Make parallel HTTP calls

There are cases where an application needs to make multiple HTTP calls to other services as part of handling a particular request. As a general rule-of-thumb, we recommend people to avoid premature optimisation, however sometimes the quantity of calls or performance of other services demand those to be executed in parallel.

In this example, we show how to use the extension function on [ExecutionService] to manage multiple HTTP calls in parallel, and synchronise their results to produce a single response.

This recipe also covers how to make distributed tracing work when tracing information is consumed by multiple threads.

package guide.howto.make_parallel_calls

import org.http4k.client.OkHttp
import org.http4k.core.Filter
import org.http4k.core.HttpHandler
import org.http4k.core.Method.GET
import org.http4k.core.NoOp
import org.http4k.core.Request
import org.http4k.core.Response
import org.http4k.core.Status.Companion.OK
import org.http4k.core.then
import org.http4k.filter.ClientFilters
import org.http4k.filter.DebuggingFilters
import org.http4k.filter.ServerFilters
import org.http4k.server.Undertow
import org.http4k.server.asServer
import org.http4k.util.withRequestTracing
import java.util.concurrent.Callable
import java.util.concurrent.ExecutorService
import java.util.concurrent.LinkedBlockingDeque
import java.util.concurrent.ThreadPoolExecutor
import java.util.concurrent.TimeUnit.SECONDS
import kotlin.system.measureTimeMillis

private fun serverWithTracing(handler: HttpHandler) = Filter.NoOp

private fun clientWithTracing() = Filter.NoOp

private fun serverWithParallelCallsAndTracing(
    dependencyServerPort: Int,
    executor: ExecutorService
): HttpHandler {
    val localClient = clientWithTracing()

    return serverWithTracing { _: Request ->
        val overallStatus = (1..5).map {
            executor.submit(Callable {
        }.foldRight(OK) { future, overallStatus ->
            val nextResponse = future.get(5, SECONDS)
            if (nextResponse.status.successful) nextResponse.status else overallStatus


fun main() {
    val originalExecutor = ThreadPoolExecutor(5, 5, 10, SECONDS, LinkedBlockingDeque())

    // convert the execution service to one that propagates the trace information
    // optionally pass a custom ZipkinStorage
    val executor = originalExecutor.withRequestTracing()

    val subServer = serverWithTracing { _: Request -> Thread.sleep(500); Response(OK) }
    val runningSubServer = subServer.asServer(Undertow(0)).start()

    val server = serverWithParallelCallsAndTracing(runningSubServer.port(), executor)
    val runningServer = server.asServer(Undertow(0)).start()

    val elapsedTime = measureTimeMillis {
        val client = clientWithTracing()
        client(Request(GET, "http://localhost:${runningServer.port()}/main-request"))

    println("Elapsed time in millis = $elapsedTime")