Clojure: virtual threads with ring and jetty

Java 19 introduce virtual threads as a preview feature. Virtual threads are lightweight threads that allow you to write server applications in a simple thread-per-request style (as opposed to async style) and still scale with near-optimal hardware utilisation. The thread-per-request approach is generally easier to reason about, maintain and debug than it's asynchronous counterparts. In this post we'll cover configuring Jetty to make your ring handlers use virtual threads and benchmark it against regular threads to show how it alleviates the need for tuning thread pools.

If you're running Java 19+ you can enable preview features with --enable-preview as shown below. This step won't be needed in Java 21 (which is being released this month) as virtual threads will be out of preview and available by default.

{:paths   ["src"]
 :deps    {...}
 :aliases {:dev {:jvm-opts ["--enable-preview"]}}}

Set up a ring handler that sleeps for 50 milliseconds to simulate a long request.

(ns server.core
  (:require [ring.adapter.jetty :refer [run-jetty]]
  (:import (java.util.concurrent Executors)
           (org.eclipse.jetty.util.thread QueuedThreadPool)))
(def app
         (fn [_]
           {:headers {"Content-Type" "text/html"}
            :status  200
            :body    (do
                       ;; simulate work by sleeping
                       ;; for 50 milliseconds
                       (Thread/sleep 50)
                       (h/render [:b "hello!"]))})}}]

First we will create a jetty server without virtual threads so that we can make a performance comparison.

(def jetty-server (run-jetty app {:port 8080 :join? false}))

Then benchmark it with wrk (a http server benchmarking tool).

wrk -t 12 -c 120

Running 10s test @
12 threads and 120 connections
Requests/sec:    842.92
Transfer/sec:     95.49KB

Stop the jetty server.

(.stop server)

Now, we will create a jetty server that uses virtual threads.

(def jetty-server
  (let [thread-pool (new QueuedThreadPool)
        _           (.setVirtualThreadsExecutor thread-pool
    (run-jetty app {:port        8080
                    :join?       false
                    :thread-pool thread-pool})))

Then benchmark it.

Running 10s test @
12 threads and 120 connections
Requests/sec:   2218.19
Transfer/sec:    251.28KB

That's a 2.6x increase in performance. This increase is not due to virtual threads per say but due to virtual thread pools having an unlimited number of threads versus a fixed-size thread pool (50 threads). However, what this does highlight is that with virtual thread you no longer need to right size thread pools in environments with lots of IO, you get near-optimal utilisation without the need for manual and machine specific tuning.

The full example project can be found here.

Note: The virtual thread set up above also works with ring-jetty9-adapter which supports Jetty 12 and it's new AdaptiveExecutionStrategy.

For setting up virtual threads with http-kit checkout this post.