Websockets are a way of connecting a client and a server without the request response nature of HTTP.

To create a websocket in Crow, you need a websocket route.
A websocket route differs from a normal route quite a bit. While it uses the same CROW_ROUTE(app, "/url") macro, that's about where the similarities end.
A websocket route follows the macro with .websocket() which is then followed by a series of methods (with handlers inside) for each event. These are (sorted by order of execution):


By default, Crow allows Clients to send unmasked websocket messages, which is useful for debugging but goes against the protocol specification. Production Crow applications should enforce the protocol by adding #define CROW_ENFORCE_WS_SPEC to their source code.

  • onaccept([&](const crow::request&){handler code goes here}) (This handler has to return bool)
  • onopen([&](crow::websocket::connection& conn){handler code goes here})
  • onmessage([&](crow::websocket::connection& conn, const std::string message, bool is_binary){handler code goes here})
  • onerror([&](crow::websocket::connection& conn){handler code goes here})
  • onclose([&](crow::websocket::connection& conn, const std::string reason){handler code goes here})

These event methods and their handlers can be chained. The full Route should look similar to this:

CROW_ROUTE(app, "/ws")
    .onopen([&](crow::websocket::connection& conn){
    .onclose([&](crow::websocket::connection& conn, const std::string& reason){
    .onmessage([&](crow::websocket::connection& /*conn*/, const std::string& data, bool is_binary){
                if (is_binary)

For more info go here.