Use HTMX-compatible HTML fragments on WebSocket, fix multi-connection

- WebSocket sends HTML fragments with id="price-display" instead of JSON
  for direct HTMX OOB swap on the client
- Switch from ring-undertow-adapter ws wrapper to raw Undertow WebSocket
  API (proxy) to fix single-connection limitation
- Keep JSON endpoint at /api/price/latest unchanged
- Update CLAUDE.md and README.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -44,7 +44,7 @@ Docker runs alongside local dev on different ports and databases:
 ## API Endpoints
 
 - `GET /api/health` — Health check
-- `GET /api/price/ws` — WebSocket endpoint for live JSON price updates
+- `GET /api/price/ws` — WebSocket endpoint for live HTML price fragments (HTMX-compatible)
 - `GET /api/price/latest` — Latest price as JSON
 
 ## Source Layout

README.md

@@ -1,12 +1,12 @@
 # btcdata
 
-Bitcoin data backend service. Fetches BTC prices from Binance (WebSocket) and Kraken (HTTP polling), stores them in PostgreSQL, and exposes a JSON API with Server-Sent Events for live price streaming.
+Bitcoin data backend service. Fetches BTC prices from Binance (WebSocket) and Kraken (HTTP polling), stores them in PostgreSQL, and exposes a JSON API with WebSocket for live price streaming.
 
 ## API
 
 | Endpoint | Description |
 |---|---|
-| `GET /api/price/stream` | SSE stream — emits `price-update` events with JSON `{price, prev_price, recorded_at}` |
+| `GET /api/price/ws` | WebSocket — sends HTML fragments with live price updates (HTMX-compatible) |
 | `GET /api/price/latest` | Latest price as JSON |
 | `GET /api/health` | Health check |
 

src/clj/pmagnus/btcdata/web/routes/api.clj

@@ -9,11 +9,13 @@
    [reitit.ring.coercion :as coercion]
    [reitit.ring.middleware.muuntaja :as muuntaja]
    [reitit.ring.middleware.parameters :as parameters]
-   [reitit.swagger :as swagger]
-   [ring.adapter.undertow.websocket :as ws])
+   [reitit.swagger :as swagger])
   (:import
    [io.undertow.server HttpServerExchange]
-   [io.undertow.websockets.core WebSockets WebSocketChannel]
+   [io.undertow.websockets WebSocketConnectionCallback WebSocketProtocolHandshakeHandler]
+   [io.undertow.websockets.core AbstractReceiveListener WebSockets WebSocketChannel]
+   [java.time Instant ZoneId]
+   [java.time.format DateTimeFormatter]
    [java.util.concurrent LinkedBlockingQueue TimeUnit]))
 
 (defn- price->json [{:keys [price prev-price recorded-at]}]
@@ -22,15 +24,30 @@
     :prev_price  (when prev-price (str prev-price))
     :recorded_at (str recorded-at)}))
 
-(defn- ws-price-handler [{:keys [binance]} req]
-  (let [^HttpServerExchange exchange (:server-exchange req)
-        price-atom (:latest-price binance)]
-    (ws/ws-request exchange nil
-      (ws/ws-callback
-        {:on-open
-         (fn [{:keys [^WebSocketChannel channel]}]
+(def ^:private ts-fmt (DateTimeFormatter/ofPattern "dd-MM HH:mm:ss"))
+
+(defn- price->html [{:keys [price prev-price recorded-at]}]
+  (let [p  (double (bigdec price))
+        pp (when prev-price (double (bigdec prev-price)))
+        color (cond
+                (nil? pp) "text-gray-900"
+                (> p pp)  "text-green-600"
+                (< p pp)  "text-red-600"
+                :else     "text-gray-900")
+        ts (.format (.atZone (Instant/from recorded-at) (ZoneId/systemDefault)) ts-fmt)]
+    (str "<div id=\"price-display\">"
+         "<div class=\"bg-white rounded-2xl shadow-sm border border-gray-200 p-6\">"
+         "<div class=\"text-center\">"
+         "<p class=\"text-4xl font-bold " color "\">$" (format "%,.2f" p) "</p>"
+         "<p class=\"text-xs text-gray-400 mt-2\">Updated " ts "</p>"
+         "</div></div></div>")))
+
+(defn- start-ws-send-loop! [^WebSocketChannel channel price-atom]
   (let [queue (LinkedBlockingQueue.)
         wkey  (keyword (gensym "ws-"))]
+    (.set (.getReceiveSetter channel)
+          (proxy [AbstractReceiveListener] []))
+    (.resumeReceives channel)
     (add-watch price-atom wkey
       (fn [_ _ _ v] (.offer queue v)))
     (when-let [v @price-atom]
@@ -40,14 +57,23 @@
         (loop []
           (when (.isOpen channel)
             (if-let [v (.poll queue 15 TimeUnit/SECONDS)]
-                       (WebSockets/sendTextBlocking (price->json v) channel)
-                       (WebSockets/sendTextBlocking "{\"ping\":true}" channel))
+              (WebSockets/sendTextBlocking (price->html v) channel)
+              (WebSockets/sendTextBlocking "" channel))
             (recur)))
         (catch Exception _)
         (finally
           (remove-watch price-atom wkey)
           (when (.isOpen channel)
-                     (try (.close channel) (catch Exception _))))))))}))
+            (try (.close channel) (catch Exception _))))))))
+
+(defn- ws-price-handler [{:keys [binance]} req]
+  (let [^HttpServerExchange exchange (:server-exchange req)
+        price-atom (:latest-price binance)
+        callback (proxy [WebSocketConnectionCallback] []
+                   (onConnect [_ws-exchange channel]
+                     (start-ws-send-loop! channel price-atom)))
+        handler (WebSocketProtocolHandshakeHandler. callback)]
+    (.handleRequest handler exchange)
     nil))
 
 (defn- latest-price-handler [{:keys [binance]} _req]