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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. It uses a slightly altered CROW_WEBSOCKET_ROUTE(app, "/url") macro, which is then followed by a series of methods (with handlers inside) for each event. These are (sorted by order of execution):

  • onaccept([&](const crow::request& req, void** userdata){handler code goes here})
  • 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, const std::string& error_message){handler code goes here})
  • onclose([&](crow::websocket::connection& conn, const std::string& reason){handler code goes here})


onaccept must return a boolean. In case false is returned, the connection is shut down, deleted, and no further communication is done.


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

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

    .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)

Maximum payload size


The maximum payload size that a connection accepts can be adjusted either globally by using app.websocket_max_payload(<value in bytes>) or per route by using CROW_WEBSOCKET_ROUTE(app, "/url").max_payload(<value in bytes>). In case a message was sent that exceeded the limit. The connection would be shut down and onerror would be triggered.


By default, this limit is disabled. To disable the global setting in specific routes, you only need to call CROW_WEBSOCKET_ROUTE(app, "/url").max_payload(UINT64_MAX).

For more info about websocket routes go here.

For more info about websocket connections go here.