http-kit

Simple, high-performance event-driven HTTP client+server for Clojure.

http-kit uses an event-driven, non-blocking I/O model. It's Ring-compatible, has zero dependencies, and is ~90kB with ~3k lines of code.

Setup

deps.edn:

http-kit/http-kit {:mvn/version "2.8.1"}

Leiningen:

[http-kit/http-kit "2.8.1"]

See https://clojars.org/http-kit/http-kit for the latest version.

Quick Start

Server:

(require '[org.httpkit.server :as hk-server])

(defn app [req]
  {:status 200
   :headers {"Content-Type" "text/html"}
   :body "hello HTTP!"})

(def server (hk-server/run-server app {:port 8080}))

;; Stop server
(server) ; immediate shutdown
(server :timeout 100) ; graceful shutdown

Client:

(require '[org.httpkit.client :as hk-client])

;; Promise-based (async)
(def resp-promise (hk-client/get "http://example.com"))
@resp-promise ; block for result

;; Callback-based (async)
(hk-client/get "http://example.com"
  (fn [{:keys [status headers body error]}]
    (if error
      (println "Failed:" error)
      (println "Success:" status))))

Server

Basic Handler

Ring-compatible handler:

(defn app [req]
  {:status 200
   :headers {"Content-Type" "application/json"}
   :body "{\"message\": \"Hello\"}"})

Server Options

(hk-server/run-server app
  {:port 8080
   :ip "0.0.0.0"
   :thread 4                           ; worker threads
   :queue-size 20480                   ; request queue size
   :max-body 8388608                   ; max request body size (bytes)
   :max-line 4096                      ; max HTTP line size
   :legacy-unsafe-remote-addr? false   ; secure remote-addr (see below)
   :legacy-content-length? false})     ; respect handler Content-Length

WebSockets

Two APIs available: http-kit's unified API (works for both WebSocket and long-polling) and Ring's WebSocket API.

Using http-kit's unified API:

(def channels (atom #{}))

(defn on-open    [ch]             (swap! channels conj ch))
(defn on-close   [ch status-code] (swap! channels disj ch))
(defn on-receive [ch message]
  (doseq [ch @channels]
    (hk-server/send! ch (str "Broadcasting: " message))))

(defn ws-handler [ring-req]
  (if-not (:websocket? ring-req)
    {:status 200 :body "WebSocket endpoint"}
    (hk-server/as-channel ring-req
      {:on-open    on-open
       :on-receive on-receive
       :on-close   on-close})))

Using Ring's WebSocket API:

(require '[ring.websocket :as ws])

(def sockets (atom #{}))

(defn on-open    [ch]                    (swap! sockets conj ch))
(defn on-close   [ch status-code reason] (swap! sockets disj ch))
(defn on-message [ch message]
  (doseq [ch @sockets]
    (ws/send ch (str "Broadcasting: " message))))

(defn ws-handler [ring-req]
  (if-not (:websocket? ring-req)
    {:status 200 :body "WebSocket endpoint"}
    {::ws/listener
     {:on-open    on-open
      :on-message on-message
      :on-close   on-close}}))

Sending to WebSocket (unified API):

(hk-server/send! channel "message")        ; returns true on success
(hk-server/send! channel "message" false)  ; false = don't close after send
(hk-server/close channel)                  ; explicitly close connection

Key differences between APIs:

• Unified API works with both WebSockets and HTTP long-polling
• Unified API: detect send success via return value
• Ring API: detect send success via callbacks
• Ring API provides close reason (though often empty in practice)

Streaming Responses

Use as-channel for HTTP streaming:

(defn streaming-handler [req]
  (hk-server/as-channel req
    {:on-open (fn [ch]
                (hk-server/send! ch {:status 200
                                    :headers {"Content-Type" "text/plain"}
                                    :body ""} false)
                (future
                  (dotimes [i 10]
                    (Thread/sleep 1000)
                    (hk-server/send! ch (str "Chunk " i "\n") false))
                  (hk-server/close ch)))}))

Client

Promise-Based Requests

Returns immediately with a promise (after DNS lookup):

;; GET request
(def resp (hk-client/get "http://example.com"))
@resp  ; block for result
(deref resp 5000 :timeout) ; timeout after 5s

;; Concurrent requests
(let [r1 (hk-client/get "http://example.com")
      r2 (hk-client/get "http://other.com")]
  (println (:status @r1))
  (println (:status @r2)))

Callback-Based Requests

(hk-client/get "http://example.com"
  {:timeout 200
   :basic-auth ["user" "pass"]
   :headers {"X-Custom" "value"}}
  (fn [{:keys [status headers body error opts]}]
    (if error
      (println "Failed:" error)
      (println "Success:" status))))

Request Options

All HTTP methods support these options:

(hk-client/request
  {:url "http://example.com/api"
   :method :post  ; :get :post :put :delete :head :patch
   :headers {"X-Api-Key" "secret"}
   :query-params {"q" "search term"}
   :form-params {"key" "value"}        ; form-encoded body
   :body (json/encode {:key "value"})  ; raw body (e.g., JSON)

   :basic-auth ["user" "pass"]
   :oauth-token "token"
   :user-agent "MyApp/1.0"

   :timeout 1000     ; connection + read timeout (ms)
   :keepalive 30000  ; keep-alive duration (ms), -1 to disable

   :follow-redirects true  ; follow 301/302 (default true)
   :max-redirects 10       ; max redirects to follow

   :insecure? false  ; accept untrusted SSL certs
   :as :auto})       ; output coercion (see below)

Convenience methods: get, post, put, delete, head, patch, options

Keep-Alive Connections

http-kit client uses HTTP keep-alive by default (120s):

;; Custom keep-alive duration
@(hk-client/get "http://example.com" {:keepalive 30000}) ; 30s

;; This reuses the TCP connection
@(hk-client/get "http://example.com" {:keepalive 30000})

;; Disable keep-alive
@(hk-client/get "http://example.com" {:keepalive -1})

Output Coercion

Control response body format with :as:

;; Stream (java.io.InputStream) - for large responses
{:as :stream}

;; Byte array - for binary data
{:as :byte-array}

;; String - for text responses
{:as :text}

;; Auto-detect from Content-Type (default)
{:as :auto}

Example:

(hk-client/get "http://example.com/image.png"
  {:as :stream}
  (fn [{:keys [body]}]
    ;; body is java.io.InputStream
    (io/copy body (io/file "downloaded.png"))))

Multipart File Upload

(hk-client/post "http://example.com/upload"
  {:multipart
   [{:name "comment" :content "Uploading my file"}
    {:name "file"
     :content (io/file "path/to/file.txt")
     :filename "file.txt"}]})

Content can be String, File, or InputStream. All content is read before sending, so keep files small (few MB).

Common Patterns

Passing State in Callbacks

Options map is passed to callback:

(let [start-time (System/currentTimeMillis)]
  (hk-client/get "http://example.com"
    {:my-start-time start-time}
    (fn [{:keys [status opts]}]
      (let [{:keys [method url my-start-time]} opts]
        (println method url "took"
          (- (System/currentTimeMillis) my-start-time) "ms")))))

Custom Thread Pool (Server)

Control worker threads:

(require '[org.httpkit.utils :as utils])

;; Easy way - use utils/new-worker
(hk-server/run-server app
  {:port 8080
   :worker-pool (utils/new-worker "my-worker-" 8)})

;; Java 19+ virtual threads
(hk-server/run-server app
  {:port 8080
   :worker-pool (java.util.concurrent.Executors/newVirtualThreadPerTaskExecutor)})

Request Body Size Limiting (Client)

(hk-client/get "http://example.com"
  {:filter (hk-client/max-body-filter (* 1024 100))}) ; reject if >100KB

Key Gotchas

Server Security

1. Remote address spoofing: By default :remote-addr is populated from X-Forwarded-For header, which clients can spoof. Set :legacy-unsafe-remote-addr? false to use actual socket address (usually your proxy's IP).

2. Getting real client IP: Parse X-Forwarded-For at application level with knowledge of trusted proxies. Never trust the leftmost IP blindly. Consider using the client-ip library.

3. Production deployment: Always run behind a reverse proxy (nginx, Caddy, HAProxy) for HTTPS support, load balancing, and security.

Nested Query Params

http-kit supports nested params but encoding is fragile:

;; This works but is not robust
{:query-params {:a {:b {:c 5}}}} ; => "a[b][c]=5"

;; Better: encode explicitly
(require '[clojure.data.json :as json])
{:query-params {:a (json/write-str {:b {:c 5}})}}

Content-Length Control

By default http-kit calculates Content-Length from body and overrides handler headers. Set :legacy-content-length? false to respect handler's Content-Length (useful for RFC-compliant HEAD responses).

WebSocket Compatibility

Check :websocket? in request before calling as-channel:

(defn handler [req]
  (if (:websocket? req)
    (hk-server/as-channel req {...})
    {:status 200 :body "Not a WebSocket request"}))

Advanced Topics

For these advanced features, consult the full documentation:

• Unix Domain Sockets (UDS) - Java 16+
• Server Name Indication (SNI) - auto-enabled in 2.7+
• Custom client instances with make-client
• Testing with http-kit-fake for mocking requests
• Custom request queues and monitoring

References

• API Docs: https://cljdoc.org/d/http-kit/http-kit/
• Wiki: https://github.com/http-kit/http-kit/wiki
• Benchmarks: https://github.com/http-kit/http-kit/wiki/4-Benchmarking