bb-server

name: bb-server description: Build Babashka HTTP API servers using httpkit, ruuter, and hiccup. Scaffolds server projects with routing, handlers, rendering, WebSocket support, and HTMX integration.

Babashka Server Skill

Build Babashka HTTP servers following proven patterns from production use. Uses httpkit for async HTTP, ruuter for routing, and hiccup for HTML rendering.

Stack

Dependencies (bb.edn)

{:deps {org.clojars.askonomm/ruuter {:mvn/version "1.3.4"}
        cheshire/cheshire {:mvn/version "5.13.0"}}
 :paths ["src"]}

Hiccup and httpkit are built into Babashka — no extra deps needed.

Project Structure

src/
  <name>/
    server.clj           # Entry point, routes, -main
    server/
      config.clj         # Dynamic config vars, data loading
      render.clj         # HTTP response helpers (html, json, form parsing)
      handlers.clj       # Request handler functions
      components.clj     # Hiccup UI components (if serving HTML)
      websocket.clj      # WebSocket state + broadcast (if needed)

Scale up by adding handler files per domain. Scale down by keeping everything in server.clj for small APIs.

Core Files

Entry Point (server.clj)

(ns <name>.server
  (:require [<name>.server.handlers :as handlers]
            [org.httpkit.server :as srv]
            [ruuter.core :as ruuter]))

(def routes
  [{:path "/"
    :method :get
    :response handlers/index-handler}
   {:path "/api/items"
    :method :get
    :response handlers/list-items}
   {:path "/api/items"
    :method :post
    :response handlers/create-item}
   {:path "/api/items/:id"
    :method :get
    :response handlers/get-item}
   {:path "/api/items/:id"
    :method :put
    :response handlers/update-item}
   {:path "/api/items/:id"
    :method :delete
    :response handlers/delete-item}])

(defn- wrap-errors [handler]
  (fn [req]
    (try
      (or (handler req)
          {:status 404 :body "Not found"})
      (catch Throwable e
        (println "ERROR" (:uri req) (ex-message e))
        {:status 500 :body (str "Error: " (ex-message e))}))))

(defn -main [& args]
  (let [port (or (some-> (first args) parse-long) 3000)]
    (srv/run-server (wrap-errors #(ruuter/route routes %)) {:port port})
    (println "Server running at" (str "http://localhost:" port "/"))))

(def cli? (= *file* (System/getProperty "babashka.file")))

(when cli?
  (apply -main *command-line-args*)
  @(promise))

Render Helpers (server/render.clj)

(ns <name>.server.render
  (:require [cheshire.core :as json]
            [clojure.string :as str]
            [hiccup2.core :as h])
  (:import [java.net URLDecoder]))

(defn render
  "Render hiccup to HTTP response"
  [body & [status]]
  {:status (or status 200)
   :headers {"Content-Type" "text/html; charset=utf-8"}
   :body (if (seq? body)
           (apply str (map #(if (string? %) % (str (h/html %))) body))
           (str (h/html body)))})

(defn render-json
  "Render data as JSON HTTP response"
  [data & [status]]
  {:status (or status 200)
   :headers {"Content-Type" "application/json"}
   :body (json/generate-string data)})

(defn render-html
  "Render hiccup to HTML string (for WebSocket broadcast)"
  [hiccup]
  (str (h/html hiccup)))

(defn parse-form-body
  "Parse URL-encoded form body into keyword map"
  [body]
  (when body
    (->> (str/split (slurp body) #"&")
         (map #(str/split % #"=" 2))
         (map (fn [[k v]] [(keyword k) (when v (URLDecoder/decode v "UTF-8"))]))
         (into {}))))

(defn parse-json-body
  "Parse JSON request body into keyword map"
  [body]
  (when body
    (json/parse-string (slurp body) true)))

Handlers (server/handlers.clj)

(ns <name>.server.handlers
  (:require [<name>.server.render :as render]))

(defn index-handler [_req]
  (render/render
   [:html
    [:head [:title "My App"]]
    [:body
     [:h1 "Hello"]]]))

(defn list-items [_req]
  (render/render-json {:items []}))

(defn create-item [{:keys [body]}]
  (let [data (render/parse-json-body body)]
    ;; process data
    (render/render-json {:created data} 201)))

(defn get-item [{:keys [params]}]
  (let [id (:id params)]
    (render/render-json {:id id})))

(defn update-item [{:keys [params body]}]
  (let [id (:id params)
        data (render/parse-json-body body)]
    (render/render-json {:id id :updated data})))

(defn delete-item [{:keys [params]}]
  (let [id (:id params)]
    (render/render-json {:deleted id})))

Route Definition

Ruuter routes are vectors of maps with :path, :method, and :response.

