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Add WebSocket implementation comparison doc

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Vinnie Falco
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@@ -8,9 +8,9 @@
[section:design Design choices]
The implementations are driven by business needs of cryptocurrency server
applications ([@https://ripple.com Ripple] written in C++. These
needs were not met by existing solutions so new code was written. The new
code tries to avoid design flaws encountered in the already-existing software
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.
@@ -44,11 +44,13 @@ to address those issues.
in production. That would give some evidence that the design
works in practice.""
][
Beast.HTTP and Beast.WebSocket will be used in [*rippled], an
asynchronous peer to peer server that implements the
[*Ripple Consensus Protocol]. These servers are deployed in multiple
production environments, with banks in many countries running client
applications that connect to [*rippled].
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].
]]
]
@@ -171,26 +173,453 @@ start. Other design goals:
[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<class Streambuf>
void
read(opcode& op, Streambuf& streambuf)
```]
[
/<not available>/
]
]]]]
[[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 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.
[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]]
][
[
/<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
[@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 <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.
[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<class Streambuf, class ReadHandler>
typename async_completion<ReadHandler, void(error_code)>::result_type
async_read(opcode& op, Streambuf& streambuf, 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
[@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<ReadHandler, void(error_code)> completion(handler);
read_op<Streambuf, decltype(completion.handler)>{
completion.handler, *this, op, streambuf};
return completion.result.get();
```]
[
/<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 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<class DynamicBuffer>
read(opcode& op, 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
[@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<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.
[table
[
[[@https://github.com/vinniefalco/Beast/blob/6c8b4b2f8dde72b01507e4ac7fde4ffea57ebc99/include/beast/websocket/stream.hpp#L1151 Beast]]
[websocketpp]
][
[```
template<class ConstBufferSequence>
void
write_frame(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.
[table
[
[Beast]
[[@https://github.com/zaphoyd/websocketpp/blob/378437aecdcb1dfe62096ffd5d944bf1f640ccc3/websocketpp/connection.hpp#L728 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.
[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 <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.
[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<class ConstBufferSequence>
void
accept(ConstBufferSequence const& buffers);
template<class Body, class Headers>
void
accept(http::request_v1<Body, Headers> const& request);
```]
[
/<not available>/
]
]]]]
]
]]
[[
What about message compression?
][
The feature is not currently present in the library, but the choice
of type requirements for buffers passed to the read functions have been
made with compression in mind. There is the plan to add this feature;
however, we feel that even without compression users can begin taking
advantage of the WebSocket protocol immediately with this library.
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` just 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::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
@@ -200,14 +629,11 @@ start. Other design goals:
for TLS streams. Callers may provide their own overloads of these functions
for user-defined next layer types.
]]
]
[endsect]
[endsect]