[/ Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail dot com) Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ] [section:design Design choices] The implementations are driven by business needs of cryptocurrency server applications (e.g. [@https://ripple.com Ripple]) written in C++. These needs were not met by existing solutions so Beast was written from scratch as a solution. Beast's design philosophy avoid flaws exhibited by other libraries: * Don't sacrifice performance. * Don't do too much, otherwise interfaces become rigid. * Symmetric interfaces (client and server the same, or close to it). * Emulate Boost.Asio interfaces as much as possible, since Asio is proven and it is familiar to users. * Let library users make the important decisions such as how to allocate memory or how to leverage flow control. Beast uses the [link beast.types.DynamicBuffer [*`DynamicBuffer`]] concept presented in the Netwoking TS, and relies heavily on the Boost.Asio [*`ConstBufferSequence`] and [*`MutableBufferSequence`] concepts for passing buffers to functions. The authors have found the dynamic buffer and buffer sequence interfaces to be optimal for interacting with Asio, and for other tasks such as incremental parsing of data in buffers (for example, parsing websocket frames stored in a [link beast.ref.static_streambuf `static_streambuf`]). During the development of Beast the authors have studied other software packages and in particular the comments left during the Boost Review process of other packages offering similar functionality. In this section we attempt to address those issues. [variablelist [[ "I would also like to see instances of this library being used in production. That would give some evidence that the design works in practice."" ][ Beast.HTTP and Beast.WebSocket are production ready and currently running on public servers receiving traffic and handling millions of dollars worth of financial transactions daily. The servers run [*rippled], open source software ([@https://github.com/ripple/rippled repository]) implementing the [@https://ripple.com/files/ripple_consensus_whitepaper.pdf [*Ripple Consensus Protocol]], technology provided by [@http://ripple.com Ripple]. ]] ] [section:http HTTP] For HTTP we to model the message to maximize flexibility of implementation strategies while allowing familiar verbs such as [*`read`] and [*`write`]. The HTTP interface is further driven by the needs of the WebSocket module, as a WebSocket session requires a HTTP Upgrade handshake exchange at the start. Other design goals: * Don't try to invent a complete web server or client * Have simple free functions to send and receive messages. * Allow the message object to be customized, [variablelist [[ "Some more advanced examples, e.g. including TLS with client/server certificates would help."" ][ The HTTP interface doesn't try to reinvent the wheel, it just uses the `boost::asio::ip::tcp::socket` or `boost::asio::ssl::stream` that you set up beforehand. Callers use the interfaces already existing on those objects to make outgoing connections, accept incoming connections, or establish TLS sessions with certificates. We find the available Asio examples for performing these tasks sufficient. ]] [[ "A built-in router?" ][ We presume this means a facility to match expressions against the URI in HTTP requests, and dispatch them to calling code. The authors feel that this is a responsibility of higher level code. Beast.HTTP does not try to offer a web server. ]] [[ "Cookies? Forms/File Uploads?"" ][ Cookies, or managing these types of HTTP headers in general, is the responsibility of higher levels. Beast.HTTP just tries to get complete messages to and from the calling code. It deals in the HTTP headers just enough to process the message body and leaves the rest to callers. However, for forms and file uploads the symmetric interface of the message class allows HTTP requests to include arbitrary body types including those needed to upload a file or fill out a form. ]] [[ "...supporting TLS (is this a feature? If not this would be a show-stopper), etc. ][ Beast.HTTP does not provide direct facilities for implementing TLS connections; however, the interfaces already existing on the `boost::asio::ssl::stream` are available and can be used to establish secure connections. Then, functions like `http::read` or `http::async_write` can work with those encrypted connections with no problem. ]] [[ "There should also be more examples of how to integrate the http service with getting files from the file system, generating responses CGI-style" ][ The design goal for the library is to not try to invent a web server. We feel that there is a strong need for a basic implementation that models the HTTP message and provides functions to send and receive them over Asio. Such an implementation should serve as a building block upon which higher abstractions such as the aforementioned HTTP service or cgi-gateway can be built. ]] [[ "You should send a 100-continue to ask for the rest of the body if required." ][ These behaviors are best left to the calling software. A future library can build on Beast.HTTP to provide these behaviors. ]] [[ "What about HTTP/2?"" ][ Many reviewers feel that HTTP/2 support is an essential feature of a HTTP library. The authors agree that HTTP/2 is important but also feel that the most sensible implementation is one that does not re-use the same network reading and writing interface for 2 as that for 1.0 and 1.1. The Beast.HTTP message model is suitable for HTTP/2 and can be re-used. The IEFT HTTP Working Group adopted message compatiblity with HTTP/1.x as an explicit goal. A parser can simply emit full headers after decoding the compressed HTTP/2 headers. The stream ID is not logicaly part of the message but rather message metadata and should be communicated out-of-band (see below). HTTP/2 sessions begin with a traditional HTTP/1.1 Upgrade similar in fashion to the WebSocket upgrade. A HTTP/2 implementation can use existing Beast.HTTP primitives to perform this handshake. Free functions for HTTP/2 sessions are not possible because of the requirement to maintain per-session state. For example, to decode the compressed headers. Or to remember and respect the remote peer's window settings. The authors propose that a HTTP/2 implementation be written as a separate class template, similar to the `websocket::stream` but with additional interfaces to support version 2 features. We feel that Beast.HTTP offers enough useful functionality to justify inclusion, so that developers can take advantage of it right away instead of waiting. ]] ] [endsect] [section:websocket WebSocket] [variablelist [[ How does this compare to [@https://www.zaphoyd.com/websocketpp websocketpp], an alternate header-only WebSocket implementation? ][ [variablelist [[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: [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L774 Beast]] [websocketpp] ][ [``` template void read(opcode& op, DynamicBuffer& dynabuf) ```] [ // ] ]]]] [[2. Connection Model][ websocketpp supports multiple transports by utilizing a trait, the `config::transport_type` ([@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/transport/asio/connection.hpp#L60 asio transport example]) To get an idea of the complexity involved with implementing a transport, compare the asio transport to the [@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/transport/iostream/connection.hpp#L59 `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: [@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/SyncReadStream.html SyncReadStream], [@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/SyncWriteStream.html SyncWriteStream], [@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/AsyncReadStream.html AsyncReadStream], and [@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/AsyncWriteStream.html AsyncWriteStream]. 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. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp Beast]] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/connection.hpp#L234 websocketpp]] ][ [``` template class stream { NextLayer next_layer_; ... } ```] [``` template class connection : public config::transport_type::transport_con_type , public config::connection_base { public: typedef lib::shared_ptr 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. [table [ [Beast] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/roles/server_endpoint.hpp#L39 websocketpp], [@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/roles/client_endpoint.hpp#L42 also]] ][ [ // ] [``` template class client : public endpoint,config>; template class server : public endpoint,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 [@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/asio_handler_invoke.html 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_service` associated with a `beast::websocket::stream` is single threaded, this counts as an implicit strand and no performance costs associated with mutexes are incurred. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/impl/read_frame_op.ipp#L118 Beast]] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/transport/iostream/connection.hpp#L706 websocketpp]] ][ [``` template 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. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L834 Beast]] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/connection.hpp#L281 websocketpp], [@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/connection.hpp#L473 also]] ][ [``` template typename async_completion::result_type async_read(opcode& op, DynamicBuffer& dynabuf, ReadHandler&& handler); ```] [``` typedef lib::function message_handler; void set_message_handler(message_handler h); ```] ]]]] [[6. Extensible Asynchronous Model][ Beast fully supports the [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3896.pdf 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. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/impl/stream.ipp#L378 Beast]] [websocketpp] ][ [``` beast::async_completion completion(handler); read_op{ completion.handler, *this, op, streambuf}; return completion.result.get(); ```] [ // ] ]]]] [[7. Message Buffering][ 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 [@http://vinniefalco.github.io/beast/beast/types/DynamicBuffer.html [*DynamicBuffer]] (modeled after `boost::asio::streambuf`). Beast comes with the class `beast::basic_streambuf`, 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_streambuf` 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 [@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/basic_streambuf.hpp#L21 concept requirements]. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L774 Beast]] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/message_buffer/message.hpp#L78 websocketpp]] ][ [``` template read(opcode& op, DynamicBuffer& dynabuf); ```] [``` template class con_msg_manager> class message { public: typedef lib::shared_ptr 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 [@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/ConstBufferSequence.html 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`. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L1048 Beast]] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/connection.hpp#L672 websocketpp]] ][ [``` template 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. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L1151 Beast]] [websocketpp] ][ [``` template void write_frame(bool fin, ConstBufferSequence const& buffers); ```] [ // ] ]]]] [[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. [table [ [Beast] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/connection.hpp#L728 websocketpp]] ][ [ // ] [``` 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. [table [ [[@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/async_connect.html Beast], [@http://www.boost.org/doc/libs/1_60_0/doc/html/boost_asio/reference/basic_socket_acceptor/async_accept.html also]] [[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/transport/asio/endpoint.hpp#L52 websocketpp]] ][ [``` #include ```] [``` template 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. [table [ [[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L501 Beast], [@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L401 also]] [websocketpp] ][ [``` template void accept(ConstBufferSequence const& buffers); template void accept(http::request_v1 const& request); ```] [ // ] ]]]] ] ]] [[ What about message compression? ][ The author is currently porting ZLib 1.2.8 to modern, header-only C++11 that does not use macros or try to support ancient architectures. This deflate implementation will be available as its own individually usable interface, and also will be used to power Beast WebSocket's permessage-deflate implementation, due Q4 of 2016. However, Beast currently has sufficient functionality that users can begin taking advantage of the WebSocket protocol using this library immediately. ]] [[ Where is the TLS/SSL interface? ][ The `websocket::stream` wraps the socket or stream that you provide (for example, a `boost::asio::ip::tcp::socket` or a `boost::asio::ssl::stream`). You establish your TLS connection using the interface on `ssl::stream` like shown in all of the Asio examples, they construct your `websocket::stream` around it. It works perfectly fine; Beast.WebSocket doesn't try to reinvent the wheel or put a fresh coat of interface paint on the `ssl::stream`. The WebSocket implementation [*does] provides support for shutting down the TLS connection through the use of the ADL compile-time virtual functions [link beast.ref.websocket__teardown `teardown`] and [link beast.ref.websocket__async_teardown `async_teardown`]. These will properly close the connection as per rfc6455 and overloads are available for TLS streams. Callers may provide their own overloads of these functions for user-defined next layer types. ]] ] [endsect] [endsect]