{:path "/api/items/:id"    ;; :id becomes (:id (:params req))
 :method :get               ;; :get :post :put :delete
 :response handler-fn}      ;; (fn [request] -> response-map)

Route params arrive in (:params req). Put more specific routes before catch-all patterns.

Request Shape

{:request-method :post
 :uri "/api/items/123"
 :params {:id "123"}           ;; route params from ruuter
 :query-string "foo=bar"       ;; raw query string
 :headers {"content-type" ...}
 :body <InputStream>}          ;; slurp to read

HTMX Integration

For server-rendered HTML with dynamic updates:

;; Button that posts and swaps content
[:button {:hx-post "/api/action"
          :hx-target "#result"
          :hx-swap "innerHTML"}
 "Do thing"]

;; Target div
[:div {:id "result"} "initial content"]

Out-of-Band Swaps (update multiple targets from one response)

;; Return a list — primary response + OOB updates
(render/render
 (list
  [:div {:id "main"} "primary response"]
  [:div {:id "counter" :hx-swap-oob "innerHTML"} "42"]
  [:div {:id "status" :hx-swap-oob "innerHTML"} "updated"]))

Page Template with HTMX

(defn page-template [title & body]
  [:html
   [:head
    [:title title]
    [:script {:src "https://unpkg.com/htmx.org@2.0.4"}]]
   [:body {:hx-ext "ws"}
    body]])

WebSocket Support

httpkit has built-in WebSocket support.

(ns <name>.server.websocket
  (:require [<name>.server.render :as render]
            [org.httpkit.server :as srv]))

(def ws-clients (atom #{}))

(defn ws-handler [req]
  (srv/as-channel req
    {:on-open (fn [ch] (swap! ws-clients conj ch))
     :on-close (fn [ch _] (swap! ws-clients disj ch))
     :on-receive (fn [_ch msg] (println "received:" msg))}))

(defn ws-broadcast! [html-str]
  (doseq [ch @ws-clients]
    (srv/send! ch html-str false)))

;; Broadcast hiccup as HTML to all clients
(defn broadcast-update! [hiccup]
  (ws-broadcast! (render/render-html hiccup)))

Add WebSocket route:

{:path "/ws" :method :get :response websocket/ws-handler}

Status Monitor Pattern (poll + broadcast changes)

(def state-cache (atom {}))

(defn start-status-monitor! [render-row-fn]
  (future
    (loop []
      (try
        (let [current (fetch-current-state)
              prev @state-cache
              changed (filter #(not= (get prev (:id %)) (:state %)) current)]
          (reset! state-cache (into {} (map (juxt :id :state) current)))
          (doseq [item changed]
            (broadcast-update! (render-row-fn item))))
        (catch Exception e
          (println "Monitor error:" (ex-message e))))
      (Thread/sleep 2000)
      (recur))))

JSON API Pattern

For pure JSON APIs without HTML:

(defn api-handler [{:keys [params body]}]
  (let [data (render/parse-json-body body)]
    (render/render-json {:status "ok" :data data})))

Async Operations

For long-running work, return immediately and update via WebSocket:

(defn launch-handler [{:keys [body]}]
  (let [data (render/parse-form-body body)]
    ;; Return placeholder immediately
    (future
      (try
        (let [result (do-slow-thing data)]
          (broadcast-update! [:div {:id "result"} (str result)]))
        (catch Exception e
          (broadcast-update! [:div {:id "result" :class "error"} (ex-message e)]))))
    (render/render [:div {:id "result"} "Processing..."])))

Running

# Direct
bb src/<name>/server.clj

# With custom port
bb src/<name>/server.clj 8080

# As bb task
bb server

bb.edn Task

{:tasks
 {:server {:doc "Start the server"
           :task (shell "bb src/<name>/server.clj")}}}

Conventions

• Handler functions take a request map, return a response map
• Use render/render for HTML, render/render-json for JSON
• Route params via (:params req), form body via parse-form-body, JSON via parse-json-body
• Error wrapper returns 500 with message on unhandled exceptions
• @(promise) at end of script keeps the server alive
• Use (def cli? ...) guard so the file works both as script and REPL
• Side-effecting functions use ! suffix
• Keep routes in a flat vector, ordered specific-to-general
• Split into server/handlers.clj, server/render.clj, etc. as the project grows

Checklist

1. Create bb.edn with ruuter + cheshire deps
2. Create server.clj with routes and -main
3. Create server/render.clj with response helpers
4. Create server/handlers.clj with route handlers
5. Add WebSocket if real-time updates are needed
6. Test: bb src/<name>/server.clj