docs: further setting up structure

backend/src/builtin-cli.cpp

@@ -2,15 +2,34 @@
 
 #include <iostream>
 
-#include "schedule.h"
 #include "bs-utility.h"
+#include "schedule.h"
 void BookStoreMain(bool is_server, std::string config_dir) {
   std::ios::sync_with_stdio(false);
   if (!is_server) {
     ;  // TODO: run as client
   } else {
-    throw FatalError("Server mode is not implemented yet", 1);
     std::cin.tie(nullptr);
     std::cout.rdbuf(nullptr);
+    BlockingStringStream input;
+    BlockingStringStream output;
+    BookStoreBackEndClass backend(config_dir, &input, &output);
+    std::thread backend_thread([&backend]() { backend.Run(); });
+    std::thread input_thread([&input]() {
+      std::string data;
+      while (std::getline(std::cin, data)) {
+        input << data << '\n';
+      }
+    });
+    std::thread output_thread([&output]() {
+      std::string data;
+      while (true) {
+        output.getline(data, '\n');
+        std::cout << data << std::endl;
+      }
+    });
+    input_thread.join();
+    output_thread.join();
+    backend_thread.join();
   }
 }