Kit
Out of the box Kit supports the Undertow web server via the ring-undertow-adapter
Undertow Configuration
Undertow allows setting the number of worker and IO threads using the :worker-threads and the :io-threads keys respectively. For example, we could update the default configuration in the system.edn as follows:
:server/undertow
{:port #long #or [#env PORT 3000]
:handler #ig/ref :handler/ring
:worker-threads 200
:io-threads 4}
If you want to do a custom configuration that includes calculating the io-threads at runtime, you can override the default ig/init-key for server/undertow. This is the definition in kit.edge.server.undertow
(defmethod ig/expand-key :server/undertow
[k config]
{k (merge {:port 3000
:host "0.0.0.0"}
config)})
For example,
(defmethod ig/expand-key :server/undertow
[k config]
{k (merge {:port 3000
:host "0.0.0.0"
:io-threads (* 2 (.availableProcessors (Runtime/getRuntime)))}
config)})
For a full listing of all configuration options, review the ring-undertow-adapter documentation.
Undertow uses separate thread pools for managing the IO and the worker threads. The :dispatch? flag is used to decide whether the request should be dispatched by the IO thread to a separate worker thread. Since dispatching the request to a worker carries overhead, it may be more performant to handle some requests, such as hardcoded text responses, directly in the IO thread.