randomize socket.io reconnect delays +/-50%

To prevent a "thundering herd" of clients all reconnecting on the same schedule after a server-side outage, randomize each reconnect attempt's delay by +/-50%

Author: justinwh

AndroidAsync/src/com/koushikdutta/async/http/socketio/SocketIOConnection.java

@@ -217,14 +217,23 @@ private void delayReconnect() {
             public void run() {
                 reconnect(null);
             }
-        }, reconnectDelay);
+        }, nextReconnectDelay(reconnectDelay));
 
         reconnectDelay = reconnectDelay * 2;
         if (request.config.reconnectDelayMax > 0L) {
             reconnectDelay = Math.min(reconnectDelay, request.config.reconnectDelayMax);
         }
     }
 
+    private long nextReconnectDelay(long targetDelay) {
+        if (targetDelay < 2L || targetDelay > (Long.MAX_VALUE >> 1) ||
+            !request.config.randomizeReconnectDelay)
+        {
+            return targetDelay;
+        }
+        return (targetDelay >> 1) + (long) (targetDelay * Math.random());
+    }
+
     private void reportDisconnect(final Exception ex) {
         if (ex != null) {
             request.loge("socket.io disconnected", ex);

AndroidAsync/src/com/koushikdutta/async/http/socketio/SocketIORequest.java

@@ -40,6 +40,14 @@ public SocketIORequest(String uri, String endpoint, String query, Config config)
     }
 
     public static class Config {
+        boolean randomizeReconnectDelay = false;
+        public void setRandomizeReconnectDelay(boolean randomizeReconnectDelay) {
+            this.randomizeReconnectDelay = randomizeReconnectDelay;
+        }
+        public boolean isRandomizeReconnectDelay() {
+            return randomizeReconnectDelay;
+        }
+
         long reconnectDelay = 1000L;
         public void setReconnectDelay(long reconnectDelay) {
             if (reconnectDelay < 0L) {