.

buildDispatcher {
    name = "Dispatcher Name"
    concurrentTasks = 1 
    exceptionHandler = ...// (Exception) -> Unit
    errorHandler = ... // (Throwable) -> Unit
    pollStrategy { ... }

name Sets the name of this Dispatcher. Is also used as thread names appended by a number

concurrentTasks The maximum number of tasks this Dispatcher concurrently keeps alive. Note that the actual number of threads can be lower and depends on how much work is offered. Also, during the lifetime of the Dispatcher the number ov threads instantiated can be far greater because threads can also be destroyed.

exceptionHandler Get notified of exceptions from within the Dispatcher of running tasks. Normally Kovenant handles errors on the Promise level but the Dispatcher can also be used directly and those jobs might leak exceptions.

errorHandler When things go seriously wrong, e.g. OutOfMemoryError, this is what is tried to be called.

configurePollStrategy The way you configure you poll strategy greatly influences the way the Dispatcher behaves. Poll strategies can be chained and are executed in order of configuration.

strategy parameters description
yielding numberOfPolls Keeps polling the numberOfPolls and yields the thread between polls. Yielding is most suitable on environments where the number of active threads exceeds the number of physical cores.
busy numberOfPolls Keeps polling the numberOfPolls. Use when a lot of work is expected very frequently (nano seconds)
blocking Blocks until there is new work. Note that this approach prevents the Dispatcher from shutting down. This strategy only makes sense as last of the chain since it either receives work or blocks. Can also block the producing threads for short amounts of time so effectively kills the non blocking nature of Kovenant
sleeping numberOfPolls, sleepTimeInMs Sleeps in between the numberOfPolls for the given sleepTimeInMs. Doesn't wake earlier if new is presented but sleeps the whole thing through. It is thus advised to keep sleepTimeMs very low
blockingSleep numberOfPolls, sleepTimeInMs
val dispatcher = buildDispatcher {
    name = "Bob the builder"
    concurrentTasks = 1

    pollStrategy {                
        yielding(numberOfPolls = 1000)

        sleeping(numberOfPolls = 100, 
                     sleepTimeInMs = 10)

What's best for your situation depends on your needs. So like always with concurrency: test instead of guess.

Thread Factory (JVM)

If you want to have a finer control over Thread creation you can. You can use the JVM only extension function to configure JVM specific features. Please note that Kovenant still will make the Thread a non daemon thread and tries to start it. Starting this thread early will result in an Exception.

Kovenant.context {
    callbackContext.jvmDispatcher { 
        threadFactory = {
            target, dispatcherName, id ->
            Thread(target, "custom name")

Common example

Kovenant.context {
    // Specify a new worker dispatcher.
    // this dispatcher is responsible for
    // work that is executed by `task` and
    // then functions so this is basically
    // work that is expected to run a bit
    // longer
    workerContext.dispatcher {
        // Name this dispatcher, threads
        // created by this dispatcher will
        // get this name with a number
        // appended
        name = "Bob the builder"

        // the max number tasks this
        // dispatcher keeps running in parallel.
        // This setting might be ignored on some
        // platforms
        concurrentTasks = 2

        // Configure the strategy to apply
        // to a thread when there is no work
        // left in the queue. Note that
        // when the strategy finishes the
        // thread will shutdown. Strategies are
        // applied in order of configuration and
        // resets after a thread executes any
        // new task.
        pollStrategy {
            // A busy poll strategy simple polls
            // the provided amount of polls
            // without interrupting the thread.
            yielding(numberOfPolls = 1000)

            // A sleep poll strategy simply puts
            // the thread to sleep between polls.
            sleeping(numberOfPolls = 100,
                    sleepTimeInMs = 10)

    callbackContext {
        // Specify a new callback dispatcher.
        // this dispatcher is responsible for
        // callbacks like success, fail and always.
        // it is expected that these callback do
        // very little work and never block
        dispatcher {
            name = "Tank"
            concurrentTasks = 1
        // route internal errors when invoking
        // callbacks. This is also the place to
        // route this to a preferred logging
        // framework
        errorHandler =
                fun(e: Exception)
                        = e.printStackTrace(System.err)

    // when promises are being resolved
    // multiple time, which is misuse of
    // the api this method is fired. You
    // can for instance choose to throw
    // an Exception here
    multipleCompletion =
            fun(a: Any, b: Any): Unit
                    = System.err.println(
                    "Tried resolving with $b, but is $a")