Account for backpressure when sending messages

TextractorSender/Server.cpp

@@ -1,8 +1,11 @@
 #include "Server.h"
 
+const int BACKPRESSURE_LIMIT = 1024;
+
 uWS::App* app;
 uWS::WebSocket<false, true, PerSocketData>* gws = nullptr;
 uWS::Loop* loop;
+queue<string> send_queue;
 
 void StartServer() {
     uWS::App* app = new uWS::App();
@@ -21,6 +24,22 @@ void StartServer() {
         .open = [](auto* ws) {
                     gws = ws;
         },
+        .drain = [](auto* /*ws*/) {
+            /* Check ws->getBufferedAmount() here */
+            /* If it is below threshold then send the next message on the queue */
+            if (gws->getBufferedAmount() <= BACKPRESSURE_LIMIT) {
+                string data = send_queue.front();
+                auto result = gws->send(data, uWS::OpCode::TEXT, false);
+
+                // If failed, then put back onto queue to try again next
+                if (result == 2) {
+                    send_queue.push(data);
+                }
+                else {
+                    send_queue.pop();
+                }
+            }
+        },
         .close = [](auto*/*ws*/, int /*code*/, std::string_view /*message*/) {
                     gws = nullptr;
         }
@@ -43,7 +62,17 @@ void CloseServer() {
 void BroadcastData(string data) {
     if (gws) {
         loop -> defer([data] {
-            gws -> send(data, uWS::OpCode::TEXT, false);
+            if (gws->getBufferedAmount() <= BACKPRESSURE_LIMIT) {
+                auto result = gws->send(data, uWS::OpCode::TEXT, false);
+
+                // If failed, then put back onto queue to try again next
+                if (result == 2) {
+                    send_queue.push(data);
+                }
+            }
+            else {
+                send_queue.push(data);
+            }
         });
     }
 }

TextractorSender/Server.h

@@ -2,6 +2,7 @@
 
 #include <time.h>
 #include <iostream>
+#include <queue>
 #include <uwebsockets/App.h>
 
 using namespace std;