Thanks for the extensive explanations! Regards Hartmut --------------- http://boost-spirit.com http://stellar.cct.lsu.edu

-----Original Message----- From: Boost [mailto:boost-bounces@lists.boost.org] On Behalf Of Vinnie Falco Sent: Friday, April 29, 2016 11:49 AM To: boost@lists.boost.org Subject: Re: [boost] New Lib "Beast", HTTP + WebSocket protocols

On Fri, Apr 29, 2016 at 8:31 AM, Hartmut Kaiser wrote:

...

How does this compare to www.zaphoyd.com/websocketpp'? Why should I use it and not websocketpp?

You should use Beast instead of websocketpp if: Beast has the features you need (compression and subprotocols planned), and you care about any of the library differences in the section that follows.

First, websocketpp is a great library; its clear the author paid a lot of attention to implementing broad support for websocket including Hixie-76 which is not a feature found in Beast. That library has great documentation, support, and quite a few years of presence. In this author's opinion it is one of the best available WebSocket implementation in C++, and by a wide margin.

Features already in websocketpp but planned for Beast include per-message compression and utilities for analyzing subprotocols. The section that follows provides a feature by feature comparison of websocketpp and Beast, with links to relevant source material and then exposition.

1. Synchronous Interfaces

websocketpp: <not available>

beast: template<class Streambuf> void read(opcode& op, Streambuf& streambuf)

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/stream.hpp#L774

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

2. Connection Model

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

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/connection.hpp#L234

beast: template<class NextLayer> class stream : public detail::stream_base { NextLayer next_layer_; ...

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/stream.hpp

websocketpp supports multiple transports by utilizing a trait, the config::transport_type. An example of the implementation of this concept is the asio transport, declared here: https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/transport/asio/connection.hpp#L60 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): https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/transport/iostream/connection.hpp#L59

In contrast, beast abstracts the transport by defining just one template argument, the NextLayer. The type requirements for NextLayer are already familiar to users as they are documented in Asio: http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/SyncRea dStream.html http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/SyncWri teStream.html http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/AsyncRe adStream.html http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/AsyncWr iteStream.html

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::socket. Callers can put such objects in a shared_ptr if they want to, but there is no requirement to do so.

3. Client and Server Roles

websocketpp: template <typename config> class client : public endpoint; template <typename config> class server : public endpoint {

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/roles/server_endpoint.hpp#L39 https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/roles/client_endpoint.hpp#L42

beast: <not applicable>

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.

4. Thread Safety

websocketpp: mutex_type m_read_mutex;

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/transport/iostream/connection.hpp#L706

beast: 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); }

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/impl/read_frame_op.ipp#L118

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: http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/asio_ha ndler_invoke.html

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_service associated with a beast::websocket::stream's value for NextLayer is single threaded, this counts as an implicit strand and no performance costs associated with mutexes are incurred.

5. Callback Model

websocketpp: typedef lib::function message_handler; void set_message_handler(message_handler h);

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/connection.hpp#L281 https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/connection.hpp#L473

beast: template typename async_completion::result_type async_read(opcode& op, Streambuf& streambuf, ReadHandler&& handler);

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/stream.hpp#L834

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 customizations, continuation customization, or invoke customization 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.

6. Extensible Asynchronous Model

websocketpp: <not available>

beast: ... beast::async_completion completion(handler); read_op{ completion.handler, *this, op, streambuf}; return completion.result.get();

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/impl/stream.ipp#L378

Beast fully supports the Extensible Asynchronous Model developed by Christopher Kohlhoff, author of Boost.Asio. See Section 8 in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3896.pdf

This means that Beast websocket asynchronous interface can be used with std::future, stackful/stackless coroutines, or user defined customizations.

7. Message Buffering

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

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/message_buffer/message.hpp#L78

beast: template<class Streambuf>

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/stream.hpp#L834 http://vinniefalco.github.io/beast/beast/types/Streambuf.html

websocketpp defines a message buffer, passed in arguments by shared_ptr, and an associated message manager which permits aggregation and memory 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 Streambuf (modeled after boost::asio::streambuf) and described here: http://vinniefalco.github.io/beast/beast/types/Streambuf.html

Beast comes with the class beast::basic_streambuf, an efficient implementation of the Streambuf 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_streambuf provided by beast supports standard allocators through a template argument. Use the Streambuf that comes with beast, customize the allocator if you desire, or provide your own type that meets the requirements: https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/basic_streambuf.hpp#L21

8. Sending Messages

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

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/connection.hpp#L672

beast: template typename async_completion::result_type async_write(ConstBufferSequence const& buffers, WriteHandler&& handler);

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/stream.hpp#L1048

When sending a message, websocketpp requires that the payload is packaged in a websocketpp::message object using std::string as the storage, or it makes 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 and copy buffers when sending data. The callers buffers are sent in-place. You can use any object meeting the requirements of ConstBufferSequence, permitting efficient scatter-gather I/O: http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/ConstBu fferSequence.html

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.

9. Streaming Messages

websocketpp: <not available>

beast: template typename async_completion::result_type async_write_frame(bool fin, ConstBufferSequence const& buffers, WriteHandler&& handler);

https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffe a57ebc99/include/beast/websocket/stream.hpp#L1151

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.

10. Flow Control

websocketpp: lib::error_code pause_reading();

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/connection.hpp#L728

beast: <implicit>

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 off the read pump. 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.

11. Connection Establishment

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

https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944b f1f640ccc3/websocketpp/transport/asio/endpoint.hpp#L52

beast: boost::asio

http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/async_c onnect.html http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/basic_s ocket_acceptor/async_accept.html

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.

---

The design choices of Beast.WebSocket were made to give users the familiar Asio interface while leveraging its strengths, to create a library that is lean, easy to understand, and doesn't duplicate functionality already possible in Asio. We hope that we've succeeded in this goal.

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