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### Comparison to Zaphoyd Studios WebSocket++

How does this compare to websocketpp, an alternate header-only WebSocket implementation?

  1. Synchronous Interface

    Beast offers full support for WebSockets using a synchronous interface. It uses the same style of interfaces found in Boost.Asio: versions that throw exceptions, or versions that return the error code in a reference parameter:

    Beast | websocketpp
    --- | ---

        template<class DynamicBuffer>
        void
        read(DynamicBuffer& dynabuf)

    <not available>

  2. Connection Model

    websocketpp supports multiple transports by utilizing a trait, the `config::transport_type` (asio transport example) To get an idea of the complexity involved with implementing a transport, compare the asio transport to the `iostream` transport (a layer that allows websocket communication over a `std::iostream`).

    In contrast, Beast abstracts the transport by defining just one `NextLayer` template argument The type requirements for `NextLayer` are already familiar to users as they are documented in Asio: AsyncReadStream, AsyncWriteStream, SyncReadStream, SyncWriteStream.

    The type requirements for instantiating `beast::websocket::stream` versus `websocketpp::connection` with user defined types are vastly reduced (18 functions versus 2). Note that websocketpp connections are passed by `shared_ptr`. Beast does not use `shared_ptr` anywhere in its public interface. A `beast::websocket::stream` is constructible and movable in a manner identical to a `boost::asio::ip::tcp::socket`. Callers can put such objects in a `shared_ptr` if they want to, but there is no requirement to do so.

    Beast | websocketpp
    --- | ---

        template<class NextLayer>
        class stream
        {
            NextLayer next_layer_;
            ...
        }

        template <typename config>
        class connection
        : public config::transport_type::transport_con_type
        , public config::connection_base
        {
        public:
            typedef lib::shared_ptr<type> ptr;
            ...
        }

  3. Client and Server Role

    websocketpp provides multi-role support through a hierarchy of different classes. A `beast::websocket::stream` is role-agnostic, it offers member functions to perform both client and server handshakes in the same class. The same types are used for client and server streams.

    Beast | websocketpp, also
    --- | ---

    <not needed>

        template <typename config>
        class client : public endpoint<connection<config>,config>;
        template <typename config>
        class server : public endpoint<connection<config>,config>;

  4. Thread Safety

    websocketpp uses mutexes to protect shared data from concurrent access. In contrast, Beast does not use mutexes anywhere in its implementation. Instead, it follows the Asio pattern. Calls to asynchronous initiation functions use the same method to invoke intermediate handlers as the method used to invoke the final handler, through the `asio_handler_invoke` mechanism.

    The only requirement in Beast is that calls to asynchronous initiation functions are made from the same implicit or explicit strand. For example, if the `io_context` associated with a `beast::websocket::stream` is single threaded, this counts as an implicit strand and no performance costs associated with mutexes are incurred.

    Beast | websocketpp
    --- | ---

        template <class Function>
        friend
        void asio_handler_invoke(Function&& f, read_frame_op* op)
        {
            return boost_asio_handler_invoke_helpers::invoke(f, op->d_->h);
        }

        mutex_type m_read_mutex;

  5. Callback Model

    websocketpp requires a one-time call to set the handler for each event in its interface (for example, upon message receipt). The handler is represented by a `std::function` equivalent. Its important to recognize that the websocketpp interface performs type-erasure on this handler.

    In comparison, Beast handlers are specified in a manner identical to Boost.Asio. They are function objects which can be copied or moved but most importantly they are not type erased. The compiler can see through the type directly to the implementation, permitting optimization. Furthermore, Beast follows the Asio rules for treatment of handlers. It respects any allocation, continuation, or invocation customizations associated with the handler through the use of argument dependent lookup overloads of functions such as `asio_handler_allocate`.

    The Beast completion handler is provided at the call site. For each call to an asynchronous initiation function, it is guaranteed that there will be exactly one final call to the handler. This functions exactly the same way as the asynchronous initiation functions found in Boost.Asio, allowing the composition of higher level abstractions.

    Beast | websocketpp, also
    --- | ---

        template<
            class DynamicBuffer,    // Supports user defined types
            class ReadHandler       // Handler is NOT type-erased
        >
        typename async_completion<  // Return value customization
            ReadHandler,            // supports futures and coroutines
            void(error_code)
                >::result_type
        async_read(
            DynamicBuffer& dynabuf,
            ReadHandler&& handler);

        typedef lib::function<
            void(connection_hdl,message_ptr)
                > message_handler;
        void set_message_handler(message_handler h);

  6. Extensible Asynchronous Model

    Beast fully supports the Extensible Asynchronous Model developed by Christopher Kohlhoff, author of Boost.Asio (see Section 8).

    Beast websocket asynchronous interfaces may be used seamlessly with `std::future` stackful/stackless coroutines, or user defined customizations.

    Beast | websocketpp
    --- | ---

        beast::async_completion<
            ReadHandler,
            void(error_code)> completion{handler};
        read_op<
            DynamicBuffer, decltype(completion.handler)>{
                completion.handler, *this, op, buffer};

        return completion.result.get();     // Customization point

    <not available>

  7. Message Buffering

    websocketpp defines a message buffer, passed in arguments by `shared_ptr`, and an associated message manager which permits aggregation and reuse of memory. The implementation of `websocketpp::message` uses a `std::string` to hold the payload. If an incoming message is broken up into multiple frames, the string may be reallocated for each continuation frame. The `std::string` always uses the standard allocator, it is not possible to customize the choice of allocator.

    Beast allows callers to specify the object for receiving the message or frame data, which is of any type meeting the requirements of DynamicBuffer (modeled after `boost::asio::streambuf`).

    Beast comes with the class `basic_multi_buffer`, an efficient implementation of the DynamicBuffer concept which makes use of multiple allocated octet arrays. If an incoming message is broken up into multiple pieces, no reallocation occurs. Instead, new allocations are appended to the sequence when existing allocations are filled. Beast does not impose any particular memory management model on callers. The `basic_multi_buffer` provided by beast supports standard allocators through a template argument. Use the DynamicBuffer that comes with beast, customize the allocator if you desire, or provide your own type that meets the requirements.

    Beast | websocketpp
    --- | ---

        template<class DynamicBuffer>
        read(DynamicBuffer& dynabuf);

        template <template<class> class con_msg_manager>
        class message {
        public:
            typedef lib::shared_ptr<message> ptr;
            ...
            std::string m_payload;
            ...
        };

  8. Sending Messages

    When sending a message, websocketpp requires that the payload is packaged in a `websocketpp::message` object using `std::string` as the storage, or it requires a copy of the caller provided buffer by constructing a new message object. Messages are placed onto an outgoing queue. An asynchronous write operation runs in the background to clear the queue. No user facing handler can be registered to be notified when messages or frames have completed sending.

    Beast doesn't allocate or make copies of buffers when sending data. The caller's buffers are sent in-place. You can use any object meeting the requirements of __ConstBufferSequence, permitting efficient scatter-gather I/O.

    The ConstBufferSequence interface allows callers to send data from memory-mapped regions (not possible in websocketpp). Callers can also use the same buffers to send data to multiple streams, for example broadcasting common subscription data to many clients at once. For each call to `async_write` the completion handler is called once when the data finishes sending, in a manner identical to `boost::asio::async_write`.

    Beast | websocketpp
    --- | ---

        template<class ConstBufferSequence>
        void
        write(ConstBufferSequence const& buffers);

        lib::error_code send(std::string const & payload,
            frame::opcode::value op = frame::opcode::text);
        ...
        lib::error_code send(message_ptr msg);

  9. Streaming Messages

    websocketpp requires that the entire message fit into memory, and that the size is known ahead of time.

    Beast allows callers to compose messages in individual frames. This is useful when the size of the data is not known ahead of time or if it is not desired to buffer the entire message in memory at once before sending it. For example, sending periodic output of a database query running on a coroutine. Or sending the contents of a file in pieces, without bringing it all into memory.

    Beast | websocketpp
    --- | ---

        template<class ConstBufferSequence>
        void
        write_some(bool fin,
            ConstBufferSequence const& buffers);

    <not available>

  10. Flow Control

    The websocketpp read implementation continuously reads asynchronously from the network and buffers message data. To prevent unbounded growth and leverage TCP/IP's flow control mechanism, callers can periodically turn this 'read pump' off and back on.

    In contrast a `beast::websocket::stream` does not independently begin background activity, nor does it buffer messages. It receives data only when there is a call to an asynchronous initiation function (for example `beast::websocket::stream::async_read`) with an associated handler. Applications do not need to implement explicit logic to regulate the flow of data. Instead, they follow the traditional model of issuing a read, receiving a read completion, processing the message, then issuing a new read and repeating the process.

    Beast | websocketpp
    --- | ---

    <implicit>

        lib::error_code pause_reading();
        lib::error_code resume_reading();

  11. Connection Establishment

    websocketpp offers the `endpoint` class which can handle binding and listening to a port, and spawning connection objects.

    Beast does not reinvent the wheel here, callers use the interfaces already in `boost::asio` for receiving incoming connections resolving host names, or establishing outgoing connections. After the socket (or `boost::asio::ssl::stream`) is connected, the `beast::websocket::stream` is constructed around it and the WebSocket handshake can be performed.

    Beast users are free to implement their own "connection manager", but there is no requirement to do so.

    Beast, also | websocketpp
    --- | ---

        #include <boost/asio.hpp>

        template <typename config>
        class endpoint : public config::socket_type;

  12. WebSocket Handshaking

    Callers invoke `beast::websocket::accept` to perform the WebSocket handshake, but there is no requirement to use this function. Advanced users can perform the WebSocket handshake themselves. Beast WebSocket provides the tools for composing the request or response, and the Beast HTTP interface provides the container and algorithms for sending and receiving HTTP/1 messages including the necessary HTTP Upgrade request for establishing the WebSocket session.

    Beast allows the caller to pass the incoming HTTP Upgrade request for the cases where the caller has already received an HTTP message. This flexibility permits novel and robust implementations. For example, a listening socket that can handshake in multiple protocols on the same port.

    Sometimes callers want to read some bytes on the socket before reading the WebSocket HTTP Upgrade request. Beast allows these already-received bytes to be supplied to an overload of the accepting function to permit sophisticated features. For example, a listening socket that can accept both regular WebSocket and Secure WebSocket (SSL) connections.

    Beast, also | websocketpp
    --- | ---

        template<class ConstBufferSequence>
        void
        accept(ConstBufferSequence const& buffers);

        template<class Allocator>
        void
        accept(http::header<true, http::basic_fields<Allocator>> const& req);

    <not available>