Useful tips for using libwebsockets

Maximum number of clients

There is a hard limit on the maximum number of clients the library will accept and by default it is set to 100. This limit can be changed by modifying the MAX_CLIENTS preprocessor macro in the file lib/private-libwebsockets.h. The higher the limit the more memory the library will allocate at startup.

SSL performance

It is recommended to tweak the ciphers allowed on secure connections for performance reasons, otherwise a slow algorithm may be selected by the two endpoints and the server could expend most of its time just encrypting and decrypting data, severely limiting the amount of messages it will be able to handle per second. To limit the ciphers supported on secure connections you should modify the preprocessor macro CIPHERS_LIST_STRING in the file lib/private-libwebsockets.h. For example:

#define CIPHERS_LIST_STRING "RC4-MD5:RC4-SHA:AES128-SHA:AES256-SHA:HIGH:!DSS:!aNULL"

Other tweaks

There are several preprocessor macros that could be tweaked to reduce memory usage, to increase performance or to change the behaviour of the library to suit your needs, they are all located in the file lib/private-libwebsockets.h.

Big frames

The library process data from the sockets in chunks of 4KB (defined by the macro MAX_USER_RX_BUFFER), these chunks will be passed to the client callback as LWS_CALLBACK_RECEIVE. If you want to know whether you have all the data for the current frame you need to use the function libwebsockets_remaining_packet_payload.

Fragmented messages

To support fragmented messages you need to check for the final frame of a message with libwebsocket_is_final_fragment. This check can be combined with libwebsockets_remaining_packet_payload to gather the whole contents of a message like in this example:

case LWS_CALLBACK_RECEIVE:
{
    Client * const client = (Client *)user;
    const size_t remaining = libwebsockets_remaining_packet_payload(wsi);
    if (0 == remaining &&
        libwebsocket_is_final_fragment(wsi))
    {
        if (client->HasFragments())
        {
            client->AppendMessageFragment(in, len, 0);
            in = (void *)client->GetMessage();
            len = client->GetMessageLength();
        }

        client->ProcessMessage((char *)in, len, wsi);

        client->ResetMessage();
    }
    else
    {
        client->AppendMessageFragment(in, len, remaining);
    }
}
break;

HTTP requests

If your server is going to support regular HTTP requests by handling LWS_CALLBACK_HTTP it is recommended to return 1 as the result of the callback after you write the response, this will tell the library to automatically close the connection. Closing the connection will liberate an slot for another HTTP request, otherwise it would be up to the browser to close the connection, which could be an issue because the library has a hard limit on the number of open connections, as explained before.