Boost C++ Libraries

There are a few C++ published libraries which implement some of the HTTP protocol. We analyze the message model chosen by those libraries and discuss the advantages and disadvantages relative to Beast.

The general strategy used by the author to evaluate external libraries is as follows:

	Note
	Declarations examples from external libraries have been edited: portions have been removed for simplification.

cpp-netlib

cpp-netlib is a network programming library previously intended for Boost but not having gone through formal review. As of this writing it still uses the Boost name, namespace, and directory structure although the project states that Boost acceptance is no longer a goal. The library is based on Boost.Asio and bills itself as "a collection of network related routines/implementations geared towards providing a robust cross-platform networking library". It cites "Common Message Type" as a feature. As of the branch previous linked, it uses these declarations:

template <class Tag>
struct basic_message {
 public:
  typedef Tag tag;

  typedef typename headers_container<Tag>::type headers_container_type;
  typedef typename headers_container_type::value_type header_type;
  typedef typename string<Tag>::type string_type;

  headers_container_type& headers() { return headers_; }
  headers_container_type const& headers() const { return headers_; }

  string_type& body() { return body_; }
  string_type const& body() const { return body_; }

  string_type& source() { return source_; }
  string_type const& source() const { return source_; }

  string_type& destination() { return destination_; }
  string_type const& destination() const { return destination_; }

 private:
  friend struct detail::directive_base<Tag>;
  friend struct detail::wrapper_base<Tag, basic_message<Tag> >;

  mutable headers_container_type headers_;
  mutable string_type body_;
  mutable string_type source_;
  mutable string_type destination_;
};

This container is the base class template used to represent HTTP messages. It uses a "tag" type style specializations for a variety of trait classes, allowing for customization of the various parts of the message. For example, a user specializes headers_container<T> to determine what container type holds the header fields. We note some problems with the container declaration:

The design of the message container in this library is cumbersome with its system of customization using trait specializations. The use of these customizations is extremely limited due to the way they are used in the container declaration, making the design overly complex without corresponding benefit.

Boost.HTTP

boost.http is a library resulting from the 2014 Google Summer of Code. It was submitted for a Boost formal review and rejected in 2015. It is based on Boost.Asio, and development on the library has continued to the present. As of the branch previously linked, it uses these message declarations:

template<class Headers, class Body>
struct basic_message
{
    typedef Headers headers_type;
    typedef Body body_type;

    headers_type &headers();

    const headers_type &headers() const;

    body_type &body();

    const body_type &body() const;

    headers_type &trailers();

    const headers_type &trailers() const;

private:
    headers_type headers_;
    body_type body_;
    headers_type trailers_;
};

typedef basic_message<boost::http::headers, std::vector<std::uint8_t>> message;

template<class Headers, class Body>
struct is_message<basic_message<Headers, Body>>: public std::true_type {};

This representation addresses a narrow range of use cases. It has limited potential for customization and performance. It is more difficult to use because it excludes the start line fields from the model.

C++ REST SDK (cpprestsdk)

cpprestsdk is a Microsoft project which "...aims to help C++ developers connect to and interact with services". It offers the most functionality of the libraries reviewed here, including support for Websocket services using its websocket++ dependency. It can use native APIs such as HTTP.SYS when building Windows based applications, and it can use Boost.Asio. The WebSocket module uses Boost.Asio exclusively.

As cpprestsdk is developed by a large corporation, it contains quite a bit of functionality and necessarily has more interfaces. We will break down the interfaces used to model messages into more manageable pieces. This is the container used to store the HTTP header fields:

class http_headers
{
public:
    ...

private:
    std::map<utility::string_t, utility::string_t, _case_insensitive_cmp> m_headers;
};

This declaration is quite bare-bones. We note the typical problems of most field containers:

Now we analyze the structure of the larger message container. The library uses a handle/body idiom. There are two public message container interfaces, one for requests (http_request) and one for responses (http_response). Each interface maintains a private shared pointer to an implementation class. Public member function calls are routed to the internal implementation. This is the first implementation class, which forms the base class for both the request and response implementations:

namespace details {

class http_msg_base
{
public:
    http_headers &headers() { return m_headers; }

    _ASYNCRTIMP void set_body(const concurrency::streams::istream &instream, const utf8string &contentType);

    /// Set the stream through which the message body could be read
    void set_instream(const concurrency::streams::istream &instream)  { m_inStream = instream; }

    /// Set the stream through which the message body could be written
    void set_outstream(const concurrency::streams::ostream &outstream, bool is_default)  { m_outStream = outstream; m_default_outstream = is_default; }

    const pplx::task_completion_event<utility::size64_t> & _get_data_available() const { return m_data_available; }

protected:
    /// Stream to read the message body.
    concurrency::streams::istream m_inStream;

    /// stream to write the msg body
    concurrency::streams::ostream m_outStream;

    http_headers m_headers;
    bool m_default_outstream;

    /// <summary> The TCE is used to signal the availability of the message body. </summary>
    pplx::task_completion_event<utility::size64_t> m_data_available;
};

To understand these declarations we need to first understand that cpprestsdk uses the asynchronous model defined by Microsoft's Concurrency Runtime. Identifiers from the pplx namespace define common asynchronous patterns such as tasks and events. The concurrency::streams::istream parameter and m_data_available data member indicates a lack of separation of concerns. The representation of HTTP messages should not be conflated with the asynchronous model used to serialize or parse those messages in the message declarations.

The next declaration forms the complete implementation class referenced by the handle in the public interface (which follows after):

/// Internal representation of an HTTP request message.
class _http_request final : public http::details::http_msg_base, public std::enable_shared_from_this<_http_request>
{
public:
    _ASYNCRTIMP _http_request(http::method mtd);

    _ASYNCRTIMP _http_request(std::unique_ptr<http::details::_http_server_context> server_context);

    http::method &method() { return m_method; }

    const pplx::cancellation_token &cancellation_token() const { return m_cancellationToken; }

    _ASYNCRTIMP pplx::task<void> reply(const http_response &response);

private:

    // Actual initiates sending the response, without checking if a response has already been sent.
    pplx::task<void> _reply_impl(http_response response);

    http::method m_method;

    std::shared_ptr<progress_handler> m_progress_handler;
};

} // namespace details

As before, we note that the implementation class for HTTP requests concerns itself more with the mechanics of sending the message asynchronously than it does with actually modeling the HTTP message as described in rfc7230:

Finally, here is the public class which represents an HTTP request:

class http_request
{
public:
    const http::method &method() const { return _m_impl->method(); }

    void set_method(const http::method &method) const { _m_impl->method() = method; }

    /// Extract the body of the request message as a string value, checking that the content type is a MIME text type.
    /// A body can only be extracted once because in some cases an optimization is made where the data is 'moved' out.
    pplx::task<utility::string_t> extract_string(bool ignore_content_type = false)
    {
        auto impl = _m_impl;
        return pplx::create_task(_m_impl->_get_data_available()).then([impl, ignore_content_type](utility::size64_t) { return impl->extract_string(ignore_content_type); });
    }

    /// Extracts the body of the request message into a json value, checking that the content type is application/json.
    /// A body can only be extracted once because in some cases an optimization is made where the data is 'moved' out.
    pplx::task<json::value> extract_json(bool ignore_content_type = false) const
    {
        auto impl = _m_impl;
        return pplx::create_task(_m_impl->_get_data_available()).then([impl, ignore_content_type](utility::size64_t) { return impl->_extract_json(ignore_content_type); });
    }

    /// Sets the body of the message to the contents of a byte vector. If the 'Content-Type'
    void set_body(const std::vector<unsigned char> &body_data);

    /// Defines a stream that will be relied on to provide the body of the HTTP message when it is
    /// sent.
    void set_body(const concurrency::streams::istream &stream, const utility::string_t &content_type = _XPLATSTR("application/octet-stream"));

    /// Defines a stream that will be relied on to hold the body of the HTTP response message that
    /// results from the request.
    void set_response_stream(const concurrency::streams::ostream &stream);
    {
        return _m_impl->set_response_stream(stream);
    }

    /// Defines a callback function that will be invoked for every chunk of data uploaded or downloaded
    /// as part of the request.
    void set_progress_handler(const progress_handler &handler);

private:
    friend class http::details::_http_request;
    friend class http::client::http_client;

    std::shared_ptr<http::details::_http_request> _m_impl;
};

It is clear from this declaration that the goal of the message model in this library is driven by its use-case (interacting with REST servers) and not to model HTTP messages generally. We note problems similar to the other declarations:

The general theme of the HTTP message model in cpprestsdk is "no user definable customizations". There is no allocator support, and no separation of concerns. It is designed to perform a specific set of behaviors. In other words, it does not follow the open/closed principle.

Tasks in the Concurrency Runtime operate in a fashion similar to std::future, but with some improvements such as continuations which are not yet in the C++ standard. The costs of using a task based asynchronous interface instead of completion handlers is well documented: synchronization points along the call chain of composed task operations which cannot be optimized away. See: A Universal Model for Asynchronous Operations (Kohlhoff